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 | % ================================================================ |
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6 | % Chapter Lateral Ocean Physics (LDF) |
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7 | % ================================================================ |
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8 | \chapter{Lateral Ocean Physics (LDF)} |
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9 | \label{chap:LDF} |
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10 | |
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11 | \minitoc |
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12 | |
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13 | \newpage |
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14 | |
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15 | The lateral physics terms in the momentum and tracer equations have been described in \autoref{eq:PE_zdf} and |
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16 | their discrete formulation in \autoref{sec:TRA_ldf} and \autoref{sec:DYN_ldf}). |
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17 | In this section we further discuss each lateral physics option. |
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18 | Choosing one lateral physics scheme means for the user defining, |
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19 | (1) the type of operator used (laplacian or bilaplacian operators, or no lateral mixing term); |
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20 | (2) the direction along which the lateral diffusive fluxes are evaluated |
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21 | (model level, geopotential or isopycnal surfaces); and |
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22 | (3) the space and time variations of the eddy coefficients. |
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23 | These three aspects of the lateral diffusion are set through namelist parameters |
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24 | (see the \textit{\ngn{nam\_traldf}} and \textit{\ngn{nam\_dynldf}} below). |
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25 | Note that this chapter describes the standard implementation of iso-neutral tracer mixing, |
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26 | and Griffies's implementation, which is used if \np{traldf\_grif}\forcode{ = .true.}, |
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27 | is described in Appdx\autoref{apdx:triad} |
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28 | |
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29 | %-----------------------------------nam_traldf - nam_dynldf-------------------------------------------- |
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30 | |
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31 | \nlst{namtra_ldf} |
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32 | |
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33 | \nlst{namdyn_ldf} |
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34 | %-------------------------------------------------------------------------------------------------------------- |
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35 | |
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36 | |
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37 | % ================================================================ |
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38 | % Direction of lateral Mixing |
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39 | % ================================================================ |
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40 | \section{Direction of lateral mixing (\protect\mdl{ldfslp})} |
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41 | \label{sec:LDF_slp} |
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42 | |
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43 | %%% |
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44 | \gmcomment{ |
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45 | we should emphasize here that the implementation is a rather old one. |
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46 | Better work can be achieved by using \citet{Griffies_al_JPO98, Griffies_Bk04} iso-neutral scheme. |
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47 | } |
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48 | |
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49 | A direction for lateral mixing has to be defined when the desired operator does not act along the model levels. |
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50 | This occurs when $(a)$ horizontal mixing is required on tracer or momentum |
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51 | (\np{ln\_traldf\_hor} or \np{ln\_dynldf\_hor}) in $s$- or mixed $s$-$z$- coordinates, |
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52 | and $(b)$ isoneutral mixing is required whatever the vertical coordinate is. |
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53 | This direction of mixing is defined by its slopes in the \textbf{i}- and \textbf{j}-directions at the face of |
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54 | the cell of the quantity to be diffused. |
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55 | For a tracer, this leads to the following four slopes: |
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56 | $r_{1u}$, $r_{1w}$, $r_{2v}$, $r_{2w}$ (see \autoref{eq:tra_ldf_iso}), |
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57 | while for momentum the slopes are $r_{1t}$, $r_{1uw}$, $r_{2f}$, $r_{2uw}$ for $u$ and |
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58 | $r_{1f}$, $r_{1vw}$, $r_{2t}$, $r_{2vw}$ for $v$. |
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59 | |
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60 | %gm% add here afigure of the slope in i-direction |
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61 | |
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62 | \subsection{Slopes for tracer geopotential mixing in the $s$-coordinate} |
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63 | |
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64 | In $s$-coordinates, geopotential mixing ($i.e.$ horizontal mixing) $r_1$ and $r_2$ are the slopes between |
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65 | the geopotential and computational surfaces. |
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66 | Their discrete formulation is found by locally solving \autoref{eq:tra_ldf_iso} when |
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67 | the diffusive fluxes in the three directions are set to zero and $T$ is assumed to be horizontally uniform, |
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68 | $i.e.$ a linear function of $z_T$, the depth of a $T$-point. |
