1 | \documentclass[../main/NEMO_manual]{subfiles} |
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2 | |
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3 | \begin{document} |
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4 | |
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5 | \chapter{Surface Boundary Condition (SBC, SAS, ISF, ICB)} |
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6 | \label{chap:SBC} |
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7 | |
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8 | \thispagestyle{plain} |
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9 | |
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10 | \chaptertoc |
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11 | |
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12 | \paragraph{Changes record} ~\\ |
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13 | |
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14 | {\footnotesize |
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15 | \begin{tabularx}{\textwidth}{l||X|X} |
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16 | Release & Author(s) & Modifications \\ |
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17 | \hline |
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18 | {\em 4.0} & {\em ...} & {\em ...} \\ |
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19 | {\em 3.6} & {\em ...} & {\em ...} \\ |
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20 | {\em 3.4} & {\em ...} & {\em ...} \\ |
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21 | {\em <=3.4} & {\em ...} & {\em ...} |
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22 | \end{tabularx} |
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23 | } |
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24 | |
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25 | \clearpage |
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26 | |
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27 | \begin{listing} |
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28 | \nlst{namsbc} |
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29 | \caption{\forcode{&namsbc}} |
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30 | \label{lst:namsbc} |
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31 | \end{listing} |
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32 | |
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33 | The ocean needs seven fields as surface boundary condition: |
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34 | |
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35 | \begin{itemize} |
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36 | \item the two components of the surface ocean stress $\left( {\tau_u \;,\;\tau_v} \right)$ |
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37 | \item the incoming solar and non solar heat fluxes $\left( {Q_{ns} \;,\;Q_{sr} } \right)$ |
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38 | \item the surface freshwater budget $\left( {\textit{emp}} \right)$ |
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39 | \item the surface salt flux associated with freezing/melting of seawater $\left( {\textit{sfx}} \right)$ |
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40 | \item the atmospheric pressure at the ocean surface $\left( p_a \right)$ |
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41 | \end{itemize} |
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42 | |
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43 | Four different ways are available to provide the seven fields to the ocean. They are controlled by |
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44 | namelist \nam{sbc}{sbc} variables: |
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45 | |
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46 | \begin{itemize} |
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47 | \item a bulk formulation (\np[=.true.]{ln_blk}{ln\_blk} with four possible bulk algorithms), |
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48 | \item a flux formulation (\np[=.true.]{ln_flx}{ln\_flx}), |
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49 | \item a coupled or mixed forced/coupled formulation (exchanges with a atmospheric model via the OASIS coupler), |
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50 | (\np{ln_cpl}{ln\_cpl} or \np[=.true.]{ln_mixcpl}{ln\_mixcpl}), |
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51 | \item a user defined formulation (\np[=.true.]{ln_usr}{ln\_usr}). |
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52 | \end{itemize} |
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53 | |
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54 | The frequency at which the forcing fields have to be updated is given by the \np{nn_fsbc}{nn\_fsbc} namelist parameter. |
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55 | |
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56 | When the fields are supplied from data files (bulk, flux and mixed formulations), |
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57 | the input fields do not need to be supplied on the model grid. |
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58 | Instead, a file of coordinates and weights can be supplied to map the data from the input fields grid to |
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59 | the model points (so called "Interpolation on the Fly", see \autoref{subsec:SBC_iof}). |
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60 | If the "Interpolation on the Fly" option is used, input data belonging to land points (in the native grid) |
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61 | should be masked or filled to avoid spurious results in proximity of the coasts, as |
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62 | large sea-land gradients characterize most of the atmospheric variables. |
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63 | |
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64 | In addition, the resulting fields can be further modified using several namelist options. |
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65 | These options control: |
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66 | |
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67 | \begin{itemize} |
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68 | \item the rotation of vector components supplied relative to an east-north coordinate system onto |
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69 | the local grid directions in the model, |
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70 | \item the use of a land/sea mask for input fields (\np[=.true.]{nn_lsm}{nn\_lsm}), |
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71 | \item the addition of a surface restoring term to observed SST and/or SSS (\np[=.true.]{ln_ssr}{ln\_ssr}), |
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72 | \item the modification of fluxes below ice-covered areas (using climatological ice-cover or a sea-ice model) |
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73 | (\np[=0..3]{nn_ice}{nn\_ice}), |
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74 | \item the addition of river runoffs as surface freshwater fluxes or lateral inflow (\np[=.true.]{ln_rnf}{ln\_rnf}), |
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75 | \item the addition of ice-shelf melting as lateral inflow (parameterisation) or |
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76 | as fluxes applied at the land-ice ocean interface (\np[=.true.]{ln_isf}{ln\_isf}), |
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77 | \item the addition of a freshwater flux adjustment in order to avoid a mean sea-level drift |
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78 | (\np[=0..2]{nn_fwb}{nn\_fwb}), |
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79 | \item the transformation of the solar radiation (if provided as daily mean) into an analytical diurnal cycle |
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80 | (\np[=.true.]{ln_dm2dc}{ln\_dm2dc}), |
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81 | \item the activation of wave effects from an external wave model (\np[=.true.]{ln_wave}{ln\_wave}), |
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82 | \item a neutral drag coefficient is read from an external wave model (\np[=.true.]{ln_cdgw}{ln\_cdgw}), |
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83 | \item the Stokes drift from an external wave model is accounted for (\np[=.true.]{ln_sdw}{ln\_sdw}), |
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84 | \item the choice of the Stokes drift profile parameterization (\np[=0..2]{nn_sdrift}{nn\_sdrift}), |
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85 | \item the surface stress given to the ocean is modified by surface waves (\np[=.true.]{ln_tauwoc}{ln\_tauwoc}), |
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86 | \item the surface stress given to the ocean is read from an external wave model (\np[=.true.]{ln_tauw}{ln\_tauw}), |
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87 | \item the Stokes-Coriolis term is included (\np[=.true.]{ln_stcor}{ln\_stcor}), |
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88 | \item the light penetration in the ocean (\np[=.true.]{ln_traqsr}{ln\_traqsr} with namelist \nam{tra_qsr}{tra\_qsr}), |
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89 | \item the atmospheric surface pressure gradient effect on ocean and ice dynamics (\np[=.true.]{ln_apr_dyn}{ln\_apr\_dyn} with namelist \nam{sbc_apr}{sbc\_apr}), |
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90 | \item the effect of sea-ice pressure on the ocean (\np[=.true.]{ln_ice_embd}{ln\_ice\_embd}). |
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91 | \end{itemize} |
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92 | |
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93 | In this chapter, we first discuss where the surface boundary conditions appear in the model equations. |
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94 | Then we present the three ways of providing the surface boundary conditions, |
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95 | followed by the description of the atmospheric pressure and the river runoff. |
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96 | Next, the scheme for interpolation on the fly is described. |
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97 | Finally, the different options that further modify the fluxes applied to the ocean are discussed. |
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98 | One of these is modification by icebergs (see \autoref{sec:SBC_ICB_icebergs}), |
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99 | which act as drifting sources of fresh water. |
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100 | Another example of modification is that due to the ice shelf melting/freezing (see \autoref{sec:SBC_isf}), |
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101 | which provides additional sources of fresh water. |
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102 | |
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103 | %% ================================================================================================= |
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104 | \section{Surface boundary condition for the ocean} |
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105 | \label{sec:SBC_ocean} |
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106 | |
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107 | The surface ocean stress is the stress exerted by the wind and the sea-ice on the ocean. |
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108 | It is applied in \mdl{dynzdf} module as a surface boundary condition of the computation of |
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109 | the momentum vertical mixing trend (see \autoref{eq:DYN_zdf_sbc} in \autoref{sec:DYN_zdf}). |
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110 | As such, it has to be provided as a 2D vector interpolated onto the horizontal velocity ocean mesh, |
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111 | \ie\ resolved onto the model (\textbf{i},\textbf{j}) direction at $u$- and $v$-points. |
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112 | |
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113 | The surface heat flux is decomposed into two parts, a non solar and a solar heat flux, |
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114 | $Q_{ns}$ and $Q_{sr}$, respectively. |
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115 | The former is the non penetrative part of the heat flux |
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116 | (\ie\ the sum of sensible, latent and long wave heat fluxes plus |
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117 | the heat content of the mass exchange between the ocean and sea-ice). |
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118 | It is applied in \mdl{trasbc} module as a surface boundary condition trend of |
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119 | the first level temperature time evolution equation |
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120 | (see \autoref{eq:TRA_sbc} and \autoref{eq:TRA_sbc_lin} in \autoref{subsec:TRA_sbc}). |
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121 | The latter is the penetrative part of the heat flux. |
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122 | It is applied as a 3D trend of the temperature equation (\mdl{traqsr} module) when |
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123 | \np[=.true.]{ln_traqsr}{ln\_traqsr}. |
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124 | The way the light penetrates inside the water column is generally a sum of decreasing exponentials |
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125 | (see \autoref{subsec:TRA_qsr}). |
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126 | |
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127 | The surface freshwater budget is provided by the \textit{emp} field. |
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128 | It represents the mass flux exchanged with the atmosphere (evaporation minus precipitation) and |
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129 | possibly with the sea-ice and ice shelves (freezing minus melting of ice). |
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130 | It affects the ocean in two different ways: |
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131 | $(i)$ it changes the volume of the ocean, and therefore appears in the sea surface height equation as %GS: autoref ssh equation to be added |
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132 | a volume flux, and |
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133 | $(ii)$ it changes the surface temperature and salinity through the heat and salt contents of |
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134 | the mass exchanged with atmosphere, sea-ice and ice shelves. |
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135 | |
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136 | %\colorbox{yellow}{Miss: } |
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137 | %A extensive description of all namsbc namelist (parameter that have to be |
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138 | %created!) |
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139 | %Especially the \np{nn_fsbc}{nn\_fsbc}, the \mdl{sbc\_oce} module (fluxes + mean sst sss ssu |
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140 | %ssv) \ie\ information required by flux computation or sea-ice |
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141 | %\mdl{sbc\_oce} containt the definition in memory of the 7 fields (6+runoff), add |
