Changeset 6289 for trunk/DOC/TexFiles/Chapters/Chap_SBC.tex
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trunk/DOC/TexFiles/Chapters/Chap_SBC.tex
r6140 r6289 1 1 % ================================================================ 2 % Chapter �Surface Boundary Condition (SBC, ISF, ICB)2 % Chapter —— Surface Boundary Condition (SBC, ISF, ICB) 3 3 % ================================================================ 4 4 \chapter{Surface Boundary Condition (SBC, ISF, ICB) } … … 17 17 \item the two components of the surface ocean stress $\left( {\tau _u \;,\;\tau _v} \right)$ 18 18 \item the incoming solar and non solar heat fluxes $\left( {Q_{ns} \;,\;Q_{sr} } \right)$ 19 \item the surface freshwater budget $\left( {\textit{emp},\;\textit{emp}_S } \right)$ 19 \item the surface freshwater budget $\left( {\textit{emp}} \right)$ 20 \item the surface salt flux associated with freezing/melting of seawater $\left( {\textit{sfx}} \right)$ 20 21 \end{itemize} 21 22 plus an optional field: … … 27 28 are controlled by namelist \ngn{namsbc} variables: an analytical formulation (\np{ln\_ana}~=~true), 28 29 a flux formulation (\np{ln\_flx}~=~true), a bulk formulae formulation (CORE 29 (\np{ln\_ core}~=~true), CLIO (\np{ln\_clio}~=~true) or MFS30 (\np{ln\_blk\_core}~=~true), CLIO (\np{ln\_blk\_clio}~=~true) or MFS 30 31 \footnote { Note that MFS bulk formulae compute fluxes only for the ocean component} 31 (\np{ln\_mfs}~=~true) bulk formulae) and a coupled 32 formulation (exchanges with a atmospheric model via the OASIS coupler) 33 (\np{ln\_cpl}~=~true). When used, the atmospheric pressure forces both 34 ocean and ice dynamics (\np{ln\_apr\_dyn}~=~true). 35 The frequency at which the six or seven fields have to be updated is the \np{nn\_fsbc} 36 namelist parameter. 32 (\np{ln\_blk\_mfs}~=~true) bulk formulae) and a coupled or mixed forced/coupled formulation 33 (exchanges with a atmospheric model via the OASIS coupler) (\np{ln\_cpl} or \np{ln\_mixcpl}~=~true). 34 When used ($i.e.$ \np{ln\_apr\_dyn}~=~true), the atmospheric pressure forces both ocean and ice dynamics. 35 36 The frequency at which the forcing fields have to be updated is given by the \np{nn\_fsbc} namelist parameter. 37 37 When the fields are supplied from data files (flux and bulk formulations), the input fields 38 need not be supplied on the model grid. 38 need not be supplied on the model grid. Instead a file of coordinates and weights can 39 39 be supplied which maps the data from the supplied grid to the model points 40 40 (so called "Interpolation on the Fly", see \S\ref{SBC_iof}). … … 42 42 can be masked to avoid spurious results in proximity of the coasts as large sea-land gradients characterize 43 43 most of the atmospheric variables. 44 44 45 In addition, the resulting fields can be further modified using several namelist options. 45 These options control the rotation of vector components supplied relative to an east-north 46 coordinate system onto the local grid directions in the model; the addition of a surface 47 restoring term to observed SST and/or SSS (\np{ln\_ssr}~=~true); the modification of fluxes 48 below ice-covered areas (using observed ice-cover or a sea-ice model) 49 (\np{nn\_ice}~=~0,1, 2 or 3); the addition of river runoffs as surface freshwater 50 fluxes or lateral inflow (\np{ln\_rnf}~=~true); the addition of isf melting as lateral inflow (parameterisation) 51 or as surface flux at the land-ice ocean interface (\np{ln\_isf}=~true); 52 the addition of a freshwater flux adjustment in order to avoid a mean sea-level drift (\np{nn\_fwb}~=~0,~1~or~2); the 53 transformation of the solar radiation (if provided as daily mean) into a diurnal 54 cycle (\np{ln\_dm2dc}~=~true); and a neutral drag coefficient can be read from an external wave 55 model (\np{ln\_cdgw}~=~true). The latter option is possible only in case core or mfs bulk formulas are selected. 