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69 | %gm { Steven : My version is obviously wrong since I'm left with an arbitrary constant which is the local vertical temperature gradient} |
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70 | |
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71 | \begin{equation} |
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72 | \label{eq:ldfslp_geo} |
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73 | \begin{aligned} |
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74 | r_{1u} &= \frac{e_{3u}}{ \left( e_{1u}\;\overline{\overline{e_{3w}}}^{\,i+1/2,\,k} \right)} |
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75 | \;\delta_{i+1/2}[z_t] |
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76 | &\approx \frac{1}{e_{1u}}\; \delta_{i+1/2}[z_t] \ \ \ \\ |
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77 | r_{2v} &= \frac{e_{3v}}{\left( e_{2v}\;\overline{\overline{e_{3w}}}^{\,j+1/2,\,k} \right)} |
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78 | \;\delta_{j+1/2} [z_t] |
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79 | &\approx \frac{1}{e_{2v}}\; \delta_{j+1/2}[z_t] \ \ \ \\ |
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80 | r_{1w} &= \frac{1}{e_{1w}}\;\overline{\overline{\delta_{i+1/2}[z_t]}}^{\,i,\,k+1/2} |
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81 | &\approx \frac{1}{e_{1w}}\; \delta_{i+1/2}[z_{uw}] \\ |
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82 | r_{2w} &= \frac{1}{e_{2w}}\;\overline{\overline{\delta_{j+1/2}[z_t]}}^{\,j,\,k+1/2} |
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83 | &\approx \frac{1}{e_{2w}}\; \delta_{j+1/2}[z_{vw}] |
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84 | \end{aligned} |
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85 | \end{equation} |
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86 | |
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87 | %gm% caution I'm not sure the simplification was a good idea! |
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88 | |
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89 | These slopes are computed once in \rou{ldfslp\_init} when \np{ln\_sco}\forcode{ = .true.}rue, |
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90 | and either \np{ln\_traldf\_hor}\forcode{ = .true.} or \np{ln\_dynldf\_hor}\forcode{ = .true.}. |
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91 | |
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92 | \subsection{Slopes for tracer iso-neutral mixing} |
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93 | \label{subsec:LDF_slp_iso} |
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94 | |
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95 | In iso-neutral mixing $r_1$ and $r_2$ are the slopes between the iso-neutral and computational surfaces. |
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96 | Their formulation does not depend on the vertical coordinate used. |
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97 | Their discrete formulation is found using the fact that the diffusive fluxes of |
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98 | locally referenced potential density ($i.e.$ $in situ$ density) vanish. |
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99 | So, substituting $T$ by $\rho$ in \autoref{eq:tra_ldf_iso} and setting the diffusive fluxes in |
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100 | the three directions to zero leads to the following definition for the neutral slopes: |
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101 | |
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102 | \begin{equation} |
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103 | \label{eq:ldfslp_iso} |
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104 | \begin{split} |
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105 | r_{1u} &= \frac{e_{3u}}{e_{1u}}\; \frac{\delta_{i+1/2}[\rho]} |
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106 | {\overline{\overline{\delta_{k+1/2}[\rho]}}^{\,i+1/2,\,k}} \\ |
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107 | r_{2v} &= \frac{e_{3v}}{e_{2v}}\; \frac{\delta_{j+1/2}\left[\rho \right]} |
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108 | {\overline{\overline{\delta_{k+1/2}[\rho]}}^{\,j+1/2,\,k}} \\ |
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109 | r_{1w} &= \frac{e_{3w}}{e_{1w}}\; |
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110 | \frac{\overline{\overline{\delta_{i+1/2}[\rho]}}^{\,i,\,k+1/2}} |
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111 | {\delta_{k+1/2}[\rho]} \\ |
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112 | r_{2w} &= \frac{e_{3w}}{e_{2w}}\; |
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113 | \frac{\overline{\overline{\delta_{j+1/2}[\rho]}}^{\,j,\,k+1/2}} |
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114 | {\delta_{k+1/2}[\rho]} |
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115 | \end{split} |
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116 | \end{equation} |
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117 | |
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118 | %gm% rewrite this as the explanation is not very clear !!! |
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119 | %In practice, \autoref{eq:ldfslp_iso} is of little help in evaluating the neutral surface slopes. Indeed, for an unsimplified equation of state, the density has a strong dependancy on pressure (here approximated as the depth), therefore applying \autoref{eq:ldfslp_iso} using the $in situ$ density, $\rho$, computed at T-points leads to a flattening of slopes as the depth increases. This is due to the strong increase of the $in situ$ density with depth. |
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120 | |
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121 | %By definition, neutral surfaces are tangent to the local $in situ$ density \citep{McDougall1987}, therefore in \autoref{eq:ldfslp_iso}, all the derivatives have to be evaluated at the same local pressure (which in decibars is approximated by the depth in meters). |
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122 | |
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123 | %In the $z$-coordinate, the derivative of the \autoref{eq:ldfslp_iso} numerator is evaluated at the same depth \nocite{as what?} ($T$-level, which is the same as the $u$- and $v$-levels), so the $in situ$ density can be used for its evaluation. |
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124 | |
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125 | As the mixing is performed along neutral surfaces, the gradient of $\rho$ in \autoref{eq:ldfslp_iso} has to |
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126 | be evaluated at the same local pressure |
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127 | (which, in decibars, is approximated by the depth in meters in the model). |