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142 | %a word on runoff: included in surface bc or add as lateral obc{\ldots}. |
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143 | %Sbcmod manage the ``providing'' (fourniture) to the ocean the 7 fields |
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144 | %Fluxes update only each nf\_sbc time step (namsbc) explain relation |
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145 | %between nf\_sbc and nf\_ice, do we define nf\_blk??? ? only one |
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146 | %nf\_sbc |
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147 | %Explain here all the namlist namsbc variable{\ldots}. |
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148 | % explain : use or not of surface currents |
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149 | %\colorbox{yellow}{End Miss } |
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150 | |
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151 | The ocean model provides, at each time step, to the surface module (\mdl{sbcmod}) |
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152 | the surface currents, temperature and salinity. |
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153 | These variables are averaged over \np{nn_fsbc}{nn\_fsbc} time-step (\autoref{tab:SBC_ssm}), and |
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154 | these averaged fields are used to compute the surface fluxes at the frequency of \np{nn_fsbc}{nn\_fsbc} time-steps. |
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155 | |
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156 | \begin{table}[tb] |
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157 | \centering |
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158 | \begin{tabular}{|l|l|l|l|} |
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159 | \hline |
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160 | Variable description & Model variable & Units & point \\ |
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161 | \hline |
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162 | i-component of the surface current & ssu\_m & $m.s^{-1}$ & U \\ |
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163 | \hline |
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164 | j-component of the surface current & ssv\_m & $m.s^{-1}$ & V \\ |
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165 | \hline |
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166 | Sea surface temperature & sst\_m & \r{}$K$ & T \\\hline |
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167 | Sea surface salinty & sss\_m & $psu$ & T \\ \hline |
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168 | \end{tabular} |
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169 | \caption[Ocean variables provided to the surface module)]{ |
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170 | Ocean variables provided to the surface module (\texttt{SBC}). |
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171 | The variable are averaged over \protect\np{nn_fsbc}{nn\_fsbc} time-step, |
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172 | \ie\ the frequency of computation of surface fluxes.} |
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173 | \label{tab:SBC_ssm} |
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174 | \end{table} |
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175 | |
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176 | %\colorbox{yellow}{Penser a} mettre dans le restant l'info nn\_fsbc ET nn\_fsbc*rdt de sorte de reinitialiser la moyenne si on change la frequence ou le pdt |
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177 | |
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178 | %% ================================================================================================= |
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179 | \section{Input data generic interface} |
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180 | \label{sec:SBC_input} |
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181 | |
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182 | A generic interface has been introduced to manage the way input data |
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183 | (2D or 3D fields, like surface forcing or ocean T and S) are specified in \NEMO. |
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184 | This task is achieved by \mdl{fldread}. |
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185 | The module is designed with four main objectives in mind: |
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186 | \begin{enumerate} |
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187 | \item optionally provide a time interpolation of the input data every specified model time-step, whatever their input frequency is, |
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188 | and according to the different calendars available in the model. |
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189 | \item optionally provide an on-the-fly space interpolation from the native input data grid to the model grid. |
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190 | \item make the run duration independent from the period cover by the input files. |
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191 | \item provide a simple user interface and a rather simple developer interface by |
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192 | limiting the number of prerequisite informations. |
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193 | \end{enumerate} |
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194 | |
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195 | As a result, the user has only to fill in for each variable a structure in the namelist file to |
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196 | define the input data file and variable names, the frequency of the data (in hours or months), |
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197 | whether its is climatological data or not, the period covered by the input file (one year, month, week or day), |
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198 | and three additional parameters for the on-the-fly interpolation. |
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199 | When adding a new input variable, the developer has to add the associated structure in the namelist, |
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200 | read this information by mirroring the namelist read in \rou{sbc\_blk\_init} for example, |
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201 | and simply call \rou{fld\_read} to obtain the desired input field at the model time-step and grid points. |
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202 | |
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203 | The only constraints are that the input file is a NetCDF file, the file name follows a nomenclature |
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204 | (see \autoref{subsec:SBC_fldread}), the period it cover is one year, month, week or day, and, |
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205 | if on-the-fly interpolation is used, a file of weights must be supplied (see \autoref{subsec:SBC_iof}). |
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206 | |
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207 | Note that when an input data is archived on a disc which is accessible directly from the workspace where |
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208 | the code is executed, then the user can set the \np{cn_dir}{cn\_dir} to the pathway leading to the data. |
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209 | By default, the data are assumed to be in the same directory as the executable, so that cn\_dir='./'. |
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210 | |
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211 | %% ================================================================================================= |
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212 | \subsection[Input data specification (\textit{fldread.F90})]{Input data specification (\protect\mdl{fldread})} |
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213 | \label{subsec:SBC_fldread} |
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214 | |
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215 | The structure associated with an input variable contains the following information: |
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216 | \begin{forlines} |
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217 | ! file name ! frequency (hours) ! variable ! time interp. ! clim ! 'yearly'/ ! weights ! rotation ! land/sea mask ! |
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218 | ! ! (if <0 months) ! name ! (logical) ! (T/F) ! 'monthly' ! filename ! pairing ! filename ! |
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219 | \end{forlines} |
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220 | where |
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221 | \begin{description} |
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222 | \item [File name]: the stem name of the NetCDF file to be opened. |
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223 | This stem will be completed automatically by the model, with the addition of a '.nc' at its end and |
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224 | by date information and possibly a prefix (when using AGRIF). |
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225 | \autoref{tab:SBC_fldread} provides the resulting file name in all possible cases according to |
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226 | whether it is a climatological file or not, and to the open/close frequency (see below for definition). |
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227 | \begin{table}[htbp] |
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228 | \centering |
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229 | \begin{tabular}{|l|c|c|c|} |
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230 | \hline |
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231 | & daily or weekLL & monthly & yearly \\ |
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232 | \hline |
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233 | \np[=.false.]{clim}{clim} & fn\_yYYYYmMMdDD.nc & fn\_yYYYYmMM.nc & fn\_yYYYY.nc \\ |
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234 | \hline |
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235 | \np[=.true.]{clim}{clim} & not possible & fn\_m??.nc & fn \\ |
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236 | \hline |
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237 | \end{tabular} |
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238 | \caption[Naming nomenclature for climatological or interannual input file]{ |
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239 | Naming nomenclature for climatological or interannual input file, |
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240 | as a function of the open/close frequency. |
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241 | The stem name is assumed to be 'fn'. |
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242 | For weekly files, the 'LLL' corresponds to the first three letters of the first day of the week |
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243 | (\ie\ 'sun','sat','fri','thu','wed','tue','mon'). |
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244 | The 'YYYY', 'MM' and 'DD' should be replaced by the actual year/month/day, |
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245 | always coded with 4 or 2 digits. |
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246 | Note that (1) in mpp, if the file is split over each subdomain, |
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247 | the suffix '.nc' is replaced by '\_PPPP.nc', |
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248 | where 'PPPP' is the process number coded with 4 digits; |
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249 | (2) when using AGRIF, the prefix '\_N' is added to files, where 'N' is the child grid number. |
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250 | } |
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251 | \label{tab:SBC_fldread} |
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252 | \end{table} |
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253 | \item [Record frequency]: the frequency of the records contained in the input file. |
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254 | Its unit is in hours if it is positive (for example 24 for daily forcing) or in months if negative |
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255 | (for example -1 for monthly forcing or -12 for annual forcing). |
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256 | Note that this frequency must REALLY be an integer and not a real. |
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257 | On some computers, setting it to '24.' can be interpreted as 240! |
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258 | \item [Variable name]: the name of the variable to be read in the input NetCDF file. |
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259 | \item [Time interpolation]: a logical to activate, or not, the time interpolation. |
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260 | If set to 'false', the forcing will have a steplike shape remaining constant during each forcing period. |
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261 | For example, when using a daily forcing without time interpolation, the forcing remaining constant from |
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262 | 00h00'00'' to 23h59'59". |
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263 | If set to 'true', the forcing will have a broken line shape. |
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264 | Records are assumed to be dated at the middle of the forcing period. |
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265 | For example, when using a daily forcing with time interpolation, |
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266 | linear interpolation will be performed between mid-day of two consecutive days. |
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267 | \item [Climatological forcing]: a logical to specify if a input file contains climatological forcing which can be cycle in time, |
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268 | or an interannual forcing which will requires additional files if |
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269 | the period covered by the simulation exceeds the one of the file. |
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270 | See the above file naming strategy which impacts the expected name of the file to be opened. |
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271 | \item [Open/close frequency]: the frequency at which forcing files must be opened/closed. |
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272 | Four cases are coded: |
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273 | 'daily', 'weekLLL' (with 'LLL' the first 3 letters of the first day of the week), 'monthly' and 'yearly' which |
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274 | means the forcing files will contain data for one day, one week, one month or one year. |
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275 | Files are assumed to contain data from the beginning of the open/close period. |
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276 | For example, the first record of a yearly file containing daily data is Jan 1st even if |
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277 | the experiment is not starting at the beginning of the year. |
