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1\documentclass[NEMO_book]{subfiles}
2\begin{document}
3% ================================================================
4% Chapter —— Surface Boundary Condition (SBC, ISF, ICB)
5% ================================================================
6\chapter{Surface Boundary Condition (SBC, ISF, ICB) }
7\label{SBC}
8\minitoc
9
10\newpage
11$\ $\newline    % force a new ligne
12%---------------------------------------namsbc--------------------------------------------------
13\fortranfile{namelists/namsbc}
14%--------------------------------------------------------------------------------------------------------------
15$\ $\newline    % force a new ligne
16
17The ocean needs six fields as surface boundary condition:
18\begin{itemize}
19   \item the two components of the surface ocean stress $\left( {\tau _u \;,\;\tau _v} \right)$
20   \item the incoming solar and non solar heat fluxes $\left( {Q_{ns} \;,\;Q_{sr} } \right)$
21   \item the surface freshwater budget $\left( {\textit{emp}} \right)$
22   \item the surface salt flux associated with freezing/melting of seawater $\left( {\textit{sfx}} \right)$
23\end{itemize}
24plus an optional field:
25\begin{itemize}
26   \item the atmospheric pressure at the ocean surface $\left( p_a \right)$
27\end{itemize}
28
29Five different ways to provide the first six fields to the ocean are available which
30are controlled by namelist \ngn{namsbc} variables: an analytical formulation (\np{ln\_ana}~=~true),
31a flux formulation (\np{ln\_flx}~=~true), a bulk formulae formulation (CORE
32(\np{ln\_blk\_core}~=~true), CLIO (\np{ln\_blk\_clio}~=~true) or MFS
33\footnote { Note that MFS bulk formulae compute fluxes only for the ocean component}
34(\np{ln\_blk\_mfs}~=~true) bulk formulae) and a coupled or mixed forced/coupled formulation
35(exchanges with a atmospheric model via the OASIS coupler) (\np{ln\_cpl} or \np{ln\_mixcpl}~=~true).
36When used ($i.e.$ \np{ln\_apr\_dyn}~=~true), the atmospheric pressure forces both ocean and ice dynamics.
37
38The frequency at which the forcing fields have to be updated is given by the \np{nn\_fsbc} namelist parameter.
39When the fields are supplied from data files (flux and bulk formulations), the input fields
40need not be supplied on the model grid. Instead a file of coordinates and weights can
41be supplied which maps the data from the supplied grid to the model points
42(so called "Interpolation on the Fly", see \S\ref{SBC_iof}).
43If the Interpolation on the Fly option is used, input data belonging to land points (in the native grid),
44can be masked to avoid spurious results in proximity of the coasts  as large sea-land gradients characterize
45most of the atmospheric variables.
46
47In addition, the resulting fields can be further modified using several namelist options.
48These options control
49\begin{itemize}
50\item the rotation of vector components supplied relative to an east-north
51coordinate system onto the local grid directions in the model ;
52\item the addition of a surface restoring term to observed SST and/or SSS (\np{ln\_ssr}~=~true) ;
53\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) ;
54\item the addition of river runoffs as surface freshwater fluxes or lateral inflow (\np{ln\_rnf}~=~true) ;
55\item the addition of isf melting as lateral inflow (parameterisation) or as fluxes applied at the land-ice ocean interface (\np{ln\_isf}) ;
56\item the addition of a freshwater flux adjustment in order to avoid a mean sea-level drift (\np{nn\_fwb}~=~0,~1~or~2) ;
57\item the transformation of the solar radiation (if provided as daily mean) into a diurnal cycle (\np{ln\_dm2dc}~=~true) ;
58and a neutral drag coefficient can be read from an external wave model (\np{ln\_cdgw}~=~true).
59\end{itemize}
60The latter option is possible only in case core or mfs bulk formulas are selected.
61
62In this chapter, we first discuss where the surface boundary condition appears in the
63model equations. Then we present the five ways of providing the surface boundary condition,
64followed by the description of the atmospheric pressure and the river runoff.
65Next the scheme for interpolation on the fly is described.
66Finally, the different options that further modify the fluxes applied to the ocean are discussed.
67One of these is modification by icebergs (see \S\ref{ICB_icebergs}), which act as drifting sources of fresh water.
68Another example of modification is that due to the ice shelf melting/freezing (see \S\ref{SBC_isf}),
69which provides additional sources of fresh water.
70
71
72% ================================================================
73% Surface boundary condition for the ocean
74% ================================================================
75\section{Surface boundary condition for the ocean}
76\label{SBC_general}
77
78The surface ocean stress is the stress exerted by the wind and the sea-ice
79on the ocean. It is applied in \mdl{dynzdf} module as a surface boundary condition of the
80computation of the momentum vertical mixing trend (see \eqref{Eq_dynzdf_sbc} in \S\ref{DYN_zdf}).
81As such, it has to be provided as a 2D vector interpolated
82onto the horizontal velocity ocean mesh, $i.e.$ resolved onto the model
83(\textbf{i},\textbf{j}) direction at $u$- and $v$-points.
84
85The surface heat flux is decomposed into two parts, a non solar and a solar heat
86flux, $Q_{ns}$ and $Q_{sr}$, respectively. The former is the non penetrative part
87of the heat flux ($i.e.$ the sum of sensible, latent and long wave heat fluxes
88plus the heat content of the mass exchange with the atmosphere and sea-ice).