46 These options control 47 \begin{itemize} 48 \item the rotation of vector components supplied relative to an east-north 49 coordinate system onto the local grid directions in the model ; 50 \item the addition of a surface restoring term to observed SST and/or SSS (\np{ln\_ssr}~=~true) ; 51 \item the modification of fluxes below ice-covered areas (using observed ice-cover or a sea-ice model) (\np{nn\_ice}~=~0,1, 2 or 3) ; 52 \item the addition of river runoffs as surface freshwater fluxes or lateral inflow (\np{ln\_rnf}~=~true) ; 53 \item the addition of isf melting as lateral inflow (parameterisation) (\np{nn\_isf}~=~2 or 3 and \np{ln\_isfcav}~=~false) 54 or as fluxes applied at the land-ice ocean interface (\np{nn\_isf}~=~1 or 4 and \np{ln\_isfcav}~=~true) ; 55 \item the addition of a freshwater flux adjustment in order to avoid a mean sea-level drift (\np{nn\_fwb}~=~0,~1~or~2) ; 56 \item the transformation of the solar radiation (if provided as daily mean) into a diurnal cycle (\np{ln\_dm2dc}~=~true) ; 57 and a neutral drag coefficient can be read from an external wave model (\np{ln\_cdgw}~=~true). 58 \end{itemize} 59 The latter option is possible only in case core or mfs bulk formulas are selected. 56 60 57 61 In this chapter, we first discuss where the surface boundary condition appears in the … … 72 76 73 77 The surface ocean stress is the stress exerted by the wind and the sea-ice 74 on the ocean. The two components of stress are assumed to be interpolated 75 onto the ocean mesh, $i.e.$ resolved onto the model (\textbf{i},\textbf{j}) direction 76 at $u$- and $v$-points They are applied as a surface boundary condition of the 77 computation of the momentum vertical mixing trend (\mdl{dynzdf} module) : 78 \begin{equation} \label{Eq_sbc_dynzdf} 79 \left.{\left( {\frac{A^{vm} }{e_3 }\ \frac{\partial \textbf{U}_h}{\partial k}} \right)} \right|_{z=1} 80 = \frac{1}{\rho _o} \binom{\tau _u}{\tau _v } 81 \end{equation} 82 where $(\tau _u ,\;\tau _v )=(utau,vtau)$ are the two components of the wind 83 stress vector in the $(\textbf{i},\textbf{j})$ coordinate system. 78 on the ocean. It is applied in \mdl{dynzdf} module as a surface boundary condition of the 79 computation of the momentum vertical mixing trend (see \eqref{Eq_dynzdf_sbc} in \S\ref{DYN_zdf}). 80 As such, it has to be provided as a 2D vector interpolated 81 onto the horizontal velocity ocean mesh, $i.e.$ resolved onto the model 82 (\textbf{i},\textbf{j}) direction at $u$- and $v$-points. 84 83 85 84 The surface heat flux is decomposed into two parts, a non solar and a solar heat 86 85 flux, $Q_{ns}$ and $Q_{sr}$, respectively. The former is the non penetrative part 87 of the heat flux ($i.e.$ the sum of sensible, latent and long wave heat fluxes). 88 It is applied as a surface boundary condition trend of the first level temperature 89 time evolution equation (\mdl{trasbc} module). 90 \begin{equation} \label{Eq_sbc_trasbc_q} 91 \frac{\partial T}{\partial t}\equiv \cdots \;+\;\left. {\frac{Q_{ns} }{\rho 92 _o \;C_p \;e_{3t} }} \right|_{k=1} \quad 93 \end{equation} 94 $Q_{sr}$ is the penetrative part of the heat flux. It is applied as a 3D 95 trends of the temperature equation (\mdl{traqsr} module) when \np{ln\_traqsr}=True. 96 97 \begin{equation} \label{Eq_sbc_traqsr} 98 \frac{\partial T}{\partial t}\equiv \cdots \;+\frac{Q_{sr} }{\rho_o C_p \,e_{3t} }\delta _k \left[ {I_w } \right] 99 \end{equation} 100 where $I_w$ is a non-dimensional function that describes the way the light 101 penetrates inside the water column. It is generally a sum of decreasing 102 exponentials (see \S\ref{TRA_qsr}). 103 104 The surface freshwater budget is provided by fields: \textit{emp} and $\textit{emp}_S$ which 105 may or may not be identical. Indeed, a surface freshwater flux has two effects: 106 it changes the volume of the ocean and it changes the surface concentration of 107 salt (and other tracers). Therefore it appears in the sea surface height as a volume 108 flux, \textit{emp} (\textit{dynspg\_xxx} modules), and in the salinity time evolution equations 109 as a concentration/dilution effect, 110 $\textit{emp}_{S}$ (\mdl{trasbc} module). 