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278 | \item [Others]: 'weights filename', 'pairing rotation' and 'land/sea mask' are associated with |
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279 | on-the-fly interpolation which is described in \autoref{subsec:SBC_iof}. |
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280 | \end{description} |
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281 | |
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282 | Additional remarks:\\ |
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283 | (1) The time interpolation is a simple linear interpolation between two consecutive records of the input data. |
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284 | The only tricky point is therefore to specify the date at which we need to do the interpolation and |
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285 | the date of the records read in the input files. |
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286 | Following \citet{leclair.madec_OM09}, the date of a time step is set at the middle of the time step. |
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287 | For example, for an experiment starting at 0h00'00" with a one-hour time-step, |
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288 | a time interpolation will be performed at the following time: 0h30'00", 1h30'00", 2h30'00", etc. |
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289 | However, for forcing data related to the surface module, |
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290 | values are not needed at every time-step but at every \np{nn_fsbc}{nn\_fsbc} time-step. |
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291 | For example with \np[=3]{nn_fsbc}{nn\_fsbc}, the surface module will be called at time-steps 1, 4, 7, etc. |
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292 | The date used for the time interpolation is thus redefined to the middle of \np{nn_fsbc}{nn\_fsbc} time-step period. |
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293 | In the previous example, this leads to: 1h30'00", 4h30'00", 7h30'00", etc. \\ |
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294 | (2) For code readablility and maintenance issues, we don't take into account the NetCDF input file calendar. |
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295 | The calendar associated with the forcing field is build according to the information provided by |
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296 | user in the record frequency, the open/close frequency and the type of temporal interpolation. |
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297 | For example, the first record of a yearly file containing daily data that will be interpolated in time is assumed to |
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298 | start Jan 1st at 12h00'00" and end Dec 31st at 12h00'00". \\ |
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299 | (3) If a time interpolation is requested, the code will pick up the needed data in the previous (next) file when |
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300 | interpolating data with the first (last) record of the open/close period. |
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301 | For example, if the input file specifications are ''yearly, containing daily data to be interpolated in time'', |
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302 | the values given by the code between 00h00'00" and 11h59'59" on Jan 1st will be interpolated values between |
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303 | Dec 31st 12h00'00" and Jan 1st 12h00'00". |
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304 | If the forcing is climatological, Dec and Jan will be keep-up from the same year. |
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305 | However, if the forcing is not climatological, at the end of |
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306 | the open/close period, the code will automatically close the current file and open the next one. |
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307 | Note that, if the experiment is starting (ending) at the beginning (end) of |
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308 | an open/close period, we do accept that the previous (next) file is not existing. |
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309 | In this case, the time interpolation will be performed between two identical values. |
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310 | For example, when starting an experiment on Jan 1st of year Y with yearly files and daily data to be interpolated, |
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311 | we do accept that the file related to year Y-1 is not existing. |
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312 | The value of Jan 1st will be used as the missing one for Dec 31st of year Y-1. |
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313 | If the file of year Y-1 exists, the code will read its last record. |
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314 | Therefore, this file can contain only one record corresponding to Dec 31st, |
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315 | a useful feature for user considering that it is too heavy to manipulate the complete file for year Y-1. |
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316 | |
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317 | %% ================================================================================================= |
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318 | \subsection{Interpolation on-the-fly} |
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319 | \label{subsec:SBC_iof} |
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320 | |
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321 | Interpolation on the Fly allows the user to supply input files required for the surface forcing on |
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322 | grids other than the model grid. |
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323 | To do this, he or she must supply, in addition to the source data file(s), a file of weights to be used to |
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324 | interpolate from the data grid to the model grid. |
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325 | The original development of this code used the SCRIP package |
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326 | (freely available \href{http://climate.lanl.gov/Software/SCRIP}{here} under a copyright agreement). |
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327 | In principle, any package such as CDO can be used to generate the weights, but the variables in |
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328 | the input weights file must have the same names and meanings as assumed by the model. |
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329 | Two methods are currently available: bilinear and bicubic interpolations. |
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330 | Prior to the interpolation, providing a land/sea mask file, the user can decide to remove land points from |
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331 | the input file and substitute the corresponding values with the average of the 8 neighbouring points in |
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332 | the native external grid. |
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333 | Only "sea points" are considered for the averaging. |
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334 | The land/sea mask file must be provided in the structure associated with the input variable. |
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335 | The netcdf land/sea mask variable name must be 'LSM' and must have the same horizontal and vertical dimensions as |
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336 | the associated variables and should be equal to 1 over land and 0 elsewhere. |
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337 | The procedure can be recursively applied by setting nn\_lsm > 1 in namsbc namelist. |
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338 | Note that nn\_lsm=0 forces the code to not apply the procedure, even if a land/sea mask file is supplied. |
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339 | |
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340 | %% ================================================================================================= |
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341 | \subsubsection{Bilinear interpolation} |
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342 | \label{subsec:SBC_iof_bilinear} |
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343 | |
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344 | The input weights file in this case has two sets of variables: |
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345 | src01, src02, src03, src04 and wgt01, wgt02, wgt03, wgt04. |
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346 | The "src" variables correspond to the point in the input grid to which the weight "wgt" is applied. |
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347 | Each src value is an integer corresponding to the index of a point in the input grid when |
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348 | written as a one dimensional array. |
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349 | For example, for an input grid of size 5x10, point (3,2) is referenced as point 8, since (2-1)*5+3=8. |
---|
350 | There are four of each variable because bilinear interpolation uses the four points defining |
---|
351 | the grid box containing the point to be interpolated. |
---|
352 | All of these arrays are on the model grid, so that values src01(i,j) and wgt01(i,j) are used to |
---|
353 | generate a value for point (i,j) in the model. |
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354 | |
---|
355 | Symbolically, the algorithm used is: |
---|
356 | \[ |
---|
357 | f_{m}(i,j) = f_{m}(i,j) + \sum_{k=1}^{4} {wgt(k)f(idx(src(k)))} |
---|
358 | \] |
---|
359 | where function idx() transforms a one dimensional index src(k) into a two dimensional index, |
---|
360 | and wgt(1) corresponds to variable "wgt01" for example. |
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361 | |
---|
362 | %% ================================================================================================= |
---|
363 | \subsubsection{Bicubic interpolation} |
---|
364 | \label{subsec:SBC_iof_bicubic} |
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365 | |
---|
366 | Again, there are two sets of variables: "src" and "wgt". |
---|
367 | But in this case, there are 16 of each. |
---|
368 | The symbolic algorithm used to calculate values on the model grid is now: |
---|
369 | |
---|
370 | \[ |
---|
371 | \begin{split} |
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372 | f_{m}(i,j) = f_{m}(i,j) +& \sum_{k=1}^{4} {wgt(k)f(idx(src(k)))} |
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373 | + \sum_{k=5 }^{8 } {wgt(k)\left.\frac{\partial f}{\partial i}\right| _{idx(src(k))} } \\ |
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374 | +& \sum_{k=9 }^{12} {wgt(k)\left.\frac{\partial f}{\partial j}\right| _{idx(src(k))} } |
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375 | + \sum_{k=13}^{16} {wgt(k)\left.\frac{\partial ^2 f}{\partial i \partial j}\right| _{idx(src(k))} } |
---|
376 | \end{split} |
---|
377 | \] |
---|
378 | The gradients here are taken with respect to the horizontal indices and not distances since |
---|
379 | the spatial dependency has been included into the weights. |
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380 | |
---|
381 | %% ================================================================================================= |
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382 | \subsubsection{Implementation} |
---|
383 | \label{subsec:SBC_iof_imp} |
---|
384 | |
---|
385 | To activate this option, a non-empty string should be supplied in |
---|
386 | the weights filename column of the relevant namelist; |
---|
387 | if this is left as an empty string no action is taken. |
---|
388 | In the model, weights files are read in and stored in a structured type (WGT) in the fldread module, |
---|
389 | as and when they are first required. |
---|
390 | This initialisation procedure determines whether the input data grid should be treated as cyclical or not by |
---|
391 | inspecting a global attribute stored in the weights input file. |
---|
392 | This attribute must be called "ew\_wrap" and be of integer type. |
---|
393 | If it is negative, the input non-model grid is assumed to be not cyclic. |
---|
394 | If zero or greater, then the value represents the number of columns that overlap. |
---|
395 | $E.g.$ if the input grid has columns at longitudes 0, 1, 2, .... , 359, then ew\_wrap should be set to 0; |
---|
396 | if longitudes are 0.5, 2.5, .... , 358.5, 360.5, 362.5, ew\_wrap should be 2. |
---|
397 | If the model does not find attribute ew\_wrap, then a value of -999 is assumed. |
---|
398 | In this case, the \rou{fld\_read} routine defaults ew\_wrap to value 0 and |
---|
399 | therefore the grid is assumed to be cyclic with no overlapping columns. |
---|
400 | (In fact, this only matters when bicubic interpolation is required.) |
---|
401 | Note that no testing is done to check the validity in the model, |
---|
402 | since there is no way of knowing the name used for the longitude variable, |
---|
403 | so it is up to the user to make sure his or her data is correctly represented. |
---|
404 | |
---|
405 | Next the routine reads in the weights. |
---|
406 | Bicubic interpolation is assumed if it finds a variable with name "src05", otherwise bilinear interpolation is used. |
---|
407 | The WGT structure includes dynamic arrays both for the storage of the weights (on the model grid), |
---|
408 | and when required, for reading in the variable to be interpolated (on the input data grid). |
---|
409 | The size of the input data array is determined by examining the values in the "src" arrays to |
---|
410 | find the minimum and maximum i and j values required. |
---|
411 | Since bicubic interpolation requires the calculation of gradients at each point on the grid, |
---|
412 | the corresponding arrays are dimensioned with a halo of width one grid point all the way around. |
---|
413 | When the array of points from the data file is adjacent to an edge of the data grid, |
---|
414 | the halo is either a copy of the row/column next to it (non-cyclical case), |
---|
415 | or is a copy of one from the first few columns on the opposite side of the grid (cyclical case). |
---|
416 | |
---|
417 | %% ================================================================================================= |
---|
418 | \subsubsection{Limitations} |
---|
419 | \label{subsec:SBC_iof_lim} |
---|
420 | |
---|
421 | \begin{enumerate} |
---|
422 | \item The case where input data grids are not logically rectangular (irregular grid case) has not been tested. |
---|
423 | \item This code is not guaranteed to produce positive definite answers from positive definite inputs when |
---|
424 | a bicubic interpolation method is used. |
---|
425 | \item The cyclic condition is only applied on left and right columns, and not to top and bottom rows. |
---|
426 | \item The gradients across the ends of a cyclical grid assume that the grid spacing between |
---|
427 | the two columns involved are consistent with the weights used. |
---|
428 | \item Neither interpolation scheme is conservative. (There is a conservative scheme available in SCRIP, |
---|
429 | but this has not been implemented.) |
---|
430 | \end{enumerate} |
---|
431 | |
---|
432 | %% ================================================================================================= |
---|
433 | \subsubsection{Utilities} |