89It is applied in \mdl{trasbc} module as a surface boundary condition trend of
90the first level temperature time evolution equation (see \eqref{Eq_tra_sbc} 
91and \eqref{Eq_tra_sbc_lin} in \S\ref{TRA_sbc}).
92The latter is the penetrative part of the heat flux. It is applied as a 3D
93trends of the temperature equation (\mdl{traqsr} module) when \np{ln\_traqsr}=\textit{true}.
94The way the light penetrates inside the water column is generally a sum of decreasing
95exponentials (see \S\ref{TRA_qsr}).
96
97The surface freshwater budget is provided by the \textit{emp} field.
98It represents the mass flux exchanged with the atmosphere (evaporation minus precipitation)
99and possibly with the sea-ice and ice shelves (freezing minus melting of ice).
100It affects both the ocean in two different ways:
101$(i)$   it changes the volume of the ocean and therefore appears in the sea surface height
102equation as a volume flux, and
103$(ii)$  it changes the surface temperature and salinity through the heat and salt contents
104of the mass exchanged with the atmosphere, the sea-ice and the ice shelves.
105
106
107%\colorbox{yellow}{Miss: }
108%
109%A extensive description of all namsbc namelist (parameter that have to be
110%created!)
111%
112%Especially the \np{nn\_fsbc}, the \mdl{sbc\_oce} module (fluxes + mean sst sss ssu
113%ssv) i.e. information required by flux computation or sea-ice
114%
115%\mdl{sbc\_oce} containt the definition in memory of the 7 fields (6+runoff), add
116%a word on runoff: included in surface bc or add as lateral obc{\ldots}.
117%
118%Sbcmod manage the ``providing'' (fourniture) to the ocean the 7 fields
119%
120%Fluxes update only each nf{\_}sbc time step (namsbc) explain relation
121%between nf{\_}sbc and nf{\_}ice, do we define nf{\_}blk??? ? only one
122%nf{\_}sbc
123%
124%Explain here all the namlist namsbc variable{\ldots}.
125%
126% explain : use or not of surface currents
127%
128%\colorbox{yellow}{End Miss }
129
130The ocean model provides, at each time step, to the surface module (\mdl{sbcmod})
131the surface currents, temperature and salinity. 
132These variables are averaged over \np{nn\_fsbc} time-step (\ref{Tab_ssm}),
133and it is these averaged fields which are used to computes the surface fluxes
134at a frequency of \np{nn\_fsbc} time-step.
135
136
137%-------------------------------------------------TABLE---------------------------------------------------
138\begin{table}[tb]   \begin{center}   \begin{tabular}{|l|l|l|l|}
139\hline
140Variable description             & Model variable  & Units  & point \\  \hline
141i-component of the surface current  & ssu\_m & $m.s^{-1}$   & U \\   \hline
142j-component of the surface current  & ssv\_m & $m.s^{-1}$   & V \\   \hline
143Sea surface temperature          & sst\_m & \r{}$K$      & T \\   \hline
144Sea surface salinty              & sss\_m & $psu$        & T \\   \hline
145\end{tabular}
146\caption{  \protect\label{Tab_ssm}   
147Ocean variables provided by the ocean to the surface module (SBC).
148The variable are averaged over nn{\_}fsbc time step,
149$i.e.$ the frequency of computation of surface fluxes.}
150\end{center}   \end{table}
151%--------------------------------------------------------------------------------------------------------------
152
153%\colorbox{yellow}{Penser a} mettre dans le restant l'info nn{\_}fsbc ET nn{\_}fsbc*rdt de sorte de reinitialiser la moyenne si on change la frequence ou le pdt
154
155
156% ================================================================
157%       Input Data
158% ================================================================
159\section{Input Data generic interface}
160\label{SBC_input}
161
162A generic interface has been introduced to manage the way input data (2D or 3D fields,
163like surface forcing or ocean T and S) are specify in \NEMO. This task is archieved by fldread.F90.
164The module was design with four main objectives in mind:
165\begin{enumerate} 
166\item optionally provide a time interpolation of the input data at model time-step,
167whatever their input frequency is, and according to the different calendars available in the model.
168\item optionally provide an on-the-fly space interpolation from the native input data grid to the model grid.
169\item make the run duration independent from the period cover by the input files.
170\item provide a simple user interface and a rather simple developer interface by limiting the
171 number of prerequisite information.
172\end{enumerate} 
173
174As a results the user have only to fill in for each variable a structure in the namelist file
175to defined the input data file and variable names, the frequency of the data (in hours or months),
176whether its is climatological data or not, the period covered by the input file (one year, month, week or day),
177and three additional parameters for on-the-fly interpolation. When adding a new input variable,
178the developer has to add the associated structure in the namelist, read this information
179by mirroring the namelist read in \rou{sbc\_blk\_init} for example, and simply call \rou{fld\_read} 
180to obtain the desired input field at the model time-step and grid points.
181
182The only constraints are that the input file is a NetCDF file, the file name follows a nomenclature
183(see \S\ref{SBC_fldread}), the period it cover is one year, month, week or day, and, if on-the-fly
184interpolation is used, a file of weights must be supplied (see \S\ref{SBC_iof}).
185
186Note that when an input data is archived on a disc which is accessible directly
187from the workspace where the code is executed, then the use can set the \np{cn\_dir} 
188to the pathway leading to the data. By default, the data are assumed to have been
189copied so that cn\_dir='./'.