111 \begin{equation} \label{Eq_trasbc_emp} 112 \begin{aligned} 113 &\frac{\partial \eta }{\partial t}\equiv \cdots \;+\;\textit{emp}\quad \\ 114 \\ 115 &\frac{\partial S}{\partial t}\equiv \cdots \;+\left. {\frac{\textit{emp}_S \;S}{e_{3t} }} \right|_{k=1} \\ 116 \end{aligned} 117 \end{equation} 118 119 In the real ocean, $\textit{emp}=\textit{emp}_S$ and the ocean salt content is conserved, 120 but it exist several numerical reasons why this equality should be broken. 121 For example, when the ocean is coupled to a sea-ice model, the water exchanged between 122 ice and ocean is slightly salty (mean sea-ice salinity is $\sim $\textit{4 psu}). In this case, 123 $\textit{emp}_{S}$ take into account both concentration/dilution effect associated with 124 freezing/melting and the salt flux between ice and ocean, while \textit{emp} is 125 only the volume flux. In addition, in the current version of \NEMO, the sea-ice is 126 assumed to be above the ocean (the so-called levitating sea-ice). Freezing/melting does 127 not change the ocean volume (no impact on \textit{emp}) but it modifies the SSS. 128 %gm \colorbox{yellow}{(see {\S} on LIM sea-ice model)}. 129 130 Note that SST can also be modified by a freshwater flux. Precipitation (in 131 particular solid precipitation) may have a temperature significantly different from 132 the SST. Due to the lack of information about the temperature of 133 precipitation, we assume it is equal to the SST. Therefore, no 134 concentration/dilution term appears in the temperature equation. It has to 135 be emphasised that this absence does not mean that there is no heat flux 136 associated with precipitation! Precipitation can change the ocean volume and thus the 137 ocean heat content. It is therefore associated with a heat flux (not yet 138 diagnosed in the model) \citep{Roullet_Madec_JGR00}). 86 of the heat flux ($i.e.$ the sum of sensible, latent and long wave heat fluxes 87 plus the heat content of the mass exchange with the atmosphere and sea-ice). 88 It is applied in \mdl{trasbc} module as a surface boundary condition trend of 89 the first level temperature time evolution equation (see \eqref{Eq_tra_sbc} 90 and \eqref{Eq_tra_sbc_lin} in \S\ref{TRA_sbc}). 91 The latter is the penetrative part of the heat flux. It is applied as a 3D 92 trends of the temperature equation (\mdl{traqsr} module) when \np{ln\_traqsr}=\textit{true}. 93 The way the light penetrates inside the water column is generally a sum of decreasing 94 exponentials (see \S\ref{TRA_qsr}). 95 96 The surface freshwater budget is provided by the \textit{emp} field. 97 It represents the mass flux exchanged with the atmosphere (evaporation minus precipitation) 98 and possibly with the sea-ice and ice shelves (freezing minus melting of ice). 99 It affects both the ocean in two different ways: 100 $(i)$ it changes the volume of the ocean and therefore appears in the sea surface height 101 equation as a volume flux, and 102 $(ii)$ it changes the surface temperature and salinity through the heat and salt contents 103 of the mass exchanged with the atmosphere, the sea-ice and the ice shelves. 104 139 105 140 106 %\colorbox{yellow}{Miss: } … … 156 122 % 157 123 %Explain here all the namlist namsbc variable{\ldots}. 124 % 125 % explain : use or not of surface currents 158 126 % 159 127 %\colorbox{yellow}{End Miss } 160 128 161 The ocean model provides the surface currents, temperature and salinity 162 averaged over \np{nf\_sbc} time-step (\ref{Tab_ssm}).The computation of the 163 mean is done in \mdl{sbcmod} module. 129 The ocean model provides, at each time step, to the surface module (\mdl{sbcmod}) 130 the surface currents, temperature and salinity. 131 These variables are averaged over \np{nf\_sbc} time-step (\ref{Tab_ssm}), 132 and it is these averaged fields which are used to computes the surface fluxes 133 at a frequency of \np{nf\_sbc} time-step. 134 164 135 165 136 %-------------------------------------------------TABLE--------------------------------------------------- … … 458 429 %-------------------------------------------------------------------------------------------------------------- 459 430 460 In some circumstances it may be useful to avoid calculating the 3D temperature, salinity and velocity fields and simply read them in from a previous run.461 Options are defined through the \ngn{namsbc\_sas} namelist variables. 