---|
434 | \label{subsec:SBC_iof_util} |
---|
435 | |
---|
436 | % to be completed |
---|
437 | A set of utilities to create a weights file for a rectilinear input grid is available |
---|
438 | (see the directory NEMOGCM/TOOLS/WEIGHTS). |
---|
439 | |
---|
440 | %% ================================================================================================= |
---|
441 | \subsection{Standalone surface boundary condition scheme (SAS)} |
---|
442 | \label{subsec:SBC_SAS} |
---|
443 | |
---|
444 | \begin{listing} |
---|
445 | \nlst{namsbc_sas} |
---|
446 | \caption{\forcode{&namsbc_sas}} |
---|
447 | \label{lst:namsbc_sas} |
---|
448 | \end{listing} |
---|
449 | |
---|
450 | In some circumstances, it may be useful to avoid calculating the 3D temperature, |
---|
451 | salinity and velocity fields and simply read them in from a previous run or receive them from OASIS. |
---|
452 | For example: |
---|
453 | |
---|
454 | \begin{itemize} |
---|
455 | \item Multiple runs of the model are required in code development to |
---|
456 | see the effect of different algorithms in the bulk formulae. |
---|
457 | \item The effect of different parameter sets in the ice model is to be examined. |
---|
458 | \item Development of sea-ice algorithms or parameterizations. |
---|
459 | \item Spinup of the iceberg floats |
---|
460 | \item Ocean/sea-ice simulation with both models running in parallel (\np[=.true.]{ln_mixcpl}{ln\_mixcpl}) |
---|
461 | \end{itemize} |
---|
462 | |
---|
463 | The Standalone Surface scheme provides this capacity. |
---|
464 | Its options are defined through the \nam{sbc_sas}{sbc\_sas} namelist variables. |
---|
465 | A new copy of the model has to be compiled with a configuration based on ORCA2\_SAS\_LIM. |
---|
466 | However, no namelist parameters need be changed from the settings of the previous run (except perhaps nn\_date0). |
---|
467 | In this configuration, a few routines in the standard model are overriden by new versions. |
---|
468 | Routines replaced are: |
---|
469 | |
---|
470 | \begin{itemize} |
---|
471 | \item \mdl{nemogcm}: This routine initialises the rest of the model and repeatedly calls the stp time stepping routine (\mdl{step}). |
---|
472 | Since the ocean state is not calculated all associated initialisations have been removed. |
---|
473 | \item \mdl{step}: The main time stepping routine now only needs to call the sbc routine (and a few utility functions). |
---|
474 | \item \mdl{sbcmod}: This has been cut down and now only calculates surface forcing and the ice model required. |
---|
475 | New surface modules that can function when only the surface level of the ocean state is defined can also be added |
---|
476 | (\eg\ icebergs). |
---|
477 | \item \mdl{daymod}: No ocean restarts are read or written (though the ice model restarts are retained), |
---|
478 | so calls to restart functions have been removed. |
---|
479 | This also means that the calendar cannot be controlled by time in a restart file, |
---|
480 | so the user must check that nn\_date0 in the model namelist is correct for his or her purposes. |
---|
481 | \item \mdl{stpctl}: Since there is no free surface solver, references to it have been removed from \rou{stp\_ctl} module. |
---|
482 | \item \mdl{diawri}: All 3D data have been removed from the output. |
---|
483 | The surface temperature, salinity and velocity components (which have been read in) are written along with |
---|
484 | relevant forcing and ice data. |
---|
485 | \end{itemize} |
---|
486 | |
---|
487 | One new routine has been added: |
---|
488 | |
---|
489 | \begin{itemize} |
---|
490 | \item \mdl{sbcsas}: This module initialises the input files needed for reading temperature, salinity and |
---|
491 | velocity arrays at the surface. |
---|
492 | These filenames are supplied in namelist namsbc\_sas. |
---|
493 | Unfortunately, because of limitations with the \mdl{iom} module, |
---|
494 | the full 3D fields from the mean files have to be read in and interpolated in time, |
---|
495 | before using just the top level. |
---|
496 | Since fldread is used to read in the data, Interpolation on the Fly may be used to change input data resolution. |
---|
497 | \end{itemize} |
---|
498 | |
---|
499 | The user can also choose in the \nam{sbc_sas}{sbc\_sas} namelist to read the mean (nn\_fsbc time-step) fraction of solar net radiation absorbed in the 1st T level using |
---|
500 | (\np[=.true.]{ln_flx}{ln\_flx}) and to provide 3D oceanic velocities instead of 2D ones (\np{ln_flx}{ln\_flx}\forcode{=.true.}). In that last case, only the 1st level will be read in. |
---|
501 | |
---|
502 | %% ================================================================================================= |
---|
503 | \section[Flux formulation (\textit{sbcflx.F90})]{Flux formulation (\protect\mdl{sbcflx})} |
---|
504 | \label{sec:SBC_flx} |
---|
505 | |
---|
506 | \begin{listing} |
---|
507 | \nlst{namsbc_flx} |
---|
508 | \caption{\forcode{&namsbc_flx}} |
---|
509 | \label{lst:namsbc_flx} |
---|
510 | \end{listing} |
---|
511 | |
---|
512 | In the flux formulation (\np[=.true.]{ln_flx}{ln\_flx}), |
---|
513 | the surface boundary condition fields are directly read from input files. |
---|
514 | The user has to define in the namelist \nam{sbc_flx}{sbc\_flx} the name of the file, |
---|
515 | the name of the variable read in the file, the time frequency at which it is given (in hours), |
---|
516 | and a logical setting whether a time interpolation to the model time step is required for this field. |
---|
517 | See \autoref{subsec:SBC_fldread} for a more detailed description of the parameters. |
---|
518 | |
---|
519 | Note that in general, a flux formulation is used in associated with a restoring term to observed SST and/or SSS. |
---|
520 | See \autoref{subsec:SBC_ssr} for its specification. |
---|
521 | |
---|
522 | %% ================================================================================================= |
---|
523 | \section[Bulk formulation (\textit{sbcblk.F90})]{Bulk formulation (\protect\mdl{sbcblk})} |
---|
524 | \label{sec:SBC_blk} |
---|
525 | |
---|
526 | \begin{listing} |
---|
527 | \nlst{namsbc_blk} |
---|
528 | \caption{\forcode{&namsbc_blk}} |
---|
529 | \label{lst:namsbc_blk} |
---|
530 | \end{listing} |
---|
531 | |
---|
532 | In the bulk formulation, the surface boundary condition fields are computed with bulk formulae using atmospheric fields |
---|
533 | and ocean (and sea-ice) variables averaged over \np{nn_fsbc}{nn\_fsbc} time-step. |
---|
534 | |
---|
535 | The atmospheric fields used depend on the bulk formulae used. |
---|
536 | In forced mode, when a sea-ice model is used, a specific bulk formulation is used. |
---|
537 | Therefore, different bulk formulae are used for the turbulent fluxes computation |
---|
538 | over the ocean and over sea-ice surface. |
---|
539 | For the ocean, four bulk formulations are available thanks to the \href{https://brodeau.github.io/aerobulk/}{Aerobulk} package (\citet{brodeau.barnier.ea_JPO16}): |
---|
540 | the NCAR (formerly named CORE), COARE 3.0, COARE 3.5 and ECMWF bulk formulae. |
---|
541 | The choice is made by setting to true one of the following namelist variable: |
---|
542 | \np{ln_NCAR}{ln\_NCAR}, \np{ln_COARE_3p0}{ln\_COARE\_3p0}, \np{ln_COARE_3p5}{ln\_COARE\_3p5} and \np{ln_ECMWF}{ln\_ECMWF}. |
---|
543 | For sea-ice, three possibilities can be selected: |
---|
544 | a constant transfer coefficient (1.4e-3; default value), \citet{lupkes.gryanik.ea_JGR12} (\np{ln_Cd_L12}{ln\_Cd\_L12}), and \citet{lupkes.gryanik_JGR15} (\np{ln_Cd_L15}{ln\_Cd\_L15}) parameterizations |
---|
545 | |
---|
546 | Common options are defined through the \nam{sbc_blk}{sbc\_blk} namelist variables. |
---|
547 | The required 9 input fields are: |
---|
548 | |
---|
549 | \begin{table}[htbp] |
---|
550 | \centering |
---|
551 | \begin{tabular}{|l|c|c|c|} |
---|
552 | \hline |
---|
553 | Variable description & Model variable & Units & point \\ |
---|
554 | \hline |
---|
555 | i-component of the 10m air velocity & utau & $m.s^{-1}$ & T \\ |
---|
556 | \hline |
---|
557 | j-component of the 10m air velocity & vtau & $m.s^{-1}$ & T \\ |
---|
558 | \hline |
---|
559 | 10m air temperature & tair & \r{}$K$ & T \\ |
---|
560 | \hline |
---|
561 | Specific humidity & humi & \% & T \\ |
---|
562 | \hline |
---|
563 | Incoming long wave radiation & qlw & $W.m^{-2}$ & T \\ |
---|
564 | \hline |
---|
565 | Incoming short wave radiation & qsr & $W.m^{-2}$ & T \\ |
---|
566 | \hline |
---|
567 | Total precipitation (liquid + solid) & precip & $Kg.m^{-2}.s^{-1}$ & T \\ |
---|
568 | \hline |
---|
569 | Solid precipitation & snow & $Kg.m^{-2}.s^{-1}$ & T \\ |
---|
570 | \hline |
---|
571 | Mean sea-level pressure & slp & $hPa$ & T \\ |
---|
572 | \hline |
---|
573 | \end{tabular} |
---|
574 | \label{tab:SBC_BULK} |
---|
575 | \end{table} |
---|
576 | |
---|
577 | Note that the air velocity is provided at a tracer ocean point, not at a velocity ocean point ($u$- and $v$-points). |
---|
578 | It is simpler and faster (less fields to be read), but it is not the recommended method when |
---|
579 | the ocean grid size is the same or larger than the one of the input atmospheric fields. |
---|
580 | |
---|
581 | The \np{sn_wndi}{sn\_wndi}, \np{sn_wndj}{sn\_wndj}, \np{sn_qsr}{sn\_qsr}, \np{sn_qlw}{sn\_qlw}, \np{sn_tair}{sn\_tair}, \np{sn_humi}{sn\_humi}, \np{sn_prec}{sn\_prec}, |
---|
582 | \np{sn_snow}{sn\_snow}, \np{sn_tdif}{sn\_tdif} parameters describe the fields and the way they have to be used |
---|
583 | (spatial and temporal interpolations). |
---|
584 | |
---|
585 | \np{cn_dir}{cn\_dir} is the directory of location of bulk files |
---|
586 | \np{ln_taudif}{ln\_taudif} is the flag to specify if we use Hight Frequency (HF) tau information (.true.) or not (.false.) |
---|
587 | \np{rn_zqt}{rn\_zqt}: is the height of humidity and temperature measurements (m) |
---|
588 | \np{rn_zu}{rn\_zu}: is the height of wind measurements (m) |
---|
589 | |
---|
590 | Three multiplicative factors are available: |
---|
591 | \np{rn_pfac}{rn\_pfac} and \np{rn_efac}{rn\_efac} allow to adjust (if necessary) the global freshwater budget by |
---|
592 | increasing/reducing the precipitations (total and snow) and or evaporation, respectively. |
---|
593 | The third one,\np{rn_vfac}{rn\_vfac}, control to which extend the ice/ocean velocities are taken into account in |
---|
594 | the calculation of surface wind stress. |
---|
595 | Its range must be between zero and one, and it is recommended to set it to 0 at low-resolution (ORCA2 configuration). |
---|
596 | |
---|
597 | As for the flux formulation, information about the input data required by the model is provided in |
---|
598 | the namsbc\_blk namelist (see \autoref{subsec:SBC_fldread}). |
---|
599 | |
---|
600 | %% ================================================================================================= |
---|
601 | \subsection[Ocean-Atmosphere Bulk formulae (\textit{sbcblk\_algo\_coare.F90, sbcblk\_algo\_coare3p5.F90, sbcblk\_algo\_ecmwf.F90, sbcblk\_algo\_ncar.F90})]{Ocean-Atmosphere Bulk formulae (\mdl{sbcblk\_algo\_coare}, \mdl{sbcblk\_algo\_coare3p5}, \mdl{sbcblk\_algo\_ecmwf}, \mdl{sbcblk\_algo\_ncar})} |
---|
602 | \label{subsec:SBC_blk_ocean} |
---|
603 | |
---|
604 | Four different bulk algorithms are available to compute surface turbulent momentum and heat fluxes over the ocean. |
---|
605 | COARE 3.0, COARE 3.5 and ECMWF schemes mainly differ by their roughness lenghts computation and consequently |
---|
606 | their neutral transfer coefficients relationships with neutral wind. |
---|
607 | \begin{itemize} |
---|
608 | \item NCAR (\np[=.true.]{ln_NCAR}{ln\_NCAR}): The NCAR bulk formulae have been developed by \citet{large.yeager_rpt04}. |
---|
609 | They have been designed to handle the NCAR forcing, a mixture of NCEP reanalysis and satellite data. |
---|
610 | They use an inertial dissipative method to compute the turbulent transfer coefficients |
---|
611 | (momentum, sensible heat and evaporation) from the 10m wind speed, air temperature and specific humidity. |
---|
612 | This \citet{large.yeager_rpt04} dataset is available through |
---|
613 | the \href{http://nomads.gfdl.noaa.gov/nomads/forms/mom4/NCAR.html}{GFDL web site}. |
---|
614 | Note that substituting ERA40 to NCEP reanalysis fields does not require changes in the bulk formulea themself. |
---|
615 | This is the so-called DRAKKAR Forcing Set (DFS) \citep{brodeau.barnier.ea_OM10}. |
---|
616 | \item COARE 3.0 (\np[=.true.]{ln_COARE_3p0}{ln\_COARE\_3p0}): See \citet{fairall.bradley.ea_JC03} for more details |
---|
617 | \item COARE 3.5 (\np[=.true.]{ln_COARE_3p5}{ln\_COARE\_3p5}): See \citet{edson.jampana.ea_JPO13} for more details |
---|
618 | \item ECMWF (\np[=.true.]{ln_ECMWF}{ln\_ECMWF}): Based on \href{https://www.ecmwf.int/node/9221}{IFS (Cy31)} implementation and documentation. |
---|
619 | Surface roughness lengths needed for the Obukhov length are computed following \citet{beljaars_QJRMS95}. |
---|
620 | \end{itemize} |
---|
621 | |
---|
622 | %% ================================================================================================= |
---|
623 | \subsection{Ice-Atmosphere Bulk formulae} |
---|
624 | \label{subsec:SBC_blk_ice} |
---|
625 | |
---|
626 | Surface turbulent fluxes between sea-ice and the atmosphere can be computed in three different ways: |
---|
627 | |
---|
628 | \begin{itemize} |
---|
629 | \item Constant value (\np[ Cd_ice=1.4e-3 ]{constant value}{constant\ value}): |
---|
630 | default constant value used for momentum and heat neutral transfer coefficients |
---|
631 | \item \citet{lupkes.gryanik.ea_JGR12} (\np[=.true.]{ln_Cd_L12}{ln\_Cd\_L12}): |
---|
632 | This scheme adds a dependency on edges at leads, melt ponds and flows |
---|
633 | of the constant neutral air-ice drag. After some approximations, |
---|
634 | this can be resumed to a dependency on ice concentration (A). |
---|
635 | This drag coefficient has a parabolic shape (as a function of ice concentration) |
---|
636 | starting at 1.5e-3 for A=0, reaching 1.97e-3 for A=0.5 and going down 1.4e-3 for A=1. |
---|
637 | It is theoretically applicable to all ice conditions (not only MIZ). |
---|
638 | \item \citet{lupkes.gryanik_JGR15} (\np[=.true.]{ln_Cd_L15}{ln\_Cd\_L15}): |
---|
639 | Alternative turbulent transfer coefficients formulation between sea-ice |
---|
640 | and atmosphere with distinct momentum and heat coefficients depending |
---|
641 | on sea-ice concentration and atmospheric stability (no melt-ponds effect for now). |
---|
642 | The parameterization is adapted from ECHAM6 atmospheric model. |
---|
643 | Compared to Lupkes2012 scheme, it considers specific skin and form drags |
---|
644 | to compute neutral transfer coefficients for both heat and momentum fluxes. |
---|
645 | Atmospheric stability effect on transfer coefficient is also taken into account. |
---|
646 | \end{itemize} |
---|
647 | |
---|
648 | %% ================================================================================================= |
---|
649 | \section[Coupled formulation (\textit{sbccpl.F90})]{Coupled formulation (\protect\mdl{sbccpl})} |
---|
650 | \label{sec:SBC_cpl} |
---|
651 | |
---|
652 | \begin{listing} |
---|
653 | \nlst{namsbc_cpl} |
---|
654 | \caption{\forcode{&namsbc_cpl}} |
---|
655 | \label{lst:namsbc_cpl} |
---|
656 | \end{listing} |
---|
657 | |
---|
658 | In the coupled formulation of the surface boundary condition, |
---|
659 | the fluxes are provided by the OASIS coupler at a frequency which is defined in the OASIS coupler namelist, |
---|
660 | while sea and ice surface temperature, ocean and ice albedo, and ocean currents are sent to |
---|
661 | the atmospheric component. |
---|
662 | |
---|
663 | A generalised coupled interface has been developed. |
---|
664 | It is currently interfaced with OASIS-3-MCT versions 1 to 4 (\key{oasis3}). |
---|
665 | An additional specific CPP key (\key{oa3mct\_v1v2}) is needed for OASIS-3-MCT versions 1 and 2. |
---|
666 | It has been successfully used to interface \NEMO\ to most of the European atmospheric GCM |