190
191% -------------------------------------------------------------------------------------------------------------
192% Input Data specification (\mdl{fldread})
193% -------------------------------------------------------------------------------------------------------------
194\subsection{Input Data specification (\protect\mdl{fldread})}
195\label{SBC_fldread}
196
197The structure associated with an input variable contains the following information:
198\begin{fortrancode}
199!  file name  ! frequency (hours) ! variable  ! time interp. !  clim  ! 'yearly'/ ! weights  ! rotation ! land/sea mask !
200!             !  (if <0  months)  !   name    !   (logical)  !  (T/F) ! 'monthly' ! filename ! pairing  ! filename      !
201\end{fortrancode}
202where
203\begin{description} 
204\item[File name]: the stem name of the NetCDF file to be open.
205This stem will be completed automatically by the model, with the addition of a '.nc' at its end
206and by date information and possibly a prefix (when using AGRIF).
207Tab.\ref{Tab_fldread} provides the resulting file name in all possible cases according to whether
208it is a climatological file or not, and to the open/close frequency (see below for definition).
209
210%--------------------------------------------------TABLE--------------------------------------------------
211\begin{table}[htbp]
212\begin{center}
213\begin{tabular}{|l|c|c|c|}
214\hline
215                         & daily or weekLLL          & monthly                   &   yearly          \\   \hline
216clim = false   & fn\_yYYYYmMMdDD  &   fn\_yYYYYmMM   &   fn\_yYYYY  \\   \hline
217clim = true       & not possible                  &  fn\_m??.nc             &   fn                \\   \hline
218\end{tabular}
219\end{center}
220\caption{ \protect\label{Tab_fldread}   naming nomenclature for climatological or interannual input file,
221as a function of the Open/close frequency. The stem name is assumed to be 'fn'.
222For weekly files, the 'LLL' corresponds to the first three letters of the first day of the week ($i.e.$ 'sun','sat','fri','thu','wed','tue','mon'). The 'YYYY', 'MM' and 'DD' should be replaced by the
223actual year/month/day, always coded with 4 or 2 digits. Note that (1) in mpp, if the file is split
224over each subdomain, the suffix '.nc' is replaced by '\_PPPP.nc', where 'PPPP' is the
225process number coded with 4 digits; (2) when using AGRIF, the prefix
226'\_N' is added to files,
227where 'N'  is the child grid number.}
228\end{table}
229%--------------------------------------------------------------------------------------------------------------
230 
231
232\item[Record frequency]: the frequency of the records contained in the input file.
233Its unit is in hours if it is positive (for example 24 for daily forcing) or in months if negative
234(for example -1 for monthly forcing or -12 for annual forcing).
235Note that this frequency must really be an integer and not a real.
236On some computers, seting it to '24.' can be interpreted as 240!
237
238\item[Variable name]: the name of the variable to be read in the input NetCDF file.
239
240\item[Time interpolation]: a logical to activate, or not, the time interpolation. If set to 'false',
241the forcing will have a steplike shape remaining constant during each forcing period.
242For example, when using a daily forcing without time interpolation, the forcing remaining
243constant from 00h00'00'' to 23h59'59". If set to 'true', the forcing will have a broken line shape.
244Records are assumed to be dated the middle of the forcing period.
245For example, when using a daily forcing with time interpolation, linear interpolation will
246be performed between mid-day of two consecutive days.
247
248\item[Climatological forcing]: a logical to specify if a input file contains climatological forcing
249which can be cycle in time, or an interannual forcing which will requires additional files
250if the period covered by the simulation exceed the one of the file. See the above the file
251naming strategy which impacts the expected name of the file to be opened.
252
253\item[Open/close frequency]: the frequency at which forcing files must be opened/closed.
254Four cases are coded: 'daily', 'weekLLL' (with 'LLL' the first 3 letters of the first day of the week),
255'monthly' and 'yearly' which means the forcing files will contain data for one day, one week,
256one month or one year. Files are assumed to contain data from the beginning of the open/close period.
257For example, the first record of a yearly file containing daily data is Jan 1st even if the experiment
258is not starting at the beginning of the year.
259
260\item[Others]: 'weights filename', 'pairing rotation' and 'land/sea mask' are associted with on-the-fly interpolation
261which is described in \S\ref{SBC_iof}.
262
263\end{description}
264
265Additional remarks:\\
266(1) The time interpolation is a simple linear interpolation between two consecutive records of
267the input data. The only tricky point is therefore to specify the date at which we need to do
268the interpolation and the date of the records read in the input files.
269Following \citet{Leclair_Madec_OM09}, the date of a time step is set at the middle of the
270time step. For example, for an experiment starting at 0h00'00" with a one hour time-step,
271a time interpolation will be performed at the following time: 0h30'00", 1h30'00", 2h30'00", etc.
272However, for forcing data related to the surface module, values are not needed at every
273time-step but at every \np{nn\_fsbc} time-step. For example with \np{nn\_fsbc}~=~3,
274the surface module will be called at time-steps 1, 4, 7, etc. The date used for the time interpolation
275is thus redefined to be at the middle of \np{nn\_fsbc} time-step period. In the previous example,
276this leads to: 1h30'00", 4h30'00", 7h30'00", etc. \\ 
277(2) For code readablility and maintenance issues, we don't take into account the NetCDF input file
278calendar. The calendar associated with the forcing field is build according to the information
279provided by user in the record frequency, the open/close frequency and the type of temporal interpolation.