431 In some circumstances it may be useful to avoid calculating the 3D temperature, salinity and velocity fields 432 and simply read them in from a previous run or receive them from OASIS. 462 433 For example: 463 434 464 \begin{ enumerate}465 \item Multiple runs of the model are required in code development to see the affect of different algorithms in435 \begin{itemize} 436 \item Multiple runs of the model are required in code development to see the effect of different algorithms in 466 437 the bulk formulae. 467 438 \item The effect of different parameter sets in the ice model is to be examined. 468 \end{enumerate} 439 \item Development of sea-ice algorithms or parameterizations. 440 \item spinup of the iceberg floats 441 \item ocean/sea-ice simulation with both media running in parallel (\np{ln\_mixcpl}~=~\textit{true}) 442 \end{itemize} 469 443 470 444 The StandAlone Surface scheme provides this utility. 445 Its options are defined through the \ngn{namsbc\_sas} namelist variables. 471 446 A new copy of the model has to be compiled with a configuration based on ORCA2\_SAS\_LIM. 472 447 However no namelist parameters need be changed from the settings of the previous run (except perhaps nn{\_}date0) … … 474 449 Routines replaced are: 475 450 476 \begin{enumerate} 477 \item \mdl{nemogcm} 478 479 This routine initialises the rest of the model and repeatedly calls the stp time stepping routine (step.F90) 451 \begin{itemize} 452 \item \mdl{nemogcm} : This routine initialises the rest of the model and repeatedly calls the stp time stepping routine (step.F90) 480 453 Since the ocean state is not calculated all associated initialisations have been removed. 481 \item \mdl{step} 482 483 The main time stepping routine now only needs to call the sbc routine (and a few utility functions). 484 \item \mdl{sbcmod} 485 486 This has been cut down and now only calculates surface forcing and the ice model required. New surface modules 454 \item \mdl{step} : The main time stepping routine now only needs to call the sbc routine (and a few utility functions). 455 \item \mdl{sbcmod} : This has been cut down and now only calculates surface forcing and the ice model required. New surface modules 487 456 that can function when only the surface level of the ocean state is defined can also be added (e.g. icebergs). 488 \item \mdl{daymod} 489 490 No ocean restarts are read or written (though the ice model restarts are retained), so calls to restart functions 457 \item \mdl{daymod} : No ocean restarts are read or written (though the ice model restarts are retained), so calls to restart functions 491 458 have been removed. This also means that the calendar cannot be controlled by time in a restart file, so the user 492 459 must make sure that nn{\_}date0 in the model namelist is correct for his or her purposes. 493 \item \mdl{stpctl} 494 495 Since there is no free surface solver, references to it have been removed from \rou{stp\_ctl} module. 496 \item \mdl{diawri} 497 498 All 3D data have been removed from the output. The surface temperature, salinity and velocity components (which 460 \item \mdl{stpctl} : Since there is no free surface solver, references to it have been removed from \rou{stp\_ctl} module. 461 \item \mdl{diawri} : All 3D data have been removed from the output. The surface temperature, salinity and velocity components (which 499 462 have been read in) are written along with relevant forcing and ice data. 500 \end{ enumerate}463 \end{itemize} 501 464 502 465 One new routine has been added: 503 466 504 \begin{enumerate} 505 \item \mdl{sbcsas} 506 This module initialises the input files needed for reading temperature, salinity and velocity arrays at the surface. 467 \begin{itemize} 468 \item \mdl{sbcsas} : This module initialises the input files needed for reading temperature, salinity and velocity arrays at the surface. 