---|
667 | (ARPEGE, ECHAM, ECMWF, HadAM, HadGAM, LMDz), as well as to \href{http://wrf-model.org/}{WRF} |
---|
668 | (Weather Research and Forecasting Model). |
---|
669 | |
---|
670 | When PISCES biogeochemical model (\key{top}) is also used in the coupled system, |
---|
671 | the whole carbon cycle is computed. |
---|
672 | In this case, CO$_2$ fluxes will be exchanged between the atmosphere and the ice-ocean system |
---|
673 | (and need to be activated in \nam{sbc_cpl}{sbc\_cpl} ). |
---|
674 | |
---|
675 | The namelist above allows control of various aspects of the coupling fields (particularly for vectors) and |
---|
676 | now allows for any coupling fields to have multiple sea ice categories (as required by LIM3 and CICE). |
---|
677 | When indicating a multi-category coupling field in \nam{sbc_cpl}{sbc\_cpl}, the number of categories will be determined by |
---|
678 | the number used in the sea ice model. |
---|
679 | In some limited cases, it may be possible to specify single category coupling fields even when |
---|
680 | the sea ice model is running with multiple categories - |
---|
681 | in this case, the user should examine the code to be sure the assumptions made are satisfactory. |
---|
682 | In cases where this is definitely not possible, the model should abort with an error message. |
---|
683 | |
---|
684 | %% ================================================================================================= |
---|
685 | \section[Atmospheric pressure (\textit{sbcapr.F90})]{Atmospheric pressure (\protect\mdl{sbcapr})} |
---|
686 | \label{sec:SBC_apr} |
---|
687 | |
---|
688 | \begin{listing} |
---|
689 | \nlst{namsbc_apr} |
---|
690 | \caption{\forcode{&namsbc_apr}} |
---|
691 | \label{lst:namsbc_apr} |
---|
692 | \end{listing} |
---|
693 | |
---|
694 | The optional atmospheric pressure can be used to force ocean and ice dynamics |
---|
695 | (\np[=.true.]{ln_apr_dyn}{ln\_apr\_dyn}, \nam{sbc}{sbc} namelist). |
---|
696 | The input atmospheric forcing defined via \np{sn_apr}{sn\_apr} structure (\nam{sbc_apr}{sbc\_apr} namelist) |
---|
697 | can be interpolated in time to the model time step, and even in space when the interpolation on-the-fly is used. |
---|
698 | When used to force the dynamics, the atmospheric pressure is further transformed into |
---|
699 | an equivalent inverse barometer sea surface height, $\eta_{ib}$, using: |
---|
700 | \[ |
---|
701 | % \label{eq:SBC_ssh_ib} |
---|
702 | \eta_{ib} = - \frac{1}{g\,\rho_o} \left( P_{atm} - P_o \right) |
---|
703 | \] |
---|
704 | where $P_{atm}$ is the atmospheric pressure and $P_o$ a reference atmospheric pressure. |
---|
705 | A value of $101,000~N/m^2$ is used unless \np{ln_ref_apr}{ln\_ref\_apr} is set to true. |
---|
706 | In this case, $P_o$ is set to the value of $P_{atm}$ averaged over the ocean domain, |
---|
707 | \ie\ the mean value of $\eta_{ib}$ is kept to zero at all time steps. |
---|
708 | |
---|
709 | The gradient of $\eta_{ib}$ is added to the RHS of the ocean momentum equation (see \mdl{dynspg} for the ocean). |
---|
710 | For sea-ice, the sea surface height, $\eta_m$, which is provided to the sea ice model is set to $\eta - \eta_{ib}$ |
---|
711 | (see \mdl{sbcssr} module). |
---|
712 | $\eta_{ib}$ can be written in the output. |
---|
713 | This can simplify altimetry data and model comparison as |
---|
714 | inverse barometer sea surface height is usually removed from these date prior to their distribution. |
---|
715 | |
---|
716 | When using time-splitting and BDY package for open boundaries conditions, |
---|
717 | the equivalent inverse barometer sea surface height $\eta_{ib}$ can be added to BDY ssh data: |
---|
718 | \np{ln_apr_obc}{ln\_apr\_obc} might be set to true. |
---|
719 | |
---|
720 | %% ================================================================================================= |
---|
721 | \section[Surface tides (\textit{sbctide.F90})]{Surface tides (\protect\mdl{sbctide})} |
---|
722 | \label{sec:SBC_tide} |
---|
723 | |
---|
724 | \begin{listing} |
---|
725 | \nlst{nam_tide} |
---|
726 | \caption{\forcode{&nam_tide}} |
---|
727 | \label{lst:nam_tide} |
---|
728 | \end{listing} |
---|
729 | |
---|
730 | The tidal forcing, generated by the gravity forces of the Earth-Moon and Earth-Sun sytems, |
---|
731 | is activated if \np{ln_tide}{ln\_tide} and \np{ln_tide_pot}{ln\_tide\_pot} are both set to \forcode{.true.} in \nam{_tide}{\_tide}. |
---|
732 | This translates as an additional barotropic force in the momentum \autoref{eq:MB_PE_dyn} such that: |
---|
733 | \[ |
---|
734 | % \label{eq:SBC_PE_dyn_tides} |
---|
735 | \frac{\partial {\mathrm {\mathbf U}}_h }{\partial t}= ... |
---|
736 | +g\nabla (\Pi_{eq} + \Pi_{sal}) |
---|
737 | \] |
---|
738 | where $\Pi_{eq}$ stands for the equilibrium tidal forcing and |
---|
739 | $\Pi_{sal}$ is a self-attraction and loading term (SAL). |
---|
740 | |
---|
741 | The equilibrium tidal forcing is expressed as a sum over a subset of |
---|
742 | constituents chosen from the set of available tidal constituents |
---|
743 | defined in file \hf{SBC/tide} (this comprises the tidal |
---|
744 | constituents \textit{M2, N2, 2N2, S2, K2, K1, O1, Q1, P1, M4, Mf, Mm, |
---|
745 | Msqm, Mtm, S1, MU2, NU2, L2}, and \textit{T2}). Individual |
---|
746 | constituents are selected by including their names in the array |
---|
747 | \np{clname}{clname} in \nam{_tide}{\_tide} (e.g., \np{clname}{clname}\forcode{(1)='M2', } |
---|
748 | \np{clname}{clname}\forcode{(2)='S2'} to select solely the tidal consituents \textit{M2} |
---|
749 | and \textit{S2}). Optionally, when \np{ln_tide_ramp}{ln\_tide\_ramp} is set to |
---|
750 | \forcode{.true.}, the equilibrium tidal forcing can be ramped up |
---|
751 | linearly from zero during the initial \np{rdttideramp}{rdttideramp} days of the |
---|
752 | model run. |
---|
753 | |
---|
754 | The SAL term should in principle be computed online as it depends on |
---|
755 | the model tidal prediction itself (see \citet{arbic.garner.ea_DSR04} for a |
---|
756 | discussion about the practical implementation of this term). |
---|
757 | Nevertheless, the complex calculations involved would make this |
---|
758 | computationally too expensive. Here, two options are available: |
---|
759 | $\Pi_{sal}$ generated by an external model can be read in |
---|
760 | (\np[=.true.]{ln_read_load}{ln\_read\_load}), or a ``scalar approximation'' can be |
---|
761 | used (\np[=.true.]{ln_scal_load}{ln\_scal\_load}). In the latter case |
---|
762 | \[ |
---|
763 | \Pi_{sal} = \beta \eta, |
---|
764 | \] |
---|
765 | where $\beta$ (\np{rn_scal_load}{rn\_scal\_load} with a default value of 0.094) is a |
---|
766 | spatially constant scalar, often chosen to minimize tidal prediction |
---|
767 | errors. Setting both \np{ln_read_load}{ln\_read\_load} and \np{ln_scal_load}{ln\_scal\_load} to |
---|
768 | \forcode{.false.} removes the SAL contribution. |
---|
769 | |
---|
770 | %% ================================================================================================= |
---|
771 | \section[River runoffs (\textit{sbcrnf.F90})]{River runoffs (\protect\mdl{sbcrnf})} |
---|
772 | \label{sec:SBC_rnf} |
---|
773 | |
---|
774 | \begin{listing} |
---|
775 | \nlst{namsbc_rnf} |
---|
776 | \caption{\forcode{&namsbc_rnf}} |
---|
777 | \label{lst:namsbc_rnf} |
---|
778 | \end{listing} |
---|
779 | |
---|
780 | %River runoff generally enters the ocean at a nonzero depth rather than through the surface. |
---|
781 | %Many models, however, have traditionally inserted river runoff to the top model cell. |
---|
782 | %This was the case in \NEMO\ prior to the version 3.3. The switch toward a input of runoff |
---|
783 | %throughout a nonzero depth has been motivated by the numerical and physical problems |
---|
784 | %that arise when the top grid cells are of the order of one meter. This situation is common in |
---|
785 | %coastal modelling and becomes more and more often open ocean and climate modelling |
---|
786 | %\footnote{At least a top cells thickness of 1~meter and a 3 hours forcing frequency are |
---|
787 | %required to properly represent the diurnal cycle \citep{bernie.woolnough.ea_JC05}. see also \autoref{fig:SBC_dcy}.}. |
---|
788 | |
---|
789 | %To do this we need to treat evaporation/precipitation fluxes and river runoff differently in the |
---|
790 | %\mdl{tra\_sbc} module. We decided to separate them throughout the code, so that the variable |
---|
791 | %\textit{emp} represented solely evaporation minus precipitation fluxes, and a new 2d variable |
---|
792 | %rnf was added which represents the volume flux of river runoff (in kg/m2s to remain consistent with |
---|
793 | %emp). This meant many uses of emp and emps needed to be changed, a list of all modules which use |
---|
794 | %emp or emps and the changes made are below: |
---|
795 | |
---|
796 | %Rachel: |
---|
797 | River runoff generally enters the ocean at a nonzero depth rather than through the surface. |
---|
798 | Many models, however, have traditionally inserted river runoff to the top model cell. |
---|
799 | This was the case in \NEMO\ prior to the version 3.3, |
---|
800 | and was combined with an option to increase vertical mixing near the river mouth. |
---|
801 | |
---|
802 | However, with this method numerical and physical problems arise when the top grid cells are of the order of one meter. |
---|
803 | This situation is common in coastal modelling and is becoming more common in open ocean and climate modelling |
---|
804 | \footnote{ |
---|
805 | At least a top cells thickness of 1~meter and a 3 hours forcing frequency are required to |
---|
806 | properly represent the diurnal cycle \citep{bernie.woolnough.ea_JC05}. |
---|
807 | see also \autoref{fig:SBC_dcy}.}. |
---|
808 | |
---|
809 | As such from V~3.3 onwards it is possible to add river runoff through a non-zero depth, |
---|
810 | and for the temperature and salinity of the river to effect the surrounding ocean. |
---|
811 | The user is able to specify, in a NetCDF input file, the temperature and salinity of the river, |
---|
812 | along with the depth (in metres) which the river should be added to. |
---|
813 | |
---|
814 | Namelist variables in \nam{sbc_rnf}{sbc\_rnf}, \np{ln_rnf_depth}{ln\_rnf\_depth}, \np{ln_rnf_sal}{ln\_rnf\_sal} and |
---|
815 | \np{ln_rnf_temp}{ln\_rnf\_temp} control whether the river attributes (depth, salinity and temperature) are read in and used. |
---|
816 | If these are set as false the river is added to the surface box only, assumed to be fresh (0~psu), |
---|
817 | and/or taken as surface temperature respectively. |
---|
818 | |
---|
819 | The runoff value and attributes are read in in sbcrnf. |
---|
820 | For temperature -999 is taken as missing data and the river temperature is taken to |
---|
821 | be the surface temperatue at the river point. |
---|
822 | For the depth parameter a value of -1 means the river is added to the surface box only, |
---|
823 | and a value of -999 means the river is added through the entire water column. |
---|
824 | After being read in the temperature and salinity variables are multiplied by the amount of runoff |
---|
825 | (converted into m/s) to give the heat and salt content of the river runoff. |
---|
826 | After the user specified depth is read ini, |
---|
827 | the number of grid boxes this corresponds to is calculated and stored in the variable \np{nz_rnf}{nz\_rnf}. |
---|
828 | The variable \textit{h\_dep} is then calculated to be the depth (in metres) of |
---|
829 | the bottom of the lowest box the river water is being added to |
---|
830 | (\ie\ the total depth that river water is being added to in the model). |
---|
831 | |
---|
832 | The mass/volume addition due to the river runoff is, at each relevant depth level, added to |
---|
833 | the horizontal divergence (\textit{hdivn}) in the subroutine \rou{sbc\_rnf\_div} (called from \mdl{divhor}). |
---|
834 | This increases the diffusion term in the vicinity of the river, thereby simulating a momentum flux. |
---|
835 | The sea surface height is calculated using the sum of the horizontal divergence terms, |
---|
836 | and so the river runoff indirectly forces an increase in sea surface height. |
---|
837 | |
---|
838 | The \textit{hdivn} terms are used in the tracer advection modules to force vertical velocities. |
---|
839 | This causes a mass of water, equal to the amount of runoff, to be moved into the box above. |
---|
840 | The heat and salt content of the river runoff is not included in this step, |
---|
841 | and so the tracer concentrations are diluted as water of ocean temperature and salinity is moved upward out of |
---|
842 | the box and replaced by the same volume of river water with no corresponding heat and salt addition. |
---|
843 | |
---|
844 | For the linear free surface case, at the surface box the tracer advection causes a flux of water |
---|
845 | (of equal volume to the runoff) through the sea surface out of the domain, |
---|
846 | which causes a salt and heat flux out of the model. |
---|
847 | As such the volume of water does not change, but the water is diluted. |
---|
848 | |
---|
849 | For the non-linear free surface case, no flux is allowed through the surface. |
---|
850 | Instead in the surface box (as well as water moving up from the boxes below) a volume of runoff water is added with |
---|
851 | no corresponding heat and salt addition and so as happens in the lower boxes there is a dilution effect. |
---|
852 | (The runoff addition to the top box along with the water being moved up through |
---|
853 | boxes below means the surface box has a large increase in volume, whilst all other boxes remain the same size) |
---|
854 | |
---|
855 | In trasbc the addition of heat and salt due to the river runoff is added. |
---|
856 | This is done in the same way for both vvl and non-vvl. |
---|
857 | The temperature and salinity are increased through the specified depth according to |
---|
858 | the heat and salt content of the river. |
---|
859 | |
---|
860 | In the non-linear free surface case (vvl), |
---|
861 | near the end of the time step the change in sea surface height is redistrubuted through the grid boxes, |
---|
862 | so that the original ratios of grid box heights are restored. |
---|
863 | In doing this water is moved into boxes below, throughout the water column, |
---|
864 | so the large volume addition to the surface box is spread between all the grid boxes. |
---|
865 | |
---|
866 | It is also possible for runnoff to be specified as a negative value for modelling flow through straits, |
---|
867 | \ie\ modelling the Baltic flow in and out of the North Sea. |
---|
868 | When the flow is out of the domain there is no change in temperature and salinity, |
---|
869 | regardless of the namelist options used, |
---|
870 | as the ocean water leaving the domain removes heat and salt (at the same concentration) with it. |
---|
871 | |
---|
872 | %\colorbox{yellow}{Nevertheless, Pb of vertical resolution and 3D input : increase vertical mixing near river mouths to mimic a 3D river |
---|
873 | |
---|
874 | %All river runoff and emp fluxes are assumed to be fresh water (zero salinity) and at the same temperature as the sea surface.} |
---|
875 | |
---|
876 | %\colorbox{yellow}{river mouths{\ldots}} |
---|
877 | |
---|
878 | %IF( ln_rnf ) THEN ! increase diffusivity at rivers mouths |
---|
879 | % DO jk = 2, nkrnf ; avt(:,:,jk) = avt(:,:,jk) + rn_avt_rnf * rnfmsk(:,:) ; END DO |
---|
880 | %ENDIF |
---|
881 | |
---|
882 | \cmtgm{ word doc of runoffs: |
---|
883 | In the current \NEMO\ setup river runoff is added to emp fluxes, |
---|
884 | these are then applied at just the sea surface as a volume change (in the variable volume case |
---|
885 | this is a literal volume change, and in the linear free surface case the free surface is moved) |
---|
886 | and a salt flux due to the concentration/dilution effect. |
---|
887 | There is also an option to increase vertical mixing near river mouths; |
---|
888 | this gives the effect of having a 3d river. |
---|