280For example, the first record of a yearly file containing daily data that will be interpolated in time
281is assumed to be start Jan 1st at 12h00'00" and end Dec 31st at 12h00'00". \\
282(3) If a time interpolation is requested, the code will pick up the needed data in the previous (next) file
283when interpolating data with the first (last) record of the open/close period.
284For example, if the input file specifications are ''yearly, containing daily data to be interpolated in time'',
285the values given by the code between 00h00'00" and 11h59'59" on Jan 1st will be interpolated values
286between Dec 31st 12h00'00" and Jan 1st 12h00'00". If the forcing is climatological, Dec and Jan will
287be keep-up from the same year. However, if the forcing is not climatological, at the end of the
288open/close period the code will automatically close the current file and open the next one.
289Note that, if the experiment is starting (ending) at the beginning (end) of an open/close period
290we do accept that the previous (next) file is not existing. In this case, the time interpolation
291will be performed between two identical values. For example, when starting an experiment on
292Jan 1st of year Y with yearly files and daily data to be interpolated, we do accept that the file
293related to year Y-1 is not existing. The value of Jan 1st will be used as the missing one for
294Dec 31st of year Y-1. If the file of year Y-1 exists, the code will read its last record.
295Therefore, this file can contain only one record corresponding to Dec 31st, a useful feature for
296user considering that it is too heavy to manipulate the complete file for year Y-1.
297
298
299% -------------------------------------------------------------------------------------------------------------
300% Interpolation on the Fly
301% -------------------------------------------------------------------------------------------------------------
302\subsection [Interpolation on-the-Fly] {Interpolation on-the-Fly}
303\label{SBC_iof}
304
305Interpolation on the Fly allows the user to supply input files required
306for the surface forcing on grids other than the model grid.
307To do this he or she must supply, in addition to the source data file,
308a file of weights to be used to interpolate from the data grid to the model grid.
309The original development of this code used the SCRIP package (freely available
310\href{http://climate.lanl.gov/Software/SCRIP}{here} under a copyright agreement).
311In principle, any package can be used to generate the weights, but the
312variables in the input weights file must have the same names and meanings as
313assumed by the model.
314Two methods are currently available: bilinear and bicubic interpolation.
315Prior to the interpolation, providing a land/sea mask file, the user can decide to
316 remove land points from the input file and substitute the corresponding values
317with the average of the 8 neighbouring points in the native external grid.
318 Only "sea points" are considered for the averaging. The land/sea mask file must
319be provided in the structure associated with the input variable.
320 The netcdf land/sea mask variable name must be 'LSM' it must have the same
321horizontal and vertical dimensions of the associated variable and should
322be equal to 1 over land and 0 elsewhere.
323The procedure can be recursively applied setting nn\_lsm > 1 in namsbc namelist.
324Note that nn\_lsm=0 forces the code to not apply the procedure even if a file for land/sea mask is supplied.
325
326\subsubsection{Bilinear Interpolation}
327\label{SBC_iof_bilinear}
328
329The input weights file in this case has two sets of variables: src01, src02,
330src03, src04 and wgt01, wgt02, wgt03, wgt04.
331The "src" variables correspond to the point in the input grid to which the weight
332"wgt" is to be applied. Each src value is an integer corresponding to the index of a
333point in the input grid when written as a one dimensional array.  For example, for an input grid
334of size 5x10, point (3,2) is referenced as point 8, since (2-1)*5+3=8.
335There are four of each variable because bilinear interpolation uses the four points defining
336the grid box containing the point to be interpolated.
337All of these arrays are on the model grid, so that values src01(i,j) and
338wgt01(i,j) are used to generate a value for point (i,j) in the model.
339
340Symbolically, the algorithm used is:
341
342\begin{equation}
343f_{m}(i,j) = f_{m}(i,j) + \sum_{k=1}^{4} {wgt(k)f(idx(src(k)))}
344\end{equation}
345where function idx() transforms a one dimensional index src(k) into a two dimensional index,
346and wgt(1) corresponds to variable "wgt01" for example.
347
348\subsubsection{Bicubic Interpolation}
349\label{SBC_iof_bicubic}
350
351Again there are two sets of variables: "src" and "wgt".
352But in this case there are 16 of each.
353The symbolic algorithm used to calculate values on the model grid is now:
354
355\begin{equation*} \begin{split}
356f_{m}(i,j) =  f_{m}(i,j) +& \sum_{k=1}^{4} {wgt(k)f(idx(src(k)))}     
357              +   \sum_{k=5}^{8} {wgt(k)\left.\frac{\partial f}{\partial i}\right| _{idx(src(k))} }    \\
358              +& \sum_{k=9}^{12} {wgt(k)\left.\frac{\partial f}{\partial j}\right| _{idx(src(k))} }   
359              +   \sum_{k=13}^{16} {wgt(k)\left.\frac{\partial ^2 f}{\partial i \partial j}\right| _{idx(src(k))} }
360\end{split}
361\end{equation*}
362The gradients here are taken with respect to the horizontal indices and not distances since the spatial dependency has been absorbed into the weights.
363
364\subsubsection{Implementation}
365\label{SBC_iof_imp}
366
367To activate this option, a non-empty string should be supplied in the weights filename column
368of the relevant namelist; if this is left as an empty string no action is taken.
369In the model, weights files are read in and stored in a structured type (WGT) in the fldread
370module, as and when they are first required.
