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branches/2015/nemo_v3_6_STABLE/DOC/TexFiles/Chapters/Chap_SBC.tex
r5120 r6275 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 (\np{nn\_isf}~=~2 or 3 and \np{ln\_isfcav}~=~false) or as surface flux at the land-ice ocean interface 52 (\np{nn\_isf}~=~1 or 4 and \np{ln\_isfcav}~=~true); 53 the addition of a freshwater flux adjustment in order to avoid a mean sea-level drift (\np{nn\_fwb}~=~0,~1~or~2); the 54 transformation of the solar radiation (if provided as daily mean) into a diurnal 55 cycle (\np{ln\_dm2dc}~=~true); and a neutral drag coefficient can be read from an external wave 56 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. 57 60 58 61 In this chapter, we first discuss where the surface boundary condition appears in the … … 73 76 74 77 The surface ocean stress is the stress exerted by the wind and the sea-ice 75 on the ocean. The two components of stress are assumed to be interpolated 76 onto the ocean mesh, $i.e.$ resolved onto the model (\textbf{i},\textbf{j}) direction 77 at $u$- and $v$-points They are applied as a surface boundary condition of the 78 computation of the momentum vertical mixing trend (\mdl{dynzdf} module) : 79 \begin{equation} \label{Eq_sbc_dynzdf} 80 \left.{\left( {\frac{A^{vm} }{e_3 }\ \frac{\partial \textbf{U}_h}{\partial k}} \right)} \right|_{z=1} 81 = \frac{1}{\rho _o} \binom{\tau _u}{\tau _v } 82 \end{equation} 83 where $(\tau _u ,\;\tau _v )=(utau,vtau)$ are the two components of the wind 84 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. 85 83 86 84 The surface heat flux is decomposed into two parts, a non solar and a solar heat 87 85 flux, $Q_{ns}$ and $Q_{sr}$, respectively. The former is the non penetrative part 88 of the heat flux ($i.e.$ the sum of sensible, latent and long wave heat fluxes). 89 It is applied as a surface boundary condition trend of the first level temperature 90 time evolution equation (\mdl{trasbc} module). 91 \begin{equation} \label{Eq_sbc_trasbc_q} 92 \frac{\partial T}{\partial t}\equiv \cdots \;+\;\left. {\frac{Q_{ns} }{\rho 93 _o \;C_p \;e_{3t} }} \right|_{k=1} \quad 94 \end{equation} 95 $Q_{sr}$ is the penetrative part of the heat flux. It is applied as a 3D 96 trends of the temperature equation (\mdl{traqsr} module) when \np{ln\_traqsr}=True. 97 98 \begin{equation} \label{Eq_sbc_traqsr} 99 \frac{\partial T}{\partial t}\equiv \cdots \;+\frac{Q_{sr} }{\rho_o C_p \,e_{3t} }\delta _k \left[ {I_w } \right] 100 \end{equation} 101 where $I_w$ is a non-dimensional function that describes the way the light 102 penetrates inside the water column. It is generally a sum of decreasing 103 exponentials (see \S\ref{TRA_qsr}). 104 105 The surface freshwater budget is provided by fields: \textit{emp} and $\textit{emp}_S$ which 106 may or may not be identical. Indeed, a surface freshwater flux has two effects: 107 it changes the volume of the ocean and it changes the surface concentration of 108 salt (and other tracers). Therefore it appears in the sea surface height as a volume 109 flux, \textit{emp} (\textit{dynspg\_xxx} modules), and in the salinity time evolution equations 110 as a concentration/dilution effect, 111 $\textit{emp}_{S}$ (\mdl{trasbc} module). 112 \begin{equation} \label{Eq_trasbc_emp} 113 \begin{aligned} 114 &\frac{\partial \eta }{\partial t}\equiv \cdots \;+\;\textit{emp}\quad \\ 115 \\ 116 &\frac{\partial S}{\partial t}\equiv \cdots \;+\left. {\frac{\textit{emp}_S \;S}{e_{3t} }} \right|_{k=1} \\ 117 \end{aligned} 118 \end{equation} 119 120 In the real ocean, $\textit{emp}=\textit{emp}_S$ and the ocean salt content is conserved, 121 but it exist several numerical reasons why this equality should be broken. 122 For example, when the ocean is coupled to a sea-ice model, the water exchanged between 123 ice and ocean is slightly salty (mean sea-ice salinity is $\sim $\textit{4 psu}). In this case, 124 $\textit{emp}_{S}$ take into account both concentration/dilution effect associated with 125 freezing/melting and the salt flux between ice and ocean, while \textit{emp} is 126 only the volume flux. In addition, in the current version of \NEMO, the sea-ice is 127 assumed to be above the ocean (the so-called levitating sea-ice). Freezing/melting does 128 not change the ocean volume (no impact on \textit{emp}) but it modifies the SSS. 129 %gm \colorbox{yellow}{(see {\S} on LIM sea-ice model)}. 