889 | All river runoff and emp fluxes are assumed to be fresh water (zero salinity) and |
---|
890 | at the same temperature as the sea surface. |
---|
891 | Our aim was to code the option to specify the temperature and salinity of river runoff, |
---|
892 | (as well as the amount), along with the depth that the river water will affect. |
---|
893 | This would make it possible to model low salinity outflow, such as the Baltic, |
---|
894 | and would allow the ocean temperature to be affected by river runoff. |
---|
895 | |
---|
896 | The depth option makes it possible to have the river water affecting just the surface layer, |
---|
897 | throughout depth, or some specified point in between. |
---|
898 | |
---|
899 | To do this we need to treat evaporation/precipitation fluxes and river runoff differently in |
---|
900 | the \mdl{tra_sbc} module. |
---|
901 | We decided to separate them throughout the code, |
---|
902 | so that the variable emp represented solely evaporation minus precipitation fluxes, |
---|
903 | and a new 2d variable rnf was added which represents the volume flux of river runoff |
---|
904 | (in $kg/m^2s$ to remain consistent with $emp$). |
---|
905 | This meant many uses of emp and emps needed to be changed, |
---|
906 | a list of all modules which use $emp$ or $emps$ and the changes made are below:} |
---|
907 | |
---|
908 | %% ================================================================================================= |
---|
909 | \section[Ice shelf melting (\textit{sbcisf.F90})]{Ice shelf melting (\protect\mdl{sbcisf})} |
---|
910 | \label{sec:SBC_isf} |
---|
911 | |
---|
912 | \begin{listing} |
---|
913 | \nlst{namsbc_isf} |
---|
914 | \caption{\forcode{&namsbc_isf}} |
---|
915 | \label{lst:namsbc_isf} |
---|
916 | \end{listing} |
---|
917 | |
---|
918 | The namelist variable in \nam{sbc}{sbc}, \np{nn_isf}{nn\_isf}, controls the ice shelf representation. |
---|
919 | Description and result of sensitivity test to \np{nn_isf}{nn\_isf} are presented in \citet{mathiot.jenkins.ea_GMD17}. |
---|
920 | The different options are illustrated in \autoref{fig:SBC_isf}. |
---|
921 | |
---|
922 | \begin{description} |
---|
923 | \item [{\np[=1]{nn_isf}{nn\_isf}}]: The ice shelf cavity is represented (\np[=.true.]{ln_isfcav}{ln\_isfcav} needed). |
---|
924 | The fwf and heat flux are depending of the local water properties. |
---|
925 | |
---|
926 | Two different bulk formulae are available: |
---|
927 | |
---|
928 | \begin{description} |
---|
929 | \item [{\np[=1]{nn_isfblk}{nn\_isfblk}}]: The melt rate is based on a balance between the upward ocean heat flux and |
---|
930 | the latent heat flux at the ice shelf base. A complete description is available in \citet{hunter_rpt06}. |
---|
931 | \item [{\np[=2]{nn_isfblk}{nn\_isfblk}}]: The melt rate and the heat flux are based on a 3 equations formulation |
---|
932 | (a heat flux budget at the ice base, a salt flux budget at the ice base and a linearised freezing point temperature equation). |
---|
933 | A complete description is available in \citet{jenkins_JGR91}. |
---|
934 | \end{description} |
---|
935 | |
---|
936 | Temperature and salinity used to compute the melt are the average temperature in the top boundary layer \citet{losch_JGR08}. |
---|
937 | Its thickness is defined by \np{rn_hisf_tbl}{rn\_hisf\_tbl}. |
---|
938 | The fluxes and friction velocity are computed using the mean temperature, salinity and velocity in the the first \np{rn_hisf_tbl}{rn\_hisf\_tbl} m. |
---|
939 | Then, the fluxes are spread over the same thickness (ie over one or several cells). |
---|
940 | If \np{rn_hisf_tbl}{rn\_hisf\_tbl} larger than top $e_{3}t$, there is no more feedback between the freezing point at the interface and the the top cell temperature. |
---|
941 | This can lead to super-cool temperature in the top cell under melting condition. |
---|
942 | If \np{rn_hisf_tbl}{rn\_hisf\_tbl} smaller than top $e_{3}t$, the top boundary layer thickness is set to the top cell thickness.\\ |
---|
943 | |
---|
944 | Each melt bulk formula depends on a exchange coeficient ($\Gamma^{T,S}$) between the ocean and the ice. |
---|
945 | There are 3 different ways to compute the exchange coeficient: |
---|
946 | \begin{description} |
---|
947 | \item [{\np[=0]{nn_gammablk}{nn\_gammablk}}]: The salt and heat exchange coefficients are constant and defined by \np{rn_gammas0}{rn\_gammas0} and \np{rn_gammat0}{rn\_gammat0}. |
---|
948 | \begin{gather*} |
---|
949 | % \label{eq:SBC_isf_gamma_iso} |
---|
950 | \gamma^{T} = rn\_gammat0 \\ |
---|
951 | \gamma^{S} = rn\_gammas0 |
---|
952 | \end{gather*} |
---|
953 | This is the recommended formulation for ISOMIP. |
---|
954 | \item [{\np[=1]{nn_gammablk}{nn\_gammablk}}]: The salt and heat exchange coefficients are velocity dependent and defined as |
---|
955 | \begin{gather*} |
---|
956 | \gamma^{T} = rn\_gammat0 \times u_{*} \\ |
---|
957 | \gamma^{S} = rn\_gammas0 \times u_{*} |
---|
958 | \end{gather*} |
---|
959 | where $u_{*}$ is the friction velocity in the top boundary layer (ie first \np{rn_hisf_tbl}{rn\_hisf\_tbl} meters). |
---|
960 | See \citet{jenkins.nicholls.ea_JPO10} for all the details on this formulation. It is the recommended formulation for realistic application. |
---|
961 | \item [{\np[=2]{nn_gammablk}{nn\_gammablk}}]: The salt and heat exchange coefficients are velocity and stability dependent and defined as: |
---|
962 | \[ |
---|
963 | \gamma^{T,S} = \frac{u_{*}}{\Gamma_{Turb} + \Gamma^{T,S}_{Mole}} |
---|
964 | \] |
---|
965 | where $u_{*}$ is the friction velocity in the top boundary layer (ie first \np{rn_hisf_tbl}{rn\_hisf\_tbl} meters), |
---|
966 | $\Gamma_{Turb}$ the contribution of the ocean stability and |
---|
967 | $\Gamma^{T,S}_{Mole}$ the contribution of the molecular diffusion. |
---|
968 | See \citet{holland.jenkins_JPO99} for all the details on this formulation. |
---|
969 | This formulation has not been extensively tested in \NEMO\ (not recommended). |
---|
970 | \end{description} |
---|
971 | \item [{\np[=2]{nn_isf}{nn\_isf}}]: The ice shelf cavity is not represented. |
---|
972 | The fwf and heat flux are computed using the \citet{beckmann.goosse_OM03} parameterisation of isf melting. |
---|
973 | The fluxes are distributed along the ice shelf edge between the depth of the average grounding line (GL) |
---|
974 | (\np{sn_depmax_isf}{sn\_depmax\_isf}) and the base of the ice shelf along the calving front |
---|
975 | (\np{sn_depmin_isf}{sn\_depmin\_isf}) as in (\np[=3]{nn_isf}{nn\_isf}). |
---|
976 | The effective melting length (\np{sn_Leff_isf}{sn\_Leff\_isf}) is read from a file. |
---|
977 | \item [{\np[=3]{nn_isf}{nn\_isf}}]: The ice shelf cavity is not represented. |
---|
978 | The fwf (\np{sn_rnfisf}{sn\_rnfisf}) is prescribed and distributed along the ice shelf edge between |
---|
979 | the depth of the average grounding line (GL) (\np{sn_depmax_isf}{sn\_depmax\_isf}) and |
---|
980 | the base of the ice shelf along the calving front (\np{sn_depmin_isf}{sn\_depmin\_isf}). |
---|
981 | The heat flux ($Q_h$) is computed as $Q_h = fwf \times L_f$. |
---|
982 | \item [{\np[=4]{nn_isf}{nn\_isf}}]: The ice shelf cavity is opened (\np[=.true.]{ln_isfcav}{ln\_isfcav} needed). |
---|
983 | However, the fwf is not computed but specified from file \np{sn_fwfisf}{sn\_fwfisf}). |
---|
984 | The heat flux ($Q_h$) is computed as $Q_h = fwf \times L_f$. |
---|
985 | As in \np[=1]{nn_isf}{nn\_isf}, the fluxes are spread over the top boundary layer thickness (\np{rn_hisf_tbl}{rn\_hisf\_tbl}) |
---|
986 | \end{description} |
---|
987 | |
---|
988 | $\bullet$ \np[=1]{nn_isf}{nn\_isf} and \np[=2]{nn_isf}{nn\_isf} compute a melt rate based on |
---|
989 | the water mass properties, ocean velocities and depth. |
---|
990 | This flux is thus highly dependent of the model resolution (horizontal and vertical), |
---|
991 | realism of the water masses onto the shelf ...\\ |
---|
992 | |
---|
993 | $\bullet$ \np[=3]{nn_isf}{nn\_isf} and \np[=4]{nn_isf}{nn\_isf} read the melt rate from a file. |
---|
994 | You have total control of the fwf forcing. |
---|
995 | This can be useful if the water masses on the shelf are not realistic or |
---|
996 | the resolution (horizontal/vertical) are too coarse to have realistic melting or |
---|
997 | for studies where you need to control your heat and fw input.\\ |
---|
998 | |
---|
999 | The ice shelf melt is implemented as a volume flux as for the runoff. |
---|
1000 | The fw addition due to the ice shelf melting is, at each relevant depth level, added to |
---|
1001 | the horizontal divergence (\textit{hdivn}) in the subroutine \rou{sbc\_isf\_div}, called from \mdl{divhor}. |
---|
1002 | See the runoff section \autoref{sec:SBC_rnf} for all the details about the divergence correction.\\ |
---|
1003 | |
---|
1004 | \begin{figure}[!t] |
---|
1005 | \centering |
---|
1006 | \includegraphics[width=0.66\textwidth]{SBC_isf} |
---|
1007 | \caption[Ice shelf location and fresh water flux definition]{ |
---|
1008 | Illustration of the location where the fwf is injected and |
---|
1009 | whether or not the fwf is interactif or not depending of \protect\np{nn_isf}{nn\_isf}.} |
---|
1010 | \label{fig:SBC_isf} |
---|
1011 | \end{figure} |
---|
1012 | |
---|
1013 | %% ================================================================================================= |
---|
1014 | \section{Ice sheet coupling} |
---|
1015 | \label{sec:SBC_iscpl} |
---|
1016 | |
---|
1017 | \begin{listing} |
---|
1018 | \nlst{namsbc_iscpl} |
---|
1019 | \caption{\forcode{&namsbc_iscpl}} |
---|
1020 | \label{lst:namsbc_iscpl} |
---|
1021 | \end{listing} |
---|
1022 | |
---|
1023 | Ice sheet/ocean coupling is done through file exchange at the restart step. |
---|
1024 | At each restart step: |
---|
1025 | |
---|
1026 | \begin{enumerate} |
---|
1027 | \item the ice sheet model send a new bathymetry and ice shelf draft netcdf file. |
---|
1028 | \item a new domcfg.nc file is built using the DOMAINcfg tools. |
---|
1029 | \item \NEMO\ run for a specific period and output the average melt rate over the period. |
---|
1030 | \item the ice sheet model run using the melt rate outputed in step 4. |
---|
1031 | \item go back to 1. |
---|
1032 | \end{enumerate} |
---|
1033 | |
---|
1034 | If \np[=.true.]{ln_iscpl}{ln\_iscpl}, the isf draft is assume to be different at each restart step with |
---|
1035 | potentially some new wet/dry cells due to the ice sheet dynamics/thermodynamics. |
---|
1036 | The wetting and drying scheme applied on the restart is very simple and described below for the 6 different possible cases: |
---|
1037 | |
---|
1038 | \begin{description} |
---|
1039 | \item [Thin a cell down]: T/S/ssh are unchanged and U/V in the top cell are corrected to keep the barotropic transport (bt) constant |
---|
1040 | ($bt_b=bt_n$). |
---|
1041 | \item [Enlarge a cell]: See case "Thin a cell down" |
---|
1042 | \item [Dry a cell]: mask, T/S, U/V and ssh are set to 0. |
---|
1043 | Furthermore, U/V into the water column are modified to satisfy ($bt_b=bt_n$). |
---|
1044 | \item [Wet a cell]: mask is set to 1, T/S is extrapolated from neighbours, $ssh_n = ssh_b$ and U/V set to 0. |
---|
1045 | If no neighbours, T/S is extrapolated from old top cell value. |
---|
1046 | If no neighbours along i,j and k (both previous test failed), T/S/U/V/ssh and mask are set to 0. |
---|
1047 | \item [Dry a column]: mask, T/S, U/V are set to 0 everywhere in the column and ssh set to 0. |
---|
1048 | \item [Wet a column]: set mask to 1, T/S is extrapolated from neighbours, ssh is extrapolated from neighbours and U/V set to 0. |
---|
1049 | If no neighbour, T/S/U/V and mask set to 0. |
---|
1050 | \end{description} |
---|
1051 | |
---|
1052 | Furthermore, as the before and now fields are not compatible (modification of the geometry), |
---|
1053 | the restart time step is prescribed to be an euler time step instead of a leap frog and $fields_b = fields_n$.\\ |
---|
1054 | |
---|
1055 | The horizontal extrapolation to fill new cell with realistic value is called \np{nn_drown}{nn\_drown} times. |
---|
1056 | It means that if the grounding line retreat by more than \np{nn_drown}{nn\_drown} cells between 2 coupling steps, |
---|
1057 | the code will be unable to fill all the new wet cells properly. |
---|
1058 | The default number is set up for the MISOMIP idealised experiments. |
---|
1059 | This coupling procedure is able to take into account grounding line and calving front migration. |
---|
1060 | However, it is a non-conservative processe. |
---|
1061 | This could lead to a trend in heat/salt content and volume.\\ |
---|
1062 | |
---|
1063 | In order to remove the trend and keep the conservation level as close to 0 as possible, |
---|
1064 | a simple conservation scheme is available with \np[=.true.]{ln_hsb}{ln\_hsb}. |
---|
1065 | The heat/salt/vol. gain/loss is diagnosed, as well as the location. |
---|
1066 | A correction increment is computed and apply each time step during the next \np{rn_fiscpl}{rn\_fiscpl} time steps. |
---|
1067 | For safety, it is advised to set \np{rn_fiscpl}{rn\_fiscpl} equal to the coupling period (smallest increment possible). |
---|
1068 | The corrective increment is apply into the cell itself (if it is a wet cell), the neigbouring cells or the closest wet cell (if the cell is now dry). |
---|
1069 | |
---|
1070 | %% ================================================================================================= |
---|
1071 | \section{Handling of icebergs (ICB)} |
---|
1072 | \label{sec:SBC_ICB_icebergs} |
---|
1073 | |
---|
1074 | \begin{listing} |
---|
1075 | \nlst{namberg} |
---|
1076 | \caption{\forcode{&namberg}} |
---|
1077 | \label{lst:namberg} |
---|
1078 | \end{listing} |
---|
1079 | |
---|
1080 | Icebergs are modelled as lagrangian particles in \NEMO\ \citep{marsh.ivchenko.ea_GMD15}. |
---|
1081 | Their physical behaviour is controlled by equations as described in \citet{martin.adcroft_OM10} ). |
---|
1082 | (Note that the authors kindly provided a copy of their code to act as a basis for implementation in \NEMO). |
---|
1083 | Icebergs are initially spawned into one of ten classes which have specific mass and thickness as |
---|
1084 | described in the \nam{berg}{berg} namelist: \np{rn_initial_mass}{rn\_initial\_mass} and \np{rn_initial_thickness}{rn\_initial\_thickness}. |
---|
1085 | Each class has an associated scaling (\np{rn_mass_scaling}{rn\_mass\_scaling}), |
---|
1086 | which is an integer representing how many icebergs of this class are being described as one lagrangian point |
---|
1087 | (this reduces the numerical problem of tracking every single iceberg). |
---|
1088 | They are enabled by setting \np[=.true.]{ln_icebergs}{ln\_icebergs}. |
---|
1089 | |
---|
1090 | Two initialisation schemes are possible. |
---|
1091 | \begin{description} |
---|
1092 | \item [{\np{nn_test_icebergs}{nn\_test\_icebergs}~$>$~0}] In this scheme, the value of \np{nn_test_icebergs}{nn\_test\_icebergs} represents the class of iceberg to generate |
---|
1093 | (so between 1 and 10), and \np{nn_test_icebergs}{nn\_test\_icebergs} provides a lon/lat box in the domain at each grid point of |
---|
1094 | which an iceberg is generated at the beginning of the run. |
---|
1095 | (Note that this happens each time the timestep equals \np{nn_nit000}{nn\_nit000}.) |
---|
1096 | \np{nn_test_icebergs}{nn\_test\_icebergs} is defined by four numbers in \np{nn_test_box}{nn\_test\_box} representing the corners of |
---|
1097 | the geographical box: lonmin,lonmax,latmin,latmax |
---|
1098 | \item [{\np[=-1]{nn_test_icebergs}{nn\_test\_icebergs}}] In this scheme, the model reads a calving file supplied in the \np{sn_icb}{sn\_icb} parameter. |
---|
1099 | This should be a file with a field on the configuration grid (typically ORCA) |
---|
1100 | representing ice accumulation rate at each model point. |
---|
1101 | These should be ocean points adjacent to land where icebergs are known to calve. |
---|
1102 | Most points in this input grid are going to have value zero. |
---|
1103 | When the model runs, ice is accumulated at each grid point which has a non-zero source term. |
---|