371This initialisation procedure determines whether the input data grid should be treated
372as cyclical or not by inspecting a global attribute stored in the weights input file.
373This attribute must be called "ew\_wrap" and be of integer type.
374If it is negative, the input non-model grid is assumed not to be cyclic.
375If zero or greater, then the value represents the number of columns that overlap.
376$E.g.$ if the input grid has columns at longitudes 0, 1, 2, .... , 359, then ew\_wrap should be set to 0;
377if longitudes are 0.5, 2.5, .... , 358.5, 360.5, 362.5, ew\_wrap should be 2.
378If the model does not find attribute ew\_wrap, then a value of -999 is assumed.
379In this case the \rou{fld\_read} routine defaults ew\_wrap to value 0 and therefore the grid
380is assumed to be cyclic with no overlapping columns.
381(In fact this only matters when bicubic interpolation is required.)
382Note that no testing is done to check the validity in the model, since there is no way
383of knowing the name used for the longitude variable,
384so it is up to the user to make sure his or her data is correctly represented.
385
386Next the routine reads in the weights.
387Bicubic interpolation is assumed if it finds a variable with name "src05", otherwise
388bilinear interpolation is used. The WGT structure includes dynamic arrays both for
389the storage of the weights (on the model grid), and when required, for reading in
390the variable to be interpolated (on the input data grid).
391The size of the input data array is determined by examining the values in the "src"
392arrays to find the minimum and maximum i and j values required.
393Since bicubic interpolation requires the calculation of gradients at each point on the grid,
394the corresponding arrays are dimensioned with a halo of width one grid point all the way around.
395When the array of points from the data file is adjacent to an edge of the data grid,
396the halo is either a copy of the row/column next to it (non-cyclical case), or is a copy
397of one from the first few columns on the opposite side of the grid (cyclical case).
398
399\subsubsection{Limitations}
400\label{SBC_iof_lim}
401
402\begin{enumerate} 
403\item  The case where input data grids are not logically rectangular has not been tested.
404\item  This code is not guaranteed to produce positive definite answers from positive definite inputs
405          when a bicubic interpolation method is used.
406\item  The cyclic condition is only applied on left and right columns, and not to top and bottom rows.
407\item  The gradients across the ends of a cyclical grid assume that the grid spacing between
408          the two columns involved are consistent with the weights used.
409\item  Neither interpolation scheme is conservative. (There is a conservative scheme available
410          in SCRIP, but this has not been implemented.)
411\end{enumerate}
412
413\subsubsection{Utilities}
414\label{SBC_iof_util}
415
416% to be completed
417A set of utilities to create a weights file for a rectilinear input grid is available
418(see the directory NEMOGCM/TOOLS/WEIGHTS).
419
420% -------------------------------------------------------------------------------------------------------------
421% Standalone Surface Boundary Condition Scheme
422% -------------------------------------------------------------------------------------------------------------
423\subsection [Standalone Surface Boundary Condition Scheme] {Standalone Surface Boundary Condition Scheme}
424\label{SAS_iof}
425
426%---------------------------------------namsbc_ana--------------------------------------------------
427\fortranfile{namelists/namsbc_sas}
428%--------------------------------------------------------------------------------------------------------------
429
430In some circumstances it may be useful to avoid calculating the 3D temperature, salinity and velocity fields
431and simply read them in from a previous run or receive them from OASIS. 
432For example:
433
434\begin{itemize}
435\item  Multiple runs of the model are required in code development to see the effect of different algorithms in
436       the bulk formulae.
437\item  The effect of different parameter sets in the ice model is to be examined.
438\item  Development of sea-ice algorithms or parameterizations.
439\item  spinup of the iceberg floats
440\item  ocean/sea-ice simulation with both media running in parallel (\np{ln\_mixcpl}~=~\textit{true})
441\end{itemize}
442
443The StandAlone Surface scheme provides this utility.
444Its options are defined through the \ngn{namsbc\_sas} namelist variables.
445A new copy of the model has to be compiled with a configuration based on ORCA2\_SAS\_LIM.
446However no namelist parameters need be changed from the settings of the previous run (except perhaps nn{\_}date0)
447In this configuration, a few routines in the standard model are overriden by new versions.
448Routines replaced are:
449
450\begin{itemize}
451\item \mdl{nemogcm} : This routine initialises the rest of the model and repeatedly calls the stp time stepping routine (step.F90)
452       Since the ocean state is not calculated all associated initialisations have been removed.
453\item  \mdl{step} : The main time stepping routine now only needs to call the sbc routine (and a few utility functions).
454\item  \mdl{sbcmod} : This has been cut down and now only calculates surface forcing and the ice model required.  New surface modules
455       that can function when only the surface level of the ocean state is defined can also be added (e.g. icebergs).
456\item  \mdl{daymod} : No ocean restarts are read or written (though the ice model restarts are retained), so calls to restart functions
457       have been removed.  This also means that the calendar cannot be controlled by time in a restart file, so the user
458       must make sure that nn{\_}date0 in the model namelist is correct for his or her purposes.
459\item  \mdl{stpctl} : Since there is no free surface solver, references to it have been removed from \rou{stp\_ctl} module.
460\item  \mdl{diawri} : All 3D data have been removed from the output.  The surface temperature, salinity and velocity components (which
461       have been read in) are written along with relevant forcing and ice data.