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128 | Therefore \autoref{eq:ldfslp_iso} cannot be used as such, |
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129 | but further transformation is needed depending on the vertical coordinate used: |
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130 | |
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131 | \begin{description} |
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132 | |
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133 | \item[$z$-coordinate with full step: ] |
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134 | in \autoref{eq:ldfslp_iso} the densities appearing in the $i$ and $j$ derivatives are taken at the same depth, |
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135 | thus the $in situ$ density can be used. |
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136 | This is not the case for the vertical derivatives: $\delta_{k+1/2}[\rho]$ is replaced by $-\rho N^2/g$, |
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137 | where $N^2$ is the local Brunt-Vais\"{a}l\"{a} frequency evaluated following \citet{McDougall1987} |
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138 | (see \autoref{subsec:TRA_bn2}). |
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139 | |
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140 | \item[$z$-coordinate with partial step: ] |
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141 | this case is identical to the full step case except that at partial step level, |
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142 | the \emph{horizontal} density gradient is evaluated as described in \autoref{sec:TRA_zpshde}. |
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143 | |
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144 | \item[$s$- or hybrid $s$-$z$- coordinate: ] |
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145 | in the current release of \NEMO, iso-neutral mixing is only employed for $s$-coordinates if |
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146 | the Griffies scheme is used (\np{traldf\_grif}\forcode{ = .true.}; |
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147 | see Appdx \autoref{apdx:triad}). |
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148 | In other words, iso-neutral mixing will only be accurately represented with a linear equation of state |
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149 | (\np{nn\_eos}\forcode{ = 1..2}). |
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150 | In the case of a "true" equation of state, the evaluation of $i$ and $j$ derivatives in \autoref{eq:ldfslp_iso} |
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151 | will include a pressure dependent part, leading to the wrong evaluation of the neutral slopes. |
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152 | |
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153 | %gm% |
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154 | Note: The solution for $s$-coordinate passes trough the use of different (and better) expression for |
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155 | the constraint on iso-neutral fluxes. |
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156 | Following \citet{Griffies_Bk04}, instead of specifying directly that there is a zero neutral diffusive flux of |
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157 | locally referenced potential density, we stay in the $T$-$S$ plane and consider the balance between |
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158 | the neutral direction diffusive fluxes of potential temperature and salinity: |
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159 | \[ |
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160 | \alpha \ \textbf{F}(T) = \beta \ \textbf{F}(S) |
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161 | \] |
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162 | % gm{ where vector F is ....} |
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163 | |
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164 | This constraint leads to the following definition for the slopes: |
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165 | |
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166 | \[ |
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167 | % \label{eq:ldfslp_iso2} |
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168 | \begin{split} |
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169 | r_{1u} &= \frac{e_{3u}}{e_{1u}}\; \frac |
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170 | {\alpha_u \;\delta_{i+1/2}[T] - \beta_u \;\delta_{i+1/2}[S]} |
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171 | {\alpha_u \;\overline{\overline{\delta_{k+1/2}[T]}}^{\,i+1/2,\,k} |
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172 | -\beta_u \;\overline{\overline{\delta_{k+1/2}[S]}}^{\,i+1/2,\,k} } \\ |
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173 | r_{2v} &= \frac{e_{3v}}{e_{2v}}\; \frac |
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174 | {\alpha_v \;\delta_{j+1/2}[T] - \beta_v \;\delta_{j+1/2}[S]} |
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175 | {\alpha_v \;\overline{\overline{\delta_{k+1/2}[T]}}^{\,j+1/2,\,k} |
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176 | -\beta_v \;\overline{\overline{\delta_{k+1/2}[S]}}^{\,j+1/2,\,k} } \\ |
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177 | r_{1w} &= \frac{e_{3w}}{e_{1w}}\; \frac |
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178 | {\alpha_w \;\overline{\overline{\delta_{i+1/2}[T]}}^{\,i,\,k+1/2} |
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179 | -\beta_w \;\overline{\overline{\delta_{i+1/2}[S]}}^{\,i,\,k+1/2} } |
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180 | {\alpha_w \;\delta_{k+1/2}[T] - \beta_w \;\delta_{k+1/2}[S]} \\ |
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181 | r_{2w} &= \frac{e_{3w}}{e_{2w}}\; \frac |
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182 | {\alpha_w \;\overline{\overline{\delta_{j+1/2}[T]}}^{\,j,\,k+1/2} |
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183 | -\beta_w \;\overline{\overline{\delta_{j+1/2}[S]}}^{\,j,\,k+1/2} } |
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184 | {\alpha_w \;\delta_{k+1/2}[T] - \beta_w \;\delta_{k+1/2}[S]} \\ |
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185 | \end{split} |
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186 | \] |
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187 | where $\alpha$ and $\beta$, the thermal expansion and saline contraction coefficients introduced in |
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188 | \autoref{subsec:TRA_bn2}, have to be evaluated at the three velocity points. |
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189 | In order to save computation time, they should be approximated by the mean of their values at $T$-points |