1104 | At each time step, a test is performed to see if there is enough ice mass to |
---|
1105 | calve an iceberg of each class in order (1 to 10). |
---|
1106 | Note that this is the initial mass multiplied by the number each particle represents (\ie\ the scaling). |
---|
1107 | If there is enough ice, a new iceberg is spawned and the total available ice reduced accordingly. |
---|
1108 | \end{description} |
---|
1109 | |
---|
1110 | Icebergs are influenced by wind, waves and currents, bottom melt and erosion. |
---|
1111 | The latter act to disintegrate the iceberg. |
---|
1112 | This is either all melted freshwater, |
---|
1113 | or (if \np{rn_bits_erosion_fraction}{rn\_bits\_erosion\_fraction}~$>$~0) into melt and additionally small ice bits |
---|
1114 | which are assumed to propagate with their larger parent and thus delay fluxing into the ocean. |
---|
1115 | Melt water (and other variables on the configuration grid) are written into the main \NEMO\ model output files. |
---|
1116 | |
---|
1117 | Extensive diagnostics can be produced. |
---|
1118 | Separate output files are maintained for human-readable iceberg information. |
---|
1119 | A separate file is produced for each processor (independent of \np{ln_ctl}{ln\_ctl}). |
---|
1120 | The amount of information is controlled by two integer parameters: |
---|
1121 | \begin{description} |
---|
1122 | \item [{\np{nn_verbose_level}{nn\_verbose\_level}}] takes a value between one and four and |
---|
1123 | represents an increasing number of points in the code at which variables are written, |
---|
1124 | and an increasing level of obscurity. |
---|
1125 | \item [{\np{nn_verbose_write}{nn\_verbose\_write}}] is the number of timesteps between writes |
---|
1126 | \end{description} |
---|
1127 | |
---|
1128 | Iceberg trajectories can also be written out and this is enabled by setting \np{nn_sample_rate}{nn\_sample\_rate}~$>$~0. |
---|
1129 | A non-zero value represents how many timesteps between writes of information into the output file. |
---|
1130 | These output files are in NETCDF format. |
---|
1131 | When \key{mpp\_mpi} is defined, each output file contains only those icebergs in the corresponding processor. |
---|
1132 | Trajectory points are written out in the order of their parent iceberg in the model's "linked list" of icebergs. |
---|
1133 | So care is needed to recreate data for individual icebergs, |
---|
1134 | since its trajectory data may be spread across multiple files. |
---|
1135 | |
---|
1136 | %% ================================================================================================= |
---|
1137 | \section[Interactions with waves (\textit{sbcwave.F90}, \forcode{ln_wave})]{Interactions with waves (\protect\mdl{sbcwave}, \protect\np{ln_wave}{ln\_wave})} |
---|
1138 | \label{sec:SBC_wave} |
---|
1139 | |
---|
1140 | \begin{listing} |
---|
1141 | \nlst{namsbc_wave} |
---|
1142 | \caption{\forcode{&namsbc_wave}} |
---|
1143 | \label{lst:namsbc_wave} |
---|
1144 | \end{listing} |
---|
1145 | |
---|
1146 | Ocean waves represent the interface between the ocean and the atmosphere, so \NEMO\ is extended to incorporate |
---|
1147 | physical processes related to ocean surface waves, namely the surface stress modified by growth and |
---|
1148 | dissipation of the oceanic wave field, the Stokes-Coriolis force and the Stokes drift impact on mass and |
---|
1149 | tracer advection; moreover the neutral surface drag coefficient from a wave model can be used to evaluate |
---|
1150 | the wind stress. |
---|
1151 | |
---|
1152 | Physical processes related to ocean surface waves can be accounted by setting the logical variable |
---|
1153 | \np[=.true.]{ln_wave}{ln\_wave} in \nam{sbc}{sbc} namelist. In addition, specific flags accounting for |
---|
1154 | different processes should be activated as explained in the following sections. |
---|
1155 | |
---|
1156 | Wave fields can be provided either in forced or coupled mode: |
---|
1157 | \begin{description} |
---|
1158 | \item [forced mode]: wave fields should be defined through the \nam{sbc_wave}{sbc\_wave} namelist |
---|
1159 | for external data names, locations, frequency, interpolation and all the miscellanous options allowed by |
---|
1160 | Input Data generic Interface (see \autoref{sec:SBC_input}). |
---|
1161 | \item [coupled mode]: \NEMO\ and an external wave model can be coupled by setting \np[=.true.]{ln_cpl}{ln\_cpl} |
---|
1162 | in \nam{sbc}{sbc} namelist and filling the \nam{sbc_cpl}{sbc\_cpl} namelist. |
---|
1163 | \end{description} |
---|
1164 | |
---|
1165 | %% ================================================================================================= |
---|
1166 | \subsection[Neutral drag coefficient from wave model (\forcode{ln_cdgw})]{Neutral drag coefficient from wave model (\protect\np{ln_cdgw}{ln\_cdgw})} |
---|
1167 | \label{subsec:SBC_wave_cdgw} |
---|
1168 | |
---|
1169 | The neutral surface drag coefficient provided from an external data source (\ie\ a wave model), |
---|
1170 | can be used by setting the logical variable \np[=.true.]{ln_cdgw}{ln\_cdgw} in \nam{sbc}{sbc} namelist. |
---|
1171 | Then using the routine \rou{sbcblk\_algo\_ncar} and starting from the neutral drag coefficent provided, |
---|
1172 | the drag coefficient is computed according to the stable/unstable conditions of the |
---|
1173 | air-sea interface following \citet{large.yeager_rpt04}. |
---|
1174 | |
---|
1175 | %% ================================================================================================= |
---|
1176 | \subsection[3D Stokes Drift (\forcode{ln_sdw} \& \forcode{nn_sdrift})]{3D Stokes Drift (\protect\np{ln_sdw}{ln\_sdw} \& \np{nn_sdrift}{nn\_sdrift})} |
---|
1177 | \label{subsec:SBC_wave_sdw} |
---|
1178 | |
---|
1179 | The Stokes drift is a wave driven mechanism of mass and momentum transport \citep{stokes_ibk09}. |
---|
1180 | It is defined as the difference between the average velocity of a fluid parcel (Lagrangian velocity) |
---|
1181 | and the current measured at a fixed point (Eulerian velocity). |
---|
1182 | As waves travel, the water particles that make up the waves travel in orbital motions but |
---|
1183 | without a closed path. Their movement is enhanced at the top of the orbit and slowed slightly |
---|
1184 | at the bottom, so the result is a net forward motion of water particles, referred to as the Stokes drift. |
---|
1185 | An accurate evaluation of the Stokes drift and the inclusion of related processes may lead to improved |
---|
1186 | representation of surface physics in ocean general circulation models. %GS: reference needed |
---|
1187 | The Stokes drift velocity $\mathbf{U}_{st}$ in deep water can be computed from the wave spectrum and may be written as: |
---|
1188 | |
---|
1189 | \[ |
---|
1190 | % \label{eq:SBC_wave_sdw} |
---|
1191 | \mathbf{U}_{st} = \frac{16{\pi^3}} {g} |
---|
1192 | \int_0^\infty \int_{-\pi}^{\pi} (cos{\theta},sin{\theta}) {f^3} |
---|
1193 | \mathrm{S}(f,\theta) \mathrm{e}^{2kz}\,\mathrm{d}\theta {d}f |
---|
1194 | \] |
---|
1195 | |
---|
1196 | where: ${\theta}$ is the wave direction, $f$ is the wave intrinsic frequency, |
---|
1197 | $\mathrm{S}($f$,\theta)$ is the 2D frequency-direction spectrum, |
---|
1198 | $k$ is the mean wavenumber defined as: |
---|
1199 | $k=\frac{2\pi}{\lambda}$ (being $\lambda$ the wavelength). \\ |
---|
1200 | |
---|
1201 | In order to evaluate the Stokes drift in a realistic ocean wave field, the wave spectral shape is required |
---|
1202 | and its computation quickly becomes expensive as the 2D spectrum must be integrated for each vertical level. |
---|
1203 | To simplify, it is customary to use approximations to the full Stokes profile. |
---|
1204 | Three possible parameterizations for the calculation for the approximate Stokes drift velocity profile |
---|
1205 | are included in the code through the \np{nn_sdrift}{nn\_sdrift} parameter once provided the surface Stokes drift |
---|
1206 | $\mathbf{U}_{st |_{z=0}}$ which is evaluated by an external wave model that accurately reproduces the wave spectra |
---|
1207 | and makes possible the estimation of the surface Stokes drift for random directional waves in |
---|
1208 | realistic wave conditions: |
---|
1209 | |
---|
1210 | \begin{description} |
---|
1211 | \item [{\np{nn_sdrift}{nn\_sdrift} = 0}]: exponential integral profile parameterization proposed by |
---|
1212 | \citet{breivik.janssen.ea_JPO14}: |
---|
1213 | |
---|
1214 | \[ |
---|
1215 | % \label{eq:SBC_wave_sdw_0a} |
---|
1216 | \mathbf{U}_{st} \cong \mathbf{U}_{st |_{z=0}} \frac{\mathrm{e}^{-2k_ez}} {1-8k_ez} |
---|
1217 | \] |
---|
1218 | |
---|
1219 | where $k_e$ is the effective wave number which depends on the Stokes transport $T_{st}$ defined as follows: |
---|
1220 | |
---|
1221 | \[ |
---|
1222 | % \label{eq:SBC_wave_sdw_0b} |
---|
1223 | k_e = \frac{|\mathbf{U}_{\left.st\right|_{z=0}}|} {|T_{st}|} |
---|
1224 | \quad \text{and }\ |
---|
1225 | T_{st} = \frac{1}{16} \bar{\omega} H_s^2 |
---|
1226 | \] |
---|
1227 | |
---|
1228 | where $H_s$ is the significant wave height and $\omega$ is the wave frequency. |
---|
1229 | |
---|
1230 | \item [{\np{nn_sdrift}{nn\_sdrift} = 1}]: velocity profile based on the Phillips spectrum which is considered to be a |
---|
1231 | reasonable estimate of the part of the spectrum mostly contributing to the Stokes drift velocity near the surface |
---|
1232 | \citep{breivik.bidlot.ea_OM16}: |
---|
1233 | |
---|
1234 | \[ |
---|
1235 | % \label{eq:SBC_wave_sdw_1} |
---|
1236 | \mathbf{U}_{st} \cong \mathbf{U}_{st |_{z=0}} \Big[exp(2k_pz)-\beta \sqrt{-2 \pi k_pz} |
---|
1237 | \textit{ erf } \Big(\sqrt{-2 k_pz}\Big)\Big] |
---|
1238 | \] |
---|
1239 | |
---|
1240 | where $erf$ is the complementary error function and $k_p$ is the peak wavenumber. |
---|
1241 | |
---|
1242 | \item [{\np{nn_sdrift}{nn\_sdrift} = 2}]: velocity profile based on the Phillips spectrum as for \np{nn_sdrift}{nn\_sdrift} = 1 |
---|
1243 | but using the wave frequency from a wave model. |
---|
1244 | |
---|
1245 | \end{description} |
---|
1246 | |
---|
1247 | The Stokes drift enters the wave-averaged momentum equation, as well as the tracer advection equations |
---|
1248 | and its effect on the evolution of the sea-surface height ${\eta}$ is considered as follows: |
---|
1249 | |
---|
1250 | \[ |
---|
1251 | % \label{eq:SBC_wave_eta_sdw} |
---|
1252 | \frac{\partial{\eta}}{\partial{t}} = |
---|
1253 | -\nabla_h \int_{-H}^{\eta} (\mathbf{U} + \mathbf{U}_{st}) dz |
---|
1254 | \] |
---|
1255 | |
---|
1256 | The tracer advection equation is also modified in order for Eulerian ocean models to properly account |
---|
1257 | for unresolved wave effect. The divergence of the wave tracer flux equals the mean tracer advection |
---|
1258 | that is induced by the three-dimensional Stokes velocity. |
---|
1259 | The advective equation for a tracer $c$ combining the effects of the mean current and sea surface waves |
---|
1260 | can be formulated as follows: |
---|
1261 | |
---|
1262 | \[ |
---|
1263 | % \label{eq:SBC_wave_tra_sdw} |
---|
1264 | \frac{\partial{c}}{\partial{t}} = |
---|
1265 | - (\mathbf{U} + \mathbf{U}_{st}) \cdot \nabla{c} |
---|
1266 | \] |
---|
1267 | |
---|
1268 | %% ================================================================================================= |
---|
1269 | \subsection[Stokes-Coriolis term (\forcode{ln_stcor})]{Stokes-Coriolis term (\protect\np{ln_stcor}{ln\_stcor})} |
---|
1270 | \label{subsec:SBC_wave_stcor} |
---|
1271 | |
---|
1272 | In a rotating ocean, waves exert a wave-induced stress on the mean ocean circulation which results |
---|
1273 | in a force equal to $\mathbf{U}_{st}$×$f$, where $f$ is the Coriolis parameter. |
---|
1274 | This additional force may have impact on the Ekman turning of the surface current. |
---|
1275 | In order to include this term, once evaluated the Stokes drift (using one of the 3 possible |
---|
1276 | approximations described in \autoref{subsec:SBC_wave_sdw}), |
---|
1277 | \np[=.true.]{ln_stcor}{ln\_stcor} has to be set. |
---|
1278 | |
---|
1279 | %% ================================================================================================= |
---|
1280 | \subsection[Wave modified stress (\forcode{ln_tauwoc} \& \forcode{ln_tauw})]{Wave modified sress (\protect\np{ln_tauwoc}{ln\_tauwoc} \& \np{ln_tauw}{ln\_tauw})} |
---|
1281 | \label{subsec:SBC_wave_tauw} |
---|
1282 | |
---|
1283 | The surface stress felt by the ocean is the atmospheric stress minus the net stress going |
---|
1284 | into the waves \citep{janssen.breivik.ea_rpt13}. Therefore, when waves are growing, momentum and energy is spent and is not |
---|
1285 | available for forcing the mean circulation, while in the opposite case of a decaying sea |
---|
1286 | state, more momentum is available for forcing the ocean. |
---|
1287 | Only when the sea state is in equilibrium, the ocean is forced by the atmospheric stress, |
---|
1288 | but in practice, an equilibrium sea state is a fairly rare event. |
---|
1289 | So the atmospheric stress felt by the ocean circulation $\tau_{oc,a}$ can be expressed as: |
---|
1290 | |
---|
1291 | \[ |
---|
1292 | % \label{eq:SBC_wave_tauoc} |
---|
1293 | \tau_{oc,a} = \tau_a - \tau_w |
---|
1294 | \] |
---|
1295 | |
---|
1296 | where $\tau_a$ is the atmospheric surface stress; |
---|
1297 | $\tau_w$ is the atmospheric stress going into the waves defined as: |
---|
1298 | |
---|
1299 | \[ |
---|
1300 | % \label{eq:SBC_wave_tauw} |
---|
1301 | \tau_w = \rho g \int {\frac{dk}{c_p} (S_{in}+S_{nl}+S_{diss})} |
---|
1302 | \] |
---|
1303 | |
---|
1304 | where: $c_p$ is the phase speed of the gravity waves, |
---|
1305 | $S_{in}$, $S_{nl}$ and $S_{diss}$ are three source terms that represent |
---|
1306 | the physics of ocean waves. The first one, $S_{in}$, describes the generation |
---|
1307 | of ocean waves by wind and therefore represents the momentum and energy transfer |
---|
1308 | from air to ocean waves; the second term $S_{nl}$ denotes |
---|
1309 | the nonlinear transfer by resonant four-wave interactions; while the third term $S_{diss}$ |
---|
1310 | describes the dissipation of waves by processes such as white-capping, large scale breaking |
---|
1311 | eddy-induced damping. |
---|
1312 | |
---|
1313 | The wave stress derived from an external wave model can be provided either through the normalized |
---|
1314 | wave stress into the ocean by setting \np[=.true.]{ln_tauwoc}{ln\_tauwoc}, or through the zonal and |
---|
1315 | meridional stress components by setting \np[=.true.]{ln_tauw}{ln\_tauw}. |
---|
1316 | |
---|
1317 | %% ================================================================================================= |
---|
1318 | \section{Miscellaneous options} |
---|
1319 | \label{sec:SBC_misc} |
---|
1320 | |
---|
1321 | %% ================================================================================================= |
---|
1322 | \subsection[Diurnal cycle (\textit{sbcdcy.F90})]{Diurnal cycle (\protect\mdl{sbcdcy})} |
---|
1323 | \label{subsec:SBC_dcy} |
---|
1324 | |
---|
1325 | \begin{figure}[!t] |
---|
1326 | \centering |
---|
1327 | \includegraphics[width=0.66\textwidth]{SBC_diurnal} |
---|
1328 | \caption[Reconstruction of the diurnal cycle variation of short wave flux]{ |
---|
1329 | Example of reconstruction of the diurnal cycle variation of short wave flux from |
---|
1330 | daily mean values. |
---|
1331 | The reconstructed diurnal cycle (black line) is chosen as |
---|
1332 | the mean value of the analytical cycle (blue line) over a time step, |
---|
1333 | not as the mid time step value of the analytically cycle (red square). |
---|
1334 | From \citet{bernie.guilyardi.ea_CD07}.} |
---|
1335 | \label{fig:SBC_diurnal} |
---|
1336 | \end{figure} |
---|
1337 | |
---|
1338 | \cite{bernie.woolnough.ea_JC05} have shown that to capture 90$\%$ of the diurnal variability of SST requires a vertical resolution in upper ocean of 1~m or better and a temporal resolution of the surface fluxes of 3~h or less. |
---|
1339 | %Unfortunately high frequency forcing fields are rare, not to say inexistent. GS: not true anymore ! |
---|