462\end{itemize}
463
464One new routine has been added:
465
466\begin{itemize}
467\item  \mdl{sbcsas} : This module initialises the input files needed for reading temperature, salinity and velocity arrays at the surface.
468       These filenames are supplied in namelist namsbc{\_}sas.  Unfortunately because of limitations with the \mdl{iom} module,
469       the full 3D fields from the mean files have to be read in and interpolated in time, before using just the top level.
470       Since fldread is used to read in the data, Interpolation on the Fly may be used to change input data resolution.
471\end{itemize}
472
473
474% Missing the description of the 2 following variables:
475%   ln_3d_uve   = .true.    !  specify whether we are supplying a 3D u,v and e3 field
476%   ln_read_frq = .false.    !  specify whether we must read frq or not
477
478
479
480% ================================================================
481% Analytical formulation (sbcana module)
482% ================================================================
483\section  [Analytical formulation (\textit{sbcana}) ]
484      {Analytical formulation (\protect\mdl{sbcana} module) }
485\label{SBC_ana}
486
487%---------------------------------------namsbc_ana--------------------------------------------------
488%\fortranfile{namelists/namsbc_ana}
489%--------------------------------------------------------------------------------------------------------------
490
491The analytical formulation of the surface boundary condition is the default scheme.
492In this case, all the six fluxes needed by the ocean are assumed to
493be uniform in space. They take constant values given in the namelist
494\ngn{namsbc{\_}ana} by the variables \np{rn\_utau0}, \np{rn\_vtau0}, \np{rn\_qns0},
495\np{rn\_qsr0}, and \np{rn\_emp0} ($\textit{emp}=\textit{emp}_S$). The runoff is set to zero.
496In addition, the wind is allowed to reach its nominal value within a given number
497of time steps (\np{nn\_tau000}).
498
499If a user wants to apply a different analytical forcing, the \mdl{sbcana} 
500module can be modified to use another scheme. As an example,
501the \mdl{sbc\_ana\_gyre} routine provides the analytical forcing for the
502GYRE configuration (see GYRE configuration manual, in preparation).
503
504
505% ================================================================
506% Flux formulation
507% ================================================================
508\section  [Flux formulation (\textit{sbcflx}) ]
509      {Flux formulation (\protect\mdl{sbcflx} module) }
510\label{SBC_flx}
511%------------------------------------------namsbc_flx----------------------------------------------------
512\fortranfile{namelists/namsbc_flx} 
513%-------------------------------------------------------------------------------------------------------------
514
515In the flux formulation (\np{ln\_flx}=true), the surface boundary
516condition fields are directly read from input files. The user has to define
517in the namelist \ngn{namsbc{\_}flx} the name of the file, the name of the variable
518read in the file, the time frequency at which it is given (in hours), and a logical
519setting whether a time interpolation to the model time step is required
520for this field. See \S\ref{SBC_fldread} for a more detailed description of the parameters.
521
522Note that in general, a flux formulation is used in associated with a
523restoring term to observed SST and/or SSS. See \S\ref{SBC_ssr} for its
524specification.
525
526
527% ================================================================
528% Bulk formulation
529% ================================================================
530\section  [Bulk formulation (\textit{sbcblk\_core}, \textit{sbcblk\_clio} or \textit{sbcblk\_mfs}) ]
531      {Bulk formulation \small{(\protect\mdl{sbcblk\_core} \protect\mdl{sbcblk\_clio} \protect\mdl{sbcblk\_mfs} modules)} }
532\label{SBC_blk}
533
534In the bulk formulation, the surface boundary condition fields are computed
535using bulk formulae and atmospheric fields and ocean (and ice) variables.
536
537The atmospheric fields used depend on the bulk formulae used. Three bulk formulations
538are available : the CORE, the CLIO and the MFS bulk formulea. The choice is made by setting to true
539one of the following namelist variable : \np{ln\_core} ; \np{ln\_clio} or  \np{ln\_mfs}.
540
541Note : in forced mode, when a sea-ice model is used, a bulk formulation (CLIO or CORE) have to be used.
542Therefore the two bulk (CLIO and CORE) formulea include the computation of the fluxes over both
543an ocean and an ice surface.
544
545% -------------------------------------------------------------------------------------------------------------
546%        CORE Bulk formulea
547% -------------------------------------------------------------------------------------------------------------
548\subsection    [CORE Bulk formulea (\protect\np{ln\_core}=true)]
549            {CORE Bulk formulea (\protect\np{ln\_core}=true, \protect\mdl{sbcblk\_core})}
550\label{SBC_blk_core}
551%------------------------------------------namsbc_core----------------------------------------------------
552%\fortranfile{namelists/namsbc_core}
553%-------------------------------------------------------------------------------------------------------------
554
555The CORE bulk formulae have been developed by \citet{Large_Yeager_Rep04}.
556They have been designed to handle the CORE forcing, a mixture of NCEP
557reanalysis and satellite data. They use an inertial dissipative method to compute
558the turbulent transfer coefficients (momentum, sensible heat and evaporation)
559from the 10 metre wind speed, air temperature and specific humidity.
560This \citet{Large_Yeager_Rep04} dataset is available through the
561\href{http://nomads.gfdl.noaa.gov/nomads/forms/mom4/CORE.html}{GFDL web site}.
562
563Note that substituting ERA40 to NCEP reanalysis fields
564does not require changes in the bulk formulea themself.
565This is the so-called DRAKKAR Forcing Set (DFS) \citep{Brodeau_al_OM09}.