507 469 These filenames are supplied in namelist namsbc{\_}sas. Unfortunately because of limitations with the \mdl{iom} module, 508 470 the full 3D fields from the mean files have to be read in and interpolated in time, before using just the top level. 509 471 Since fldread is used to read in the data, Interpolation on the Fly may be used to change input data resolution. 510 \end{enumerate} 472 \end{itemize} 473 474 475 % Missing the description of the 2 following variables: 476 % ln_3d_uve = .true. ! specify whether we are supplying a 3D u,v and e3 field 477 % ln_read_frq = .false. ! specify whether we must read frq or not 478 479 511 480 512 481 % ================================================================ … … 719 688 are sent to the atmospheric component. 720 689 721 A generalised coupled interface has been developed. It is currently interfaced with OASIS 3 722 (\key{oasis3}) and does not support OASIS 4 723 \footnote{The \key{oasis4} exist. It activates portion of the code that are still under development.}. 690 A generalised coupled interface has been developed. 691 It is currently interfaced with OASIS-3-MCT (\key{oasis3}). 724 692 It has been successfully used to interface \NEMO to most of the European atmospheric 725 693 GCM (ARPEGE, ECHAM, ECMWF, HadAM, HadGAM, LMDz), … … 786 754 \label{SBC_tide} 787 755 788 A module is available to use the tidal potential forcing and is activated with with \key{tide}. 789 790 791 %------------------------------------------nam_tide---------------------------------------------------- 756 %------------------------------------------nam_tide--------------------------------------- 792 757 \namdisplay{nam_tide} 793 %------------------------------------------------------------------------------------------------------------- 794 795 Concerning the tidal potential, some parameters are available in namelist \ngn{nam\_tide}: 758 %----------------------------------------------------------------------------------------- 759 760 A module is available to compute the tidal potential and use it in the momentum equation. 761 This option is activated when \key{tide} is defined. 762 763 Some parameters are available in namelist \ngn{nam\_tide}: 796 764 797 765 - \np{ln\_tide\_pot} activate the tidal potential forcing … … 800 768 801 769 - \np{clname} is the name of constituent 802 803 770 804 771 The tide is generated by the forces of gravity ot the Earth-Moon and Earth-Sun sytem; … … 960 927 \begin{description} 961 928 \item[\np{nn\_isf}~=~1] 962 The ice shelf cavity is represented. The fwf and heat flux are computed. 2 bulk formulations are available: the ISOMIP one (\np{nn\_isfblk = 1}) described in (\np{nn\_isfblk = 2}), the 3 equation formulation described in \citet{Jenkins1991}. In addition to this, 963 3 different way to compute the exchange coefficient are available. $\gamma\_{T/S}$ is constant (\np{nn\_gammablk = 0}), $\gamma\_{T/S}$ is velocity dependant \citep{Jenkins2010} (\np{nn\_gammablk = 1}) and $\gamma\_{T/S}$ is velocity dependant and stratification dependent \citep{Holland1999} (\np{nn\_gammablk = 2}). For each of them, the thermal/salt exchange coefficient (\np{rn\_gammat0} and \np{rn\_gammas0}) have to be specified (the default values are for the ISOMIP case). 929 The ice shelf cavity is represented. The fwf and heat flux are computed. 2 bulk formulations are available: 930 the ISOMIP one (\np{nn\_isfblk = 1}) described in (\np{nn\_isfblk = 2}), 931 the 3 equation formulation described in \citet{Jenkins1991}. 932 In addition to this, 3 different ways to compute the exchange coefficient are available. 933 $\gamma\_{T/S}$ is constant (\np{nn\_gammablk = 0}), $\gamma\_{T/S}$ is velocity dependant 934 \citep{Jenkins2010} (\np{nn\_gammablk = 1}) and $\gamma\_{T/S}$ is velocity dependant 935 and stratification dependent \citep{Holland1999} (\np{nn\_gammablk = 2}). 936 For each of them, the thermal/salt exchange coefficient (\np{rn\_gammat0} and \np{rn\_gammas0}) 937 have to be specified (the default values are for the ISOMIP case). 964 938 Full description, sensitivity and validation in preparation. 965 939 … … 969 943 (\np{sn\_depmax\_isf}) and the base of the ice shelf along the calving front (\np{sn\_depmin\_isf}) as in (\np{nn\_isf}~=~3). 