1340 | Nevertheless, it is possible to obtain a reasonable diurnal cycle of the SST knowning only short wave flux (SWF) at high frequency \citep{bernie.guilyardi.ea_CD07}. |
---|
1341 | Furthermore, only the knowledge of daily mean value of SWF is needed, |
---|
1342 | as higher frequency variations can be reconstructed from them, |
---|
1343 | assuming that the diurnal cycle of SWF is a scaling of the top of the atmosphere diurnal cycle of incident SWF. |
---|
1344 | The \cite{bernie.guilyardi.ea_CD07} reconstruction algorithm is available in \NEMO\ by |
---|
1345 | setting \np[=.true.]{ln_dm2dc}{ln\_dm2dc} (a \textit{\nam{sbc}{sbc}} namelist variable) when |
---|
1346 | using a bulk formulation (\np[=.true.]{ln_blk}{ln\_blk}) or |
---|
1347 | the flux formulation (\np[=.true.]{ln_flx}{ln\_flx}). |
---|
1348 | The reconstruction is performed in the \mdl{sbcdcy} module. |
---|
1349 | The detail of the algoritm used can be found in the appendix~A of \cite{bernie.guilyardi.ea_CD07}. |
---|
1350 | The algorithm preserves the daily mean incoming SWF as the reconstructed SWF at |
---|
1351 | a given time step is the mean value of the analytical cycle over this time step (\autoref{fig:SBC_diurnal}). |
---|
1352 | The use of diurnal cycle reconstruction requires the input SWF to be daily |
---|
1353 | (\ie\ a frequency of 24 hours and a time interpolation set to true in \np{sn_qsr}{sn\_qsr} namelist parameter). |
---|
1354 | Furthermore, it is recommended to have a least 8 surface module time steps per day, |
---|
1355 | that is $\rdt \ nn\_fsbc < 10,800~s = 3~h$. |
---|
1356 | An example of recontructed SWF is given in \autoref{fig:SBC_dcy} for a 12 reconstructed diurnal cycle, |
---|
1357 | one every 2~hours (from 1am to 11pm). |
---|
1358 | |
---|
1359 | \begin{figure}[!t] |
---|
1360 | \centering |
---|
1361 | \includegraphics[width=0.66\textwidth]{SBC_dcy} |
---|
1362 | \caption[Reconstruction of the diurnal cycle variation of short wave flux on an ORCA2 grid]{ |
---|
1363 | Example of reconstruction of the diurnal cycle variation of short wave flux from |
---|
1364 | daily mean values on an ORCA2 grid with a time sampling of 2~hours (from 1am to 11pm). |
---|
1365 | The display is on (i,j) plane.} |
---|
1366 | \label{fig:SBC_dcy} |
---|
1367 | \end{figure} |
---|
1368 | |
---|
1369 | Note also that the setting a diurnal cycle in SWF is highly recommended when |
---|
1370 | the top layer thickness approach 1~m or less, otherwise large error in SST can appear due to |
---|
1371 | an inconsistency between the scale of the vertical resolution and the forcing acting on that scale. |
---|
1372 | |
---|
1373 | %% ================================================================================================= |
---|
1374 | \subsection{Rotation of vector pairs onto the model grid directions} |
---|
1375 | \label{subsec:SBC_rotation} |
---|
1376 | |
---|
1377 | When using a flux (\np[=.true.]{ln_flx}{ln\_flx}) or bulk (\np[=.true.]{ln_blk}{ln\_blk}) formulation, |
---|
1378 | pairs of vector components can be rotated from east-north directions onto the local grid directions. |
---|
1379 | This is particularly useful when interpolation on the fly is used since here any vectors are likely to |
---|
1380 | be defined relative to a rectilinear grid. |
---|
1381 | To activate this option, a non-empty string is supplied in the rotation pair column of the relevant namelist. |
---|
1382 | The eastward component must start with "U" and the northward component with "V". |
---|
1383 | The remaining characters in the strings are used to identify which pair of components go together. |
---|
1384 | So for example, strings "U1" and "V1" next to "utau" and "vtau" would pair the wind stress components together and |
---|
1385 | rotate them on to the model grid directions; |
---|
1386 | "U2" and "V2" could be used against a second pair of components, and so on. |
---|
1387 | The extra characters used in the strings are arbitrary. |
---|
1388 | The rot\_rep routine from the \mdl{geo2ocean} module is used to perform the rotation. |
---|
1389 | |
---|
1390 | %% ================================================================================================= |
---|
1391 | \subsection[Surface restoring to observed SST and/or SSS (\textit{sbcssr.F90})]{Surface restoring to observed SST and/or SSS (\protect\mdl{sbcssr})} |
---|
1392 | \label{subsec:SBC_ssr} |
---|
1393 | |
---|
1394 | \begin{listing} |
---|
1395 | \nlst{namsbc_ssr} |
---|
1396 | \caption{\forcode{&namsbc_ssr}} |
---|
1397 | \label{lst:namsbc_ssr} |
---|
1398 | \end{listing} |
---|
1399 | |
---|
1400 | Options are defined through the \nam{sbc_ssr}{sbc\_ssr} namelist variables. |
---|
1401 | On forced mode using a flux formulation (\np[=.true.]{ln_flx}{ln\_flx}), |
---|
1402 | a feedback term \emph{must} be added to the surface heat flux $Q_{ns}^o$: |
---|
1403 | \[ |
---|
1404 | % \label{eq:SBC_dmp_q} |
---|
1405 | Q_{ns} = Q_{ns}^o + \frac{dQ}{dT} \left( \left. T \right|_{k=1} - SST_{Obs} \right) |
---|
1406 | \] |
---|
1407 | where SST is a sea surface temperature field (observed or climatological), |
---|
1408 | $T$ is the model surface layer temperature and |
---|
1409 | $\frac{dQ}{dT}$ is a negative feedback coefficient usually taken equal to $-40~W/m^2/K$. |
---|
1410 | For a $50~m$ mixed-layer depth, this value corresponds to a relaxation time scale of two months. |
---|
1411 | This term ensures that if $T$ perfectly matches the supplied SST, then $Q$ is equal to $Q_o$. |
---|
1412 | |
---|
1413 | In the fresh water budget, a feedback term can also be added. |
---|
1414 | Converted into an equivalent freshwater flux, it takes the following expression : |
---|
1415 | |
---|
1416 | \begin{equation} |
---|
1417 | \label{eq:SBC_dmp_emp} |
---|
1418 | \textit{emp} = \textit{emp}_o + \gamma_s^{-1} e_{3t} \frac{ \left(\left.S\right|_{k=1}-SSS_{Obs}\right)} |
---|
1419 | {\left.S\right|_{k=1}} |
---|
1420 | \end{equation} |
---|
1421 | |
---|
1422 | where $\textit{emp}_{o }$ is a net surface fresh water flux |
---|
1423 | (observed, climatological or an atmospheric model product), |
---|
1424 | \textit{SSS}$_{Obs}$ is a sea surface salinity |
---|
1425 | (usually a time interpolation of the monthly mean Polar Hydrographic Climatology \citep{steele.morley.ea_JC01}), |
---|
1426 | $\left.S\right|_{k=1}$ is the model surface layer salinity and |
---|
1427 | $\gamma_s$ is a negative feedback coefficient which is provided as a namelist parameter. |
---|
1428 | Unlike heat flux, there is no physical justification for the feedback term in \autoref{eq:SBC_dmp_emp} as |
---|
1429 | the atmosphere does not care about ocean surface salinity \citep{madec.delecluse_IWN97}. |
---|
1430 | The SSS restoring term should be viewed as a flux correction on freshwater fluxes to |
---|
1431 | reduce the uncertainties we have on the observed freshwater budget. |
---|
1432 | |
---|
1433 | %% ================================================================================================= |
---|
1434 | \subsection{Handling of ice-covered area (\textit{sbcice\_...})} |
---|
1435 | \label{subsec:SBC_ice-cover} |
---|
1436 | |
---|
1437 | The presence at the sea surface of an ice covered area modifies all the fluxes transmitted to the ocean. |
---|
1438 | There are several way to handle sea-ice in the system depending on |
---|
1439 | the value of the \np{nn_ice}{nn\_ice} namelist parameter found in \nam{sbc}{sbc} namelist. |
---|
1440 | \begin{description} |
---|
1441 | \item [nn\_ice = 0] there will never be sea-ice in the computational domain. |
---|
1442 | This is a typical namelist value used for tropical ocean domain. |
---|
1443 | The surface fluxes are simply specified for an ice-free ocean. |
---|
1444 | No specific things is done for sea-ice. |
---|
1445 | \item [nn\_ice = 1] sea-ice can exist in the computational domain, but no sea-ice model is used. |
---|
1446 | An observed ice covered area is read in a file. |
---|
1447 | Below this area, the SST is restored to the freezing point and |
---|
1448 | the heat fluxes are set to $-4~W/m^2$ ($-2~W/m^2$) in the northern (southern) hemisphere. |
---|
1449 | The associated modification of the freshwater fluxes are done in such a way that |
---|
1450 | the change in buoyancy fluxes remains zero. |
---|
1451 | This prevents deep convection to occur when trying to reach the freezing point |
---|
1452 | (and so ice covered area condition) while the SSS is too large. |
---|
1453 | This manner of managing sea-ice area, just by using a IF case, |
---|
1454 | is usually referred as the \textit{ice-if} model. |
---|
1455 | It can be found in the \mdl{sbcice\_if} module. |
---|
1456 | \item [nn\_ice = 2 or more] A full sea ice model is used. |
---|
1457 | This model computes the ice-ocean fluxes, |
---|
1458 | that are combined with the air-sea fluxes using the ice fraction of each model cell to |
---|
1459 | provide the surface averaged ocean fluxes. |
---|
1460 | Note that the activation of a sea-ice model is done by defining a CPP key (\key{si3} or \key{cice}). |
---|
1461 | The activation automatically overwrites the read value of nn\_ice to its appropriate value |
---|
1462 | (\ie\ $2$ for SI3 or $3$ for CICE). |
---|
1463 | \end{description} |
---|
1464 | |
---|
1465 | % {Description of Ice-ocean interface to be added here or in LIM 2 and 3 doc ?} |
---|
1466 | %GS: ocean-ice (SI3) interface is not located in SBC directory anymore, so it should be included in SI3 doc |
---|
1467 | |
---|
1468 | %% ================================================================================================= |
---|
1469 | \subsection[Interface to CICE (\textit{sbcice\_cice.F90})]{Interface to CICE (\protect\mdl{sbcice\_cice})} |
---|
1470 | \label{subsec:SBC_cice} |
---|
1471 | |
---|
1472 | It is possible to couple a regional or global \NEMO\ configuration (without AGRIF) |
---|
1473 | to the CICE sea-ice model by using \key{cice}. |
---|
1474 | The CICE code can be obtained from \href{http://oceans11.lanl.gov/trac/CICE/}{LANL} and |
---|
1475 | the additional 'hadgem3' drivers will be required, even with the latest code release. |
---|
1476 | Input grid files consistent with those used in \NEMO\ will also be needed, |
---|
1477 | and CICE CPP keys \textbf{ORCA\_GRID}, \textbf{CICE\_IN\_NEMO} and \textbf{coupled} should be used |
---|
1478 | (seek advice from UKMO if necessary). |
---|
1479 | Currently, the code is only designed to work when using the NCAR forcing option for \NEMO\ %GS: still true ? |
---|
1480 | (with \textit{calc\_strair}\forcode{=.true.} and \textit{calc\_Tsfc}\forcode{=.true.} in the CICE name-list), |
---|
1481 | or alternatively when \NEMO\ is coupled to the HadGAM3 atmosphere model |
---|
1482 | (with \textit{calc\_strair}\forcode{=.false.} and \textit{calc\_Tsfc}\forcode{=false}). |
---|
1483 | The code is intended to be used with \np{nn_fsbc}{nn\_fsbc} set to 1 |
---|
1484 | (although coupling ocean and ice less frequently should work, |
---|
1485 | it is possible the calculation of some of the ocean-ice fluxes needs to be modified slightly - |
---|
1486 | the user should check that results are not significantly different to the standard case). |
---|
1487 | |
---|
1488 | There are two options for the technical coupling between \NEMO\ and CICE. |
---|
1489 | The standard version allows complete flexibility for the domain decompositions in the individual models, |
---|
1490 | but this is at the expense of global gather and scatter operations in the coupling which |
---|
1491 | become very expensive on larger numbers of processors. |
---|
1492 | The alternative option (using \key{nemocice\_decomp} for both \NEMO\ and CICE) ensures that |
---|
1493 | the domain decomposition is identical in both models (provided domain parameters are set appropriately, |
---|
1494 | and \textit{processor\_shape~=~square-ice} and \textit{distribution\_wght~=~block} in the CICE name-list) and |
---|
1495 | allows much more efficient direct coupling on individual processors. |
---|
1496 | This solution scales much better although it is at the expense of having more idle CICE processors in areas where |
---|
1497 | there is no sea ice. |
---|
1498 | |
---|
1499 | %% ================================================================================================= |
---|
1500 | \subsection[Freshwater budget control (\textit{sbcfwb.F90})]{Freshwater budget control (\protect\mdl{sbcfwb})} |
---|
1501 | \label{subsec:SBC_fwb} |
---|
1502 | |
---|
1503 | For global ocean simulation, it can be useful to introduce a control of the mean sea level in order to |
---|
1504 | prevent unrealistic drift of the sea surface height due to inaccuracy in the freshwater fluxes. |
---|
1505 | In \NEMO, two way of controlling the freshwater budget are proposed: |
---|
1506 | |
---|
1507 | \begin{description} |
---|
1508 | \item [{\np[=0]{nn_fwb}{nn\_fwb}}] no control at all. |
---|
1509 | The mean sea level is free to drift, and will certainly do so. |
---|
1510 | \item [{\np[=1]{nn_fwb}{nn\_fwb}}] global mean \textit{emp} set to zero at each model time step. |
---|
1511 | %GS: comment below still relevant ? |
---|
1512 | %Note that with a sea-ice model, this technique only controls the mean sea level with linear free surface and no mass flux between ocean and ice (as it is implemented in the current ice-ocean coupling). |
---|
1513 | \item [{\np[=2]{nn_fwb}{nn\_fwb}}] freshwater budget is adjusted from the previous year annual mean budget which |
---|
1514 | is read in the \textit{EMPave\_old.dat} file. |
---|
1515 | As the model uses the Boussinesq approximation, the annual mean fresh water budget is simply evaluated from |
---|
1516 | the change in the mean sea level at January the first and saved in the \textit{EMPav.dat} file. |
---|
1517 | \end{description} |
---|
1518 | |
---|
1519 | % Griffies doc: |
---|
1520 | % When running ocean-ice simulations, we are not explicitly representing land processes, |
---|
1521 | % such as rivers, catchment areas, snow accumulation, etc. However, to reduce model drift, |
---|
1522 | % it is important to balance the hydrological cycle in ocean-ice models. |
---|
1523 | % We thus need to prescribe some form of global normalization to the precipitation minus evaporation plus river runoff. |
---|
1524 | % The result of the normalization should be a global integrated zero net water input to the ocean-ice system over |
---|
1525 | % a chosen time scale. |
---|
1526 | % How often the normalization is done is a matter of choice. In mom4p1, we choose to do so at each model time step, |
---|
1527 | % so that there is always a zero net input of water to the ocean-ice system. |
---|
1528 | % Others choose to normalize over an annual cycle, in which case the net imbalance over an annual cycle is used |
---|
1529 | % to alter the subsequent year�s water budget in an attempt to damp the annual water imbalance. |
---|
1530 | % Note that the annual budget approach may be inappropriate with interannually varying precipitation forcing. |
---|
1531 | % When running ocean-ice coupled models, it is incorrect to include the water transport between the ocean |
---|
1532 | % and ice models when aiming to balance the hydrological cycle. |
---|
1533 | % The reason is that it is the sum of the water in the ocean plus ice that should be balanced when running ocean-ice models, |
---|
1534 | % not the water in any one sub-component. As an extreme example to illustrate the issue, |
---|
1535 | % consider an ocean-ice model with zero initial sea ice. As the ocean-ice model spins up, |
---|
1536 | % there should be a net accumulation of water in the growing sea ice, and thus a net loss of water from the ocean. |
---|
1537 | % The total water contained in the ocean plus ice system is constant, but there is an exchange of water between |
---|
1538 | % the subcomponents. This exchange should not be part of the normalization used to balance the hydrological cycle |
---|
1539 | % in ocean-ice models. |
---|
1540 | |
---|
1541 | \subinc{\input{../../global/epilogue}} |
---|
1542 | |
---|
1543 | \end{document} |
---|