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190 | (for example in the case of $\alpha$: |
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191 | $\alpha_u=\overline{\alpha_T}^{i+1/2}$, $\alpha_v=\overline{\alpha_T}^{j+1/2}$ and |
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192 | $\alpha_w=\overline{\alpha_T}^{k+1/2}$). |
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193 | |
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194 | Note that such a formulation could be also used in the $z$-coordinate and $z$-coordinate with partial steps cases. |
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195 | |
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196 | \end{description} |
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197 | |
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198 | This implementation is a rather old one. |
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199 | It is similar to the one proposed by Cox [1987], except for the background horizontal diffusion. |
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200 | Indeed, the Cox implementation of isopycnal diffusion in GFDL-type models requires |
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201 | a minimum background horizontal diffusion for numerical stability reasons. |
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202 | To overcome this problem, several techniques have been proposed in which the numerical schemes of |
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203 | the ocean model are modified \citep{Weaver_Eby_JPO97, Griffies_al_JPO98}. |
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204 | Griffies's scheme is now available in \NEMO if \np{traldf\_grif\_iso} is set true; see Appdx \autoref{apdx:triad}. |
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205 | Here, another strategy is presented \citep{Lazar_PhD97}: |
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206 | a local filtering of the iso-neutral slopes (made on 9 grid-points) prevents the development of |
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207 | grid point noise generated by the iso-neutral diffusion operator (\autoref{fig:LDF_ZDF1}). |
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208 | This allows an iso-neutral diffusion scheme without additional background horizontal mixing. |
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209 | This technique can be viewed as a diffusion operator that acts along large-scale |
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210 | (2~$\Delta$x) \gmcomment{2deltax doesnt seem very large scale} iso-neutral surfaces. |
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211 | The diapycnal diffusion required for numerical stability is thus minimized and its net effect on the flow is quite small when compared to the effect of an horizontal background mixing. |
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212 | |
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213 | Nevertheless, this iso-neutral operator does not ensure that variance cannot increase, |
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214 | contrary to the \citet{Griffies_al_JPO98} operator which has that property. |
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215 | |
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216 | %>>>>>>>>>>>>>>>>>>>>>>>>>>>> |
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217 | \begin{figure}[!ht] |
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218 | \begin{center} |
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219 | \includegraphics[width=0.70\textwidth]{Fig_LDF_ZDF1} |
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220 | \caption { |
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221 | \protect\label{fig:LDF_ZDF1} |
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222 | averaging procedure for isopycnal slope computation. |
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223 | } |
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224 | \end{center} |
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225 | \end{figure} |
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226 | %>>>>>>>>>>>>>>>>>>>>>>>>>>>> |
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227 | |
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228 | %There are three additional questions about the slope calculation. |
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229 | %First the expression for the rotation tensor has been obtain assuming the "small slope" approximation, so a bound has to be imposed on slopes. |
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230 | %Second, numerical stability issues also require a bound on slopes. |
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231 | %Third, the question of boundary condition specified on slopes... |
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232 | |
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233 | %from griffies: chapter 13.1.... |
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234 | |
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235 | |
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236 | |
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237 | % In addition and also for numerical stability reasons \citep{Cox1987, Griffies_Bk04}, |
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238 | % the slopes are bounded by $1/100$ everywhere. This limit is decreasing linearly |
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239 | % to zero fom $70$ meters depth and the surface (the fact that the eddies "feel" the |
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240 | % surface motivates this flattening of isopycnals near the surface). |
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241 | |
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242 | For numerical stability reasons \citep{Cox1987, Griffies_Bk04}, the slopes must also be bounded by |
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243 | $1/100$ everywhere. |
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244 | This constraint is applied in a piecewise linear fashion, increasing from zero at the surface to |
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245 | $1/100$ at $70$ metres and thereafter decreasing to zero at the bottom of the ocean |
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246 | (the fact that the eddies "feel" the surface motivates this flattening of isopycnals near the surface). |
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247 | |
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248 | %>>>>>>>>>>>>>>>>>>>>>>>>>>>> |
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249 | \begin{figure}[!ht] |
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250 | \begin{center} |
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251 | \includegraphics[width=0.70\textwidth]{Fig_eiv_slp} |