970 944 Furthermore the fwf is computed using the \citet{Beckmann2003} parameterisation of isf melting. 971 The effective melting length (\np{sn\_Leff\_isf}) is read from a file and the exchange coefficients are set as (\np{rn\_gammat0}) and (\np{rn\_gammas0}). 945 The effective melting length (\np{sn\_Leff\_isf}) is read from a file and the exchange coefficients 946 are set as (\np{rn\_gammat0}) and (\np{rn\_gammas0}). 972 947 973 948 \item[\np{nn\_isf}~=~3] … … 1032 1007 % Handling of icebergs 1033 1008 % ================================================================ 1034 \section{ Handling of icebergs (ICB)}1009 \section{Handling of icebergs (ICB)} 1035 1010 \label{ICB_icebergs} 1036 1011 %------------------------------------------namberg---------------------------------------------------- … … 1038 1013 %------------------------------------------------------------------------------------------------------------- 1039 1014 1040 Icebergs are modelled as lagrangian particles in NEMO. 1041 Their physical behaviour is controlled by equations as described in \citet{Martin_Adcroft_OM10} ). 1042 (Note that the authors kindly provided a copy of their code to act as a basis for implementation in NEMO.) 1043 Icebergs are initially spawned into one of ten classes which have specific mass and thickness as described in the \ngn{namberg} namelist: 1015 Icebergs are modelled as lagrangian particles in NEMO \citep{Marsh_GMD2015}. 1016 Their physical behaviour is controlled by equations as described in \citet{Martin_Adcroft_OM10} ). 1017 (Note that the authors kindly provided a copy of their code to act as a basis for implementation in NEMO). 1018 Icebergs are initially spawned into one of ten classes which have specific mass and thickness as described 1019 in the \ngn{namberg} namelist: 1044 1020 \np{rn\_initial\_mass} and \np{rn\_initial\_thickness}. 1045 1021 Each class has an associated scaling (\np{rn\_mass\_scaling}), which is an integer representing how many icebergs … … 1225 1201 The presence at the sea surface of an ice covered area modifies all the fluxes 1226 1202 transmitted to the ocean. There are several way to handle sea-ice in the system 1227 depending on the value of the \np{nn {\_}ice} namelist parameter.1203 depending on the value of the \np{nn\_ice} namelist parameter found in \ngn{namsbc} namelist. 1228 1204 \begin{description} 1229 1205 \item[nn{\_}ice = 0] there will never be sea-ice in the computational domain. … … 1300 1276 % ------------------------------------------------------------------------------------------------------------- 1301 1277 \subsection [Neutral drag coefficient from external wave model (\textit{sbcwave})] 1302 1278 {Neutral drag coefficient from external wave model (\mdl{sbcwave})} 1303 1279 \label{SBC_wave} 1304 1280 %------------------------------------------namwave---------------------------------------------------- 1305 1281 \namdisplay{namsbc_wave} 1306 1282 %------------------------------------------------------------------------------------------------------------- 1307 \begin{description} 1308 1309 \item [??] In order to read a neutral drag coeff, from an external data source (i.e. a wave model), the 1310 logical variable \np{ln\_cdgw} 1311 in $namsbc$ namelist must be defined ${.true.}$. 1283 1284 In order to read a neutral drag coeff, from an external data source ($i.e.$ a wave model), the 1285 logical variable \np{ln\_cdgw} in \ngn{namsbc} namelist must be set to \textit{true}. 1312 1286 The \mdl{sbcwave} module containing the routine \np{sbc\_wave} reads the 1313 1287 namelist \ngn{namsbc\_wave} (for external data names, locations, frequency, interpolation and all 1314 1288 the miscellanous options allowed by Input Data generic Interface see \S\ref{SBC_input}) 1315 and a 2D field of neutral drag coefficient. Then using the routine 1316 TURB\_CORE\_1Z or TURB\_CORE\_2Z, and starting from the neutral drag coefficent provided, the drag coefficient is computed according 1317 to stable/unstable conditions of the air-sea interface following \citet{Large_Yeager_Rep04}. 