130 131 Note that SST can also be modified by a freshwater flux. Precipitation (in 132 particular solid precipitation) may have a temperature significantly different from 133 the SST. Due to the lack of information about the temperature of 134 precipitation, we assume it is equal to the SST. Therefore, no 135 concentration/dilution term appears in the temperature equation. It has to 136 be emphasised that this absence does not mean that there is no heat flux 137 associated with precipitation! Precipitation can change the ocean volume and thus the 138 ocean heat content. It is therefore associated with a heat flux (not yet 139 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. 104 140 105 141 106 %\colorbox{yellow}{Miss: } … … 157 122 % 158 123 %Explain here all the namlist namsbc variable{\ldots}. 124 % 125 % explain : use or not of surface currents 159 126 % 160 127 %\colorbox{yellow}{End Miss } 161 128 162 The ocean model provides the surface currents, temperature and salinity 163 averaged over \np{nf\_sbc} time-step (\ref{Tab_ssm}).The computation of the 164 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. 134 165 135 166 136 %-------------------------------------------------TABLE--------------------------------------------------- … … 459 429 %-------------------------------------------------------------------------------------------------------------- 460 430 461 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.462 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. 463 433 For example: 464 434 465 \begin{ enumerate}466 \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 467 437 the bulk formulae. 468 438 \item The effect of different parameter sets in the ice model is to be examined. 469 \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} 470 443 471 444 The StandAlone Surface scheme provides this utility. 445 Its options are defined through the \ngn{namsbc\_sas} namelist variables. 472 446 A new copy of the model has to be compiled with a configuration based on ORCA2\_SAS\_LIM. 473 447 However no namelist parameters need be changed from the settings of the previous run (except perhaps nn{\_}date0) … … 475 449 Routines replaced are: 476 450 477 \begin{enumerate} 478 \item \mdl{nemogcm} 479 480 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) 481 453 Since the ocean state is not calculated all associated initialisations have been removed. 482 \item \mdl{step} 483 484 The main time stepping routine now only needs to call the sbc routine (and a few utility functions). 485 \item \mdl{sbcmod} 486 487 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 488 456 that can function when only the surface level of the ocean state is defined can also be added (e.g. icebergs). 489 \item \mdl{daymod} 490 491 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 492 458 have been removed. This also means that the calendar cannot be controlled by time in a restart file, so the user 493 459 must make sure that nn{\_}date0 in the model namelist is correct for his or her purposes. 494 \item \mdl{stpctl} 495 496 Since there is no free surface solver, references to it have been removed from \rou{stp\_ctl} module. 497 \item \mdl{diawri} 498 499 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 500 462 have been read in) are written along with relevant forcing and ice data. 501 \end{ enumerate}463 \end{itemize} 502 464 503 465 One new routine has been added: 504 466 505 \begin{enumerate} 506 \item \mdl{sbcsas} 507 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. 508 469 These filenames are supplied in namelist namsbc{\_}sas. Unfortunately because of limitations with the \mdl{iom} module, 509 470 the full 3D fields from the mean files have to be read in and interpolated in time, before using just the top level. 510 471 Since fldread is used to read in the data, Interpolation on the Fly may be used to change input data resolution. 