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252 | \caption{ |
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253 | \protect\label{fig:eiv_slp} |
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254 | Vertical profile of the slope used for lateral mixing in the mixed layer: |
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255 | \textit{(a)} in the real ocean the slope is the iso-neutral slope in the ocean interior, |
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256 | which has to be adjusted at the surface boundary |
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257 | (i.e. it must tend to zero at the surface since there is no mixing across the air-sea interface: |
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258 | wall boundary condition). |
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259 | Nevertheless, the profile between the surface zero value and the interior iso-neutral one is unknown, |
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260 | and especially the value at the base of the mixed layer; |
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261 | \textit{(b)} profile of slope using a linear tapering of the slope near the surface and |
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262 | imposing a maximum slope of 1/100; |
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263 | \textit{(c)} profile of slope actually used in \NEMO: a linear decrease of the slope from |
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264 | zero at the surface to its ocean interior value computed just below the mixed layer. |
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265 | Note the huge change in the slope at the base of the mixed layer between \textit{(b)} and \textit{(c)}. |
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266 | } |
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267 | \end{center} |
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268 | \end{figure} |
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269 | %>>>>>>>>>>>>>>>>>>>>>>>>>>>> |
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270 | |
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271 | \colorbox{yellow}{add here a discussion about the flattening of the slopes, vs tapering the coefficient.} |
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272 | |
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273 | \subsection{Slopes for momentum iso-neutral mixing} |
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274 | |
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275 | The iso-neutral diffusion operator on momentum is the same as the one used on tracers but |
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276 | applied to each component of the velocity separately |
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277 | (see \autoref{eq:dyn_ldf_iso} in section~\autoref{subsec:DYN_ldf_iso}). |
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278 | The slopes between the surface along which the diffusion operator acts and the surface of computation |
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279 | ($z$- or $s$-surfaces) are defined at $T$-, $f$-, and \textit{uw}- points for the $u$-component, and $T$-, $f$- and |
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280 | \textit{vw}- points for the $v$-component. |
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281 | They are computed from the slopes used for tracer diffusion, |
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282 | $i.e.$ \autoref{eq:ldfslp_geo} and \autoref{eq:ldfslp_iso} : |
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283 | |
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284 | \[ |
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285 | % \label{eq:ldfslp_dyn} |
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286 | \begin{aligned} |
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287 | &r_{1t}\ \ = \overline{r_{1u}}^{\,i} &&& r_{1f}\ \ &= \overline{r_{1u}}^{\,i+1/2} \\ |
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288 | &r_{2f} \ \ = \overline{r_{2v}}^{\,j+1/2} &&& r_{2t}\ &= \overline{r_{2v}}^{\,j} \\ |
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289 | &r_{1uw} = \overline{r_{1w}}^{\,i+1/2} &&\ \ \text{and} \ \ & r_{1vw}&= \overline{r_{1w}}^{\,j+1/2} \\ |
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290 | &r_{2uw}= \overline{r_{2w}}^{\,j+1/2} &&& r_{2vw}&= \overline{r_{2w}}^{\,j+1/2}\\ |
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291 | \end{aligned} |
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292 | \] |
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293 | |
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294 | The major issue remaining is in the specification of the boundary conditions. |
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295 | The same boundary conditions are chosen as those used for lateral diffusion along model level surfaces, |
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296 | $i.e.$ using the shear computed along the model levels and with no additional friction at the ocean bottom |
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297 | (see \autoref{sec:LBC_coast}). |
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298 | |
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299 | |
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300 | % ================================================================ |
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301 | % Lateral Mixing Operator |
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302 | % ================================================================ |
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303 | \section{Lateral mixing operators (\protect\mdl{traldf}, \protect\mdl{traldf}) } |
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304 | \label{sec:LDF_op} |
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305 | |
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306 | |
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307 | |
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308 | % ================================================================ |
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309 | % Lateral Mixing Coefficients |
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310 | % ================================================================ |
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311 | \section{Lateral mixing coefficient (\protect\mdl{ldftra}, \protect\mdl{ldfdyn}) } |
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312 | \label{sec:LDF_coef} |
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313 | |
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314 | Introducing a space variation in the lateral eddy mixing coefficients changes the model core memory requirement, |