1318 1319 \end{description} 1289 and a 2D field of neutral drag coefficient. 1290 Then using the routine TURB\_CORE\_1Z or TURB\_CORE\_2Z, and starting from the neutral drag coefficent provided, 1291 the drag coefficient is computed according to stable/unstable conditions of the air-sea interface following \citet{Large_Yeager_Rep04}. 1292 1320 1293 1321 1294 % Griffies doc: 1322 % When running ocean-ice simulations, we are not explicitly representing land processes, such as rivers, catchment areas, snow accumulation, etc. However, to reduce model drift, it is important to balance the hydrological cycle in ocean-ice models. We thus need to prescribe some form of global normalization to the precipitation minus evaporation plus river runoff. The result of the normalization should be a global integrated zero net water input to the ocean-ice system over a chosen time scale. 1323 %How often the normalization is done is a matter of choice. In mom4p1, we choose to do so at each model time step, so that there is always a zero net input of water to the ocean-ice system. Others choose to normalize over an annual cycle, in which case the net imbalance over an annual cycle is used to alter the subsequent year�s water budget in an attempt to damp the annual water imbalance. Note that the annual budget approach may be inappropriate with interannually varying precipitation forcing. 1324 %When running ocean-ice coupled models, it is incorrect to include the water transport between the ocean and ice models when aiming to balance the hydrological cycle. 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, not the water in any one sub-component. As an extreme example to illustrate the issue, consider an ocean-ice model with zero initial sea ice. As the ocean-ice model spins up, there should be a net accumulation of water in the growing sea ice, and thus a net loss of water from the ocean. The total water contained in the ocean plus ice system is constant, but there is an exchange of water between the subcomponents. This exchange should not be part of the normalization used to balance the hydrological cycle in ocean-ice models. 1325 1326 1295 % When running ocean-ice simulations, we are not explicitly representing land processes, 1296 % such as rivers, catchment areas, snow accumulation, etc. However, to reduce model drift, 1297 % it is important to balance the hydrological cycle in ocean-ice models. 1298 % We thus need to prescribe some form of global normalization to the precipitation minus evaporation plus river runoff. 1299 % The result of the normalization should be a global integrated zero net water input to the ocean-ice system over 1300 % a chosen time scale. 1301 %How often the normalization is done is a matter of choice. In mom4p1, we choose to do so at each model time step, 1302 % so that there is always a zero net input of water to the ocean-ice system. 1303 % Others choose to normalize over an annual cycle, in which case the net imbalance over an annual cycle is used 1304 % to alter the subsequent year�s water budget in an attempt to damp the annual water imbalance. 1305 % Note that the annual budget approach may be inappropriate with interannually varying precipitation forcing. 1306 % When running ocean-ice coupled models, it is incorrect to include the water transport between the ocean 1307 % and ice models when aiming to balance the hydrological cycle. 1308 % 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, 1309 % not the water in any one sub-component. As an extreme example to illustrate the issue, 1310 % consider an ocean-ice model with zero initial sea ice. As the ocean-ice model spins up, 1311 % there should be a net accumulation of water in the growing sea ice, and thus a net loss of water from the ocean. 1312 % The total water contained in the ocean plus ice system is constant, but there is an exchange of water between 1313 % the subcomponents. This exchange should not be part of the normalization used to balance the hydrological cycle 1314 % in ocean-ice models. 1315 1316
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