566
567Options are defined through the  \ngn{namsbc\_core} namelist variables.
568The required 8 input fields are:
569
570%--------------------------------------------------TABLE--------------------------------------------------
571\begin{table}[htbp]   \label{Tab_CORE}
572\begin{center}
573\begin{tabular}{|l|c|c|c|}
574\hline
575Variable desciption              & Model variable  & Units   & point \\    \hline
576i-component of the 10m air velocity & utau      & $m.s^{-1}$         & T  \\  \hline
577j-component of the 10m air velocity & vtau      & $m.s^{-1}$         & T  \\  \hline
57810m air temperature              & tair      & \r{}$K$            & T   \\ \hline
579Specific humidity             & humi      & \%              & T \\      \hline
580Incoming long wave radiation     & qlw    & $W.m^{-2}$         & T \\      \hline
581Incoming short wave radiation    & qsr    & $W.m^{-2}$         & T \\      \hline
582Total precipitation (liquid + solid)   & precip & $Kg.m^{-2}.s^{-1}$ & T \\   \hline
583Solid precipitation              & snow      & $Kg.m^{-2}.s^{-1}$ & T \\   \hline
584\end{tabular}
585\end{center}
586\end{table}
587%--------------------------------------------------------------------------------------------------------------
588
589Note that the air velocity is provided at a tracer ocean point, not at a velocity ocean
590point ($u$- and $v$-points). It is simpler and faster (less fields to be read),
591but it is not the recommended method when the ocean grid size is the same
592or larger than the one of the input atmospheric fields.
593
594The \np{sn\_wndi}, \np{sn\_wndj}, \np{sn\_qsr}, \np{sn\_qlw}, \np{sn\_tair}, \np{sn\_humi},
595\np{sn\_prec}, \np{sn\_snow}, \np{sn\_tdif} parameters describe the fields
596and the way they have to be used (spatial and temporal interpolations).
597
598\np{cn\_dir} is the directory of location of bulk files
599\np{ln\_taudif} is the flag to specify if we use Hight Frequency (HF) tau information (.true.) or not (.false.)
600\np{rn\_zqt}: is the height of humidity and temperature measurements (m)
601\np{rn\_zu}: is the height of wind measurements (m)
602
603Three multiplicative factors are availables :
604\np{rn\_pfac} and \np{rn\_efac} allows to adjust (if necessary) the global freshwater budget
605by increasing/reducing the precipitations (total and snow) and or evaporation, respectively.
606The third one,\np{rn\_vfac}, control to which extend the ice/ocean velocities are taken into account
607in the calculation of surface wind stress. Its range should be between zero and one,
608and it is recommended to set it to 0.
609
610% -------------------------------------------------------------------------------------------------------------
611%        CLIO Bulk formulea
612% -------------------------------------------------------------------------------------------------------------
613\subsection    [CLIO Bulk formulea (\protect\np{ln\_clio}=true)]
614            {CLIO Bulk formulea (\protect\np{ln\_clio}=true, \protect\mdl{sbcblk\_clio})}
615\label{SBC_blk_clio}
616%------------------------------------------namsbc_clio----------------------------------------------------
617%\fortranfile{namelists/namsbc_clio}
618%-------------------------------------------------------------------------------------------------------------
619
620The CLIO bulk formulae were developed several years ago for the
621Louvain-la-neuve coupled ice-ocean model (CLIO, \cite{Goosse_al_JGR99}).
622They are simpler bulk formulae. They assume the stress to be known and
623compute the radiative fluxes from a climatological cloud cover.
624
625Options are defined through the  \ngn{namsbc\_clio} namelist variables.
626The required 7 input fields are:
627
628%--------------------------------------------------TABLE--------------------------------------------------
629\begin{table}[htbp]   \label{Tab_CLIO}
630\begin{center}
631\begin{tabular}{|l|l|l|l|}
632\hline
633Variable desciption           & Model variable  & Units           & point \\  \hline
634i-component of the ocean stress     & utau         & $N.m^{-2}$         & U \\   \hline
635j-component of the ocean stress     & vtau         & $N.m^{-2}$         & V \\   \hline
636Wind speed module             & vatm         & $m.s^{-1}$         & T \\   \hline
63710m air temperature              & tair         & \r{}$K$            & T \\   \hline
638Specific humidity                & humi         & \%              & T \\   \hline
639Cloud cover                   &           & \%              & T \\   \hline
640Total precipitation (liquid + solid)   & precip    & $Kg.m^{-2}.s^{-1}$ & T \\   \hline
641Solid precipitation              & snow         & $Kg.m^{-2}.s^{-1}$ & T \\   \hline
642\end{tabular}
643\end{center}
644\end{table}
645%--------------------------------------------------------------------------------------------------------------
646
647As for the flux formulation, information about the input data required by the
648model is provided in the namsbc\_blk\_core or namsbc\_blk\_clio
649namelist (see \S\ref{SBC_fldread}).
650
651% -------------------------------------------------------------------------------------------------------------
652%        MFS Bulk formulae
653% -------------------------------------------------------------------------------------------------------------
654\subsection    [MFS Bulk formulea (\protect\np{ln\_mfs}=true)]
655            {MFS Bulk formulea (\protect\np{ln\_mfs}=true, \protect\mdl{sbcblk\_mfs})}
656\label{SBC_blk_mfs}
657%------------------------------------------namsbc_mfs----------------------------------------------------
658%\fortranfile{namelists/namsbc_mfs}
659%----------------------------------------------------------------------------------------------------------
660
661The MFS (Mediterranean Forecasting System) bulk formulae have been developed by
662 \citet{Castellari_al_JMS1998}.