511 \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 478 479 512 480 513 481 % ================================================================ … … 720 688 are sent to the atmospheric component. 721 689 722 A generalised coupled interface has been developed. It is currently interfaced with OASIS 3 723 (\key{oasis3}) and does not support OASIS 4 724 \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}). 725 692 It has been successfully used to interface \NEMO to most of the European atmospheric 726 693 GCM (ARPEGE, ECHAM, ECMWF, HadAM, HadGAM, LMDz), … … 787 754 \label{SBC_tide} 788 755 789 A module is available to use the tidal potential forcing and is activated with with \key{tide}. 790 791 792 %------------------------------------------nam_tide---------------------------------------------------- 756 %------------------------------------------nam_tide--------------------------------------- 793 757 \namdisplay{nam_tide} 794 %------------------------------------------------------------------------------------------------------------- 795 796 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}: 797 764 798 765 - \np{ln\_tide\_pot} activate the tidal potential forcing … … 801 768 802 769 - \np{clname} is the name of constituent 803 804 770 805 771 The tide is generated by the forces of gravity ot the Earth-Moon and Earth-Sun sytem; … … 958 924 \namdisplay{namsbc_isf} 959 925 %-------------------------------------------------------------------------------------------------------- 960 Namelist variable in \ngn{namsbc}, \np{nn\_isf}, 926 Namelist variable in \ngn{namsbc}, \np{nn\_isf}, control the kind of ice shelf representation used. 961 927 \begin{description} 962 928 \item[\np{nn\_isf}~=~1] … … 987 953 \np{nn\_isf}~=~3 and \np{nn\_isf}~=~4 read the melt rate and heat flux from a file. You have total control of the fwf scenario. 988 954 989 955 This can be usefull if the water masses on the shelf are not realistic or the resolution (horizontal/vertical) are too 990 956 coarse to have realistic melting or for sensitivity studies where you want to control your input. 991 957 Full description, sensitivity and validation in preparation. … … 1000 966 % Handling of icebergs 1001 967 % ================================================================ 1002 \section{ Handling of icebergs (ICB)}968 \section{Handling of icebergs (ICB)} 1003 969 \label{ICB_icebergs} 1004 970 %------------------------------------------namberg---------------------------------------------------- … … 1006 972 %------------------------------------------------------------------------------------------------------------- 1007 973 1008 Icebergs are modelled as lagrangian particles in NEMO. 1009 Their physical behaviour is controlled by equations as described in \citet{Martin_Adcroft_OM10} ). 1010 (Note that the authors kindly provided a copy of their code to act as a basis for implementation in NEMO.) 1011 Icebergs are initially spawned into one of ten classes which have specific mass and thickness as described in the \ngn{namberg} namelist: 974 Icebergs are modelled as lagrangian particles in NEMO \citep{Marsh_GMD2015}. 975 Their physical behaviour is controlled by equations as described in \citet{Martin_Adcroft_OM10} ). 976 (Note that the authors kindly provided a copy of their code to act as a basis for implementation in NEMO). 977 Icebergs are initially spawned into one of ten classes which have specific mass and thickness as described 978 in the \ngn{namberg} namelist: 1012 979 \np{rn\_initial\_mass} and \np{rn\_initial\_thickness}. 1013 980 Each class has an associated scaling (\np{rn\_mass\_scaling}), which is an integer representing how many icebergs … … 1193 1160 The presence at the sea surface of an ice covered area modifies all the fluxes 1194 1161 transmitted to the ocean. There are several way to handle sea-ice in the system 1195 depending on the value of the \np{nn {\_}ice} namelist parameter.1162 depending on the value of the \np{nn\_ice} namelist parameter found in \ngn{namsbc} namelist. 