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315 | adding up to four extra three-dimensional arrays for the geopotential or isopycnal second order operator applied to |
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316 | momentum. |
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317 | Six CPP keys control the space variation of eddy coefficients: three for momentum and three for tracer. |
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318 | The three choices allow: |
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319 | a space variation in the three space directions (\key{traldf\_c3d}, \key{dynldf\_c3d}), |
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320 | in the horizontal plane (\key{traldf\_c2d}, \key{dynldf\_c2d}), |
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321 | or in the vertical only (\key{traldf\_c1d}, \key{dynldf\_c1d}). |
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322 | The default option is a constant value over the whole ocean on both momentum and tracers. |
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323 | |
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324 | The number of additional arrays that have to be defined and the gridpoint position at which |
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325 | they are defined depend on both the space variation chosen and the type of operator used. |
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326 | The resulting eddy viscosity and diffusivity coefficients can be a function of more than one variable. |
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327 | Changes in the computer code when switching from one option to another have been minimized by |
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328 | introducing the eddy coefficients as statement functions |
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329 | (include file \hf{ldftra\_substitute} and \hf{ldfdyn\_substitute}). |
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330 | The functions are replaced by their actual meaning during the preprocessing step (CPP). |
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331 | The specification of the space variation of the coefficient is made in \mdl{ldftra} and \mdl{ldfdyn}, |
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332 | or more precisely in include files \hf{traldf\_cNd} and \hf{dynldf\_cNd}, with N=1, 2 or 3. |
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333 | The user can modify these include files as he/she wishes. |
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334 | The way the mixing coefficient are set in the reference version can be briefly described as follows: |
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335 | |
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336 | \subsubsection{Constant mixing coefficients (default option)} |
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337 | When none of the \key{dynldf\_...} and \key{traldf\_...} keys are defined, |
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338 | a constant value is used over the whole ocean for momentum and tracers, |
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339 | which is specified through the \np{rn\_ahm0} and \np{rn\_aht0} namelist parameters. |
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340 | |
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341 | \subsubsection{Vertically varying mixing coefficients (\protect\key{traldf\_c1d} and \key{dynldf\_c1d})} |
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342 | The 1D option is only available when using the $z$-coordinate with full step. |
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343 | Indeed in all the other types of vertical coordinate, |
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344 | the depth is a 3D function of (\textbf{i},\textbf{j},\textbf{k}) and therefore, |
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345 | introducing depth-dependent mixing coefficients will require 3D arrays. |
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346 | In the 1D option, a hyperbolic variation of the lateral mixing coefficient is introduced in which |
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347 | the surface value is \np{rn\_aht0} (\np{rn\_ahm0}), the bottom value is 1/4 of the surface value, |
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348 | and the transition takes place around z=300~m with a width of 300~m |
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349 | ($i.e.$ both the depth and the width of the inflection point are set to 300~m). |
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350 | This profile is hard coded in file \hf{traldf\_c1d}, but can be easily modified by users. |
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351 | |
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352 | \subsubsection{Horizontally varying mixing coefficients (\protect\key{traldf\_c2d} and \protect\key{dynldf\_c2d})} |
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353 | By default the horizontal variation of the eddy coefficient depends on the local mesh size and |
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354 | the type of operator used: |
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355 | \begin{equation} |
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356 | \label{eq:title} |
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357 | A_l = \left\{ |
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358 | \begin{aligned} |
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359 | & \frac{\max(e_1,e_2)}{e_{max}} A_o^l & \text{for laplacian operator } \\ |
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360 | & \frac{\max(e_1,e_2)^{3}}{e_{max}^{3}} A_o^l & \text{for bilaplacian operator } |
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361 | \end{aligned} |
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362 | \right. |
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363 | \end{equation} |
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364 | where $e_{max}$ is the maximum of $e_1$ and $e_2$ taken over the whole masked ocean domain, |
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365 | and $A_o^l$ is the \np{rn\_ahm0} (momentum) or \np{rn\_aht0} (tracer) namelist parameter. |
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366 | This variation is intended to reflect the lesser need for subgrid scale eddy mixing where |
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367 | the grid size is smaller in the domain. |
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368 | It was introduced in the context of the DYNAMO modelling project \citep{Willebrand_al_PO01}. |
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369 | Note that such a grid scale dependance of mixing coefficients significantly increase the range of stability of |
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370 | model configurations presenting large changes in grid pacing such as global ocean models. |