1196 1163 \begin{description} 1197 1164 \item[nn{\_}ice = 0] there will never be sea-ice in the computational domain. … … 1268 1235 % ------------------------------------------------------------------------------------------------------------- 1269 1236 \subsection [Neutral drag coefficient from external wave model (\textit{sbcwave})] 1270 1237 {Neutral drag coefficient from external wave model (\mdl{sbcwave})} 1271 1238 \label{SBC_wave} 1272 1239 %------------------------------------------namwave---------------------------------------------------- 1273 1240 \namdisplay{namsbc_wave} 1274 1241 %------------------------------------------------------------------------------------------------------------- 1275 \begin{description} 1276 1277 \item [??] In order to read a neutral drag coeff, from an external data source (i.e. a wave model), the 1278 logical variable \np{ln\_cdgw} 1279 in $namsbc$ namelist must be defined ${.true.}$. 1242 1243 In order to read a neutral drag coeff, from an external data source ($i.e.$ a wave model), the 1244 logical variable \np{ln\_cdgw} in \ngn{namsbc} namelist must be set to \textit{true}. 1280 1245 The \mdl{sbcwave} module containing the routine \np{sbc\_wave} reads the 1281 1246 namelist \ngn{namsbc\_wave} (for external data names, locations, frequency, interpolation and all 1282 1247 the miscellanous options allowed by Input Data generic Interface see \S\ref{SBC_input}) 1283 and a 2D field of neutral drag coefficient. Then using the routine 1284 TURB\_CORE\_1Z or TURB\_CORE\_2Z, and starting from the neutral drag coefficent provided, the drag coefficient is computed according 1285 to stable/unstable conditions of the air-sea interface following \citet{Large_Yeager_Rep04}. 1286 1287 \end{description} 1248 and a 2D field of neutral drag coefficient. 1249 Then using the routine TURB\_CORE\_1Z or TURB\_CORE\_2Z, and starting from the neutral drag coefficent provided, 1250 the drag coefficient is computed according to stable/unstable conditions of the air-sea interface following \citet{Large_Yeager_Rep04}. 1251 1288 1252 1289 1253 % Griffies doc: 1290 % 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. 1291 %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. 1292 %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. 1293 1294 1254 % When running ocean-ice simulations, we are not explicitly representing land processes, 1255 % such as rivers, catchment areas, snow accumulation, etc. However, to reduce model drift, 1256 % it is important to balance the hydrological cycle in ocean-ice models. 1257 % We thus need to prescribe some form of global normalization to the precipitation minus evaporation plus river runoff. 1258 % The result of the normalization should be a global integrated zero net water input to the ocean-ice system over 1259 % a chosen time scale. 1260 %How often the normalization is done is a matter of choice. In mom4p1, we choose to do so at each model time step, 1261 % so that there is always a zero net input of water to the ocean-ice system. 1262 % Others choose to normalize over an annual cycle, in which case the net imbalance over an annual cycle is used 1263 % to alter the subsequent year�s water budget in an attempt to damp the annual water imbalance. 1264 % Note that the annual budget approach may be inappropriate with interannually varying precipitation forcing. 1265 % When running ocean-ice coupled models, it is incorrect to include the water transport between the ocean 1266 % and ice models when aiming to balance the hydrological cycle. 1267 % 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, 1268 % not the water in any one sub-component. As an extreme example to illustrate the issue, 1269 % consider an ocean-ice model with zero initial sea ice. As the ocean-ice model spins up, 1270 % there should be a net accumulation of water in the growing sea ice, and thus a net loss of water from the ocean. 1271 % The total water contained in the ocean plus ice system is constant, but there is an exchange of water between 1272 % the subcomponents. This exchange should not be part of the normalization used to balance the hydrological cycle 1273 % in ocean-ice models. 1274 1275
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