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371 | Indeed, in such a case, a constant mixing coefficient can lead to a blow up of the model due to |
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372 | large coefficient compare to the smallest grid size (see \autoref{sec:STP_forward_imp}), |
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373 | especially when using a bilaplacian operator. |
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374 | |
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375 | Other formulations can be introduced by the user for a given configuration. |
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376 | For example, in the ORCA2 global ocean model (see Configurations), |
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377 | the laplacian viscosity operator uses \np{rn\_ahm0}~= 4.10$^4$ m$^2$/s poleward of 20$^{\circ}$ north and south and |
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378 | decreases linearly to \np{rn\_aht0}~= 2.10$^3$ m$^2$/s at the equator \citep{Madec_al_JPO96, Delecluse_Madec_Bk00}. |
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379 | This modification can be found in routine \rou{ldf\_dyn\_c2d\_orca} defined in \mdl{ldfdyn\_c2d}. |
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380 | Similar modified horizontal variations can be found with the Antarctic or Arctic sub-domain options of |
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381 | ORCA2 and ORCA05 (see \&namcfg namelist). |
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382 | |
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383 | \subsubsection{Space varying mixing coefficients (\protect\key{traldf\_c3d} and \key{dynldf\_c3d})} |
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384 | |
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385 | The 3D space variation of the mixing coefficient is simply the combination of the 1D and 2D cases, |
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386 | $i.e.$ a hyperbolic tangent variation with depth associated with a grid size dependence of |
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387 | the magnitude of the coefficient. |
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388 | |
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389 | \subsubsection{Space and time varying mixing coefficients} |
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390 | |
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391 | There is no default specification of space and time varying mixing coefficient. |
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392 | The only case available is specific to the ORCA2 and ORCA05 global ocean configurations. |
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393 | It provides only a tracer mixing coefficient for eddy induced velocity (ORCA2) or both iso-neutral and |
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394 | eddy induced velocity (ORCA05) that depends on the local growth rate of baroclinic instability. |
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395 | This specification is actually used when an ORCA key and both \key{traldf\_eiv} and \key{traldf\_c2d} are defined. |
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396 | |
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397 | The following points are relevant when the eddy coefficient varies spatially: |
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398 | |
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399 | (1) the momentum diffusion operator acting along model level surfaces is written in terms of curl and |
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400 | divergent components of the horizontal current (see \autoref{subsec:PE_ldf}). |
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401 | Although the eddy coefficient could be set to different values in these two terms, |
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402 | this option is not currently available. |
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403 | |
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404 | (2) with an horizontally varying viscosity, the quadratic integral constraints on enstrophy and on the square of |
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405 | the horizontal divergence for operators acting along model-surfaces are no longer satisfied |
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406 | (\autoref{sec:dynldf_properties}). |
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407 | |
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408 | (3) for isopycnal diffusion on momentum or tracers, an additional purely horizontal background diffusion with |
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409 | uniform coefficient can be added by setting a non zero value of \np{rn\_ahmb0} or \np{rn\_ahtb0}, |
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410 | a background horizontal eddy viscosity or diffusivity coefficient |
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411 | (namelist parameters whose default values are $0$). |
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412 | However, the technique used to compute the isopycnal slopes is intended to get rid of such a background diffusion, |
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413 | since it introduces spurious diapycnal diffusion (see \autoref{sec:LDF_slp}). |
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414 | |
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415 | (4) when an eddy induced advection term is used (\key{traldf\_eiv}), |
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416 | $A^{eiv}$, the eddy induced coefficient has to be defined. |
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417 | Its space variations are controlled by the same CPP variable as for the eddy diffusivity coefficient |
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418 | ($i.e.$ \key{traldf\_cNd}). |
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419 | |
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420 | (5) the eddy coefficient associated with a biharmonic operator must be set to a \emph{negative} value. |
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421 | |
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422 | (6) it is possible to use both the laplacian and biharmonic operators concurrently. |
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423 | |
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424 | (7) it is possible to run without explicit lateral diffusion on momentum |
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425 | (\np{ln\_dynldf\_lap}\forcode{ = .?.}\np{ln\_dynldf\_bilap}\forcode{ = .false.}). |
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426 | This is recommended when using the UBS advection scheme on momentum (\np{ln\_dynadv\_ubs}\forcode{ = .true.}, |
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427 | see \autoref{subsec:DYN_adv_ubs}) and can be useful for testing purposes. |
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428 | |
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429 | % ================================================================ |
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430 | % Eddy Induced Mixing |
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431 | % ================================================================ |
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432 | \section{Eddy induced velocity (\protect\mdl{traadv\_eiv}, \protect\mdl{ldfeiv})} |
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433 | \label{sec:LDF_eiv} |
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434 | |
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435 | %%gm from Triad appendix : to be incorporated.... |
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436 | \gmcomment{ |
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437 | Values of iso-neutral diffusivity and GM coefficient are set as described in \autoref{sec:LDF_coef}. |
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438 | If none of the keys \key{traldf\_cNd}, N=1,2,3 is set (the default), spatially constant iso-neutral $A_l$ and |
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439 | GM diffusivity $A_e$ are directly set by \np{rn\_aeih\_0} and \np{rn\_aeiv\_0}. |
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440 | If 2D-varying coefficients are set with \key{traldf\_c2d} then $A_l$ is reduced in proportion with horizontal |
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441 | scale factor according to \autoref{eq:title} |
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442 | \footnote{ |
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443 | Except in global ORCA $0.5^{\circ}$ runs with \key{traldf\_eiv}, |
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444 | where $A_l$ is set like $A_e$ but with a minimum vale of $100\;\mathrm{m}^2\;\mathrm{s}^{-1}$ |
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445 | }. |
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446 | In idealised setups with \key{traldf\_c2d}, $A_e$ is reduced similarly, but if \key{traldf\_eiv} is set in |
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447 | the global configurations with \key{traldf\_c2d}, a horizontally varying $A_e$ is instead set from |
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448 | the Held-Larichev parameterisation |
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449 | \footnote{ |
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450 | In this case, $A_e$ at low latitudes $|\theta|<20^{\circ}$ is further reduced by a factor $|f/f_{20}|$, |
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451 | where $f_{20}$ is the value of $f$ at $20^{\circ}$~N |
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452 | } (\mdl{ldfeiv}) and \np{rn\_aeiv\_0} is ignored unless it is zero. |
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453 | } |
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454 | |
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455 | When Gent and McWilliams [1990] diffusion is used (\key{traldf\_eiv} defined), |
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456 | an eddy induced tracer advection term is added, |
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457 | the formulation of which depends on the slopes of iso-neutral surfaces. |
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458 | Contrary to the case of iso-neutral mixing, the slopes used here are referenced to the geopotential surfaces, |
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459 | $i.e.$ \autoref{eq:ldfslp_geo} is used in $z$-coordinates, |
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460 | and the sum \autoref{eq:ldfslp_geo} + \autoref{eq:ldfslp_iso} in $s$-coordinates. |
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461 | The eddy induced velocity is given by: |
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462 | \begin{equation} |
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463 | \label{eq:ldfeiv} |
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464 | \begin{split} |
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465 | u^* & = \frac{1}{e_{2u}e_{3u}}\; \delta_k \left[e_{2u} \, A_{uw}^{eiv} \; \overline{r_{1w}}^{\,i+1/2} \right]\\ |
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466 | v^* & = \frac{1}{e_{1u}e_{3v}}\; \delta_k \left[e_{1v} \, A_{vw}^{eiv} \; \overline{r_{2w}}^{\,j+1/2} \right]\\ |
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467 | w^* & = \frac{1}{e_{1w}e_{2w}}\; \left\{ \delta_i \left[e_{2u} \, A_{uw}^{eiv} \; \overline{r_{1w}}^{\,i+1/2} \right] + \delta_j \left[e_{1v} \, A_{vw}^{eiv} \; \overline{r_{2w}}^{\,j+1/2} \right] \right\} \\ |
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468 | \end{split} |
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469 | \end{equation} |
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470 | where $A^{eiv}$ is the eddy induced velocity coefficient whose value is set through \np{rn\_aeiv}, |
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471 | a \textit{nam\_traldf} namelist parameter. |
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472 | The three components of the eddy induced velocity are computed and |
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473 | add to the eulerian velocity in \mdl{traadv\_eiv}. |
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474 | This has been preferred to a separate computation of the advective trends associated with the eiv velocity, |
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475 | since it allows us to take advantage of all the advection schemes offered for the tracers |
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476 | (see \autoref{sec:TRA_adv}) and not just the $2^{nd}$ order advection scheme as in |
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477 | previous releases of OPA \citep{Madec1998}. |
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478 | This is particularly useful for passive tracers where \emph{positivity} of the advection scheme is of |
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479 | paramount importance. |
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480 | |
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481 | At the surface, lateral and bottom boundaries, the eddy induced velocity, |
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482 | and thus the advective eddy fluxes of heat and salt, are set to zero. |
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483 | |
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484 | \biblio |
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485 | |
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486 | \end{document} |
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