663They have been designed to handle the ECMWF operational data and are currently
664in use in the MFS operational system \citep{Tonani_al_OS08}, \citep{Oddo_al_OS09}.
665The wind stress computation uses a drag coefficient computed according to \citet{Hellerman_Rosenstein_JPO83}.
666The surface boundary condition for temperature involves the balance between surface solar radiation,
667net long-wave radiation, the latent and sensible heat fluxes.
668Solar radiation is dependent on cloud cover and is computed by means of
669an astronomical formula \citep{Reed_JPO77}. Albedo monthly values are from \citet{Payne_JAS72} 
670as means of the values at $40^{o}N$ and $30^{o}N$ for the Atlantic Ocean (hence the same latitudinal
671band of the Mediterranean Sea). The net long-wave radiation flux
672\citep{Bignami_al_JGR95} is a function of
673air temperature, sea-surface temperature, cloud cover and relative humidity.
674Sensible heat and latent heat fluxes are computed by classical
675bulk formulae parameterised according to \citet{Kondo1975}.
676Details on the bulk formulae used can be found in \citet{Maggiore_al_PCE98} and \citet{Castellari_al_JMS1998}.
677
678Options are defined through the  \ngn{namsbc\_mfs} namelist variables.
679The required 7 input fields must be provided on the model Grid-T and  are:
680\begin{itemize}
681\item          Zonal Component of the 10m wind ($ms^{-1}$)  (\np{sn\_windi})
682\item          Meridional Component of the 10m wind ($ms^{-1}$)  (\np{sn\_windj})
683\item          Total Claud Cover (\%)  (\np{sn\_clc})
684\item          2m Air Temperature ($K$) (\np{sn\_tair})
685\item          2m Dew Point Temperature ($K$)  (\np{sn\_rhm})
686\item          Total Precipitation ${Kg} m^{-2} s^{-1}$ (\np{sn\_prec})
687\item          Mean Sea Level Pressure (${Pa}$) (\np{sn\_msl})
688\end{itemize}
689% -------------------------------------------------------------------------------------------------------------
690% ================================================================
691% Coupled formulation
692% ================================================================
693\section  [Coupled formulation (\textit{sbccpl}) ]
694      {Coupled formulation (\protect\mdl{sbccpl} module)}
695\label{SBC_cpl}
696%------------------------------------------namsbc_cpl----------------------------------------------------
697\fortranfile{namelists/namsbc_cpl} 
698%-------------------------------------------------------------------------------------------------------------
699
700In the coupled formulation of the surface boundary condition, the fluxes are
701provided by the OASIS coupler at a frequency which is defined in the OASIS coupler,
702while sea and ice surface temperature, ocean and ice albedo, and ocean currents
703are sent to the atmospheric component.
704
705A generalised coupled interface has been developed.
706It is currently interfaced with OASIS-3-MCT (\key{oasis3}).
707It has been successfully used to interface \NEMO to most of the European atmospheric
708GCM (ARPEGE, ECHAM, ECMWF, HadAM, HadGAM, LMDz),
709as well as to \href{http://wrf-model.org/}{WRF} (Weather Research and Forecasting Model).
710
711Note that in addition to the setting of \np{ln\_cpl} to true, the \key{coupled} have to be defined.
712The CPP key is mainly used in sea-ice to ensure that the atmospheric fluxes are
713actually recieved by the ice-ocean system (no calculation of ice sublimation in coupled mode).
714When PISCES biogeochemical model (\key{top} and \key{pisces}) is also used in the coupled system,
715the whole carbon cycle is computed by defining \key{cpl\_carbon\_cycle}. In this case,
716CO$_2$ fluxes will be exchanged between the atmosphere and the ice-ocean system (and need to be activated in \ngn{namsbc{\_}cpl} ).
717
718The namelist above allows control of various aspects of the coupling fields (particularly for
719vectors) and now allows for any coupling fields to have multiple sea ice categories (as required by LIM3
720and CICE).  When indicating a multi-category coupling field in namsbc{\_}cpl the number of categories will be
721determined by the number used in the sea ice model.  In some limited cases it may be possible to specify
722single category coupling fields even when the sea ice model is running with multiple categories - in this
723case the user should examine the code to be sure the assumptions made are satisfactory.  In cases where
724this is definitely not possible the model should abort with an error message.  The new code has been tested using
725ECHAM with LIM2, and HadGAM3 with CICE but although it will compile with \key{lim3} additional minor code changes
726may be required to run using LIM3.
727
728
729% ================================================================
730%        Atmospheric pressure
731% ================================================================
732\section   [Atmospheric pressure (\textit{sbcapr})]
733         {Atmospheric pressure (\protect\mdl{sbcapr})}
734\label{SBC_apr}
735%------------------------------------------namsbc_apr----------------------------------------------------
736\fortranfile{namelists/namsbc_apr} 
737%-------------------------------------------------------------------------------------------------------------
738
739The optional atmospheric pressure can be used to force ocean and ice dynamics
740(\np{ln\_apr\_dyn}~=~true, \textit{\ngn{namsbc}} namelist ).
741The input atmospheric forcing defined via \np{sn\_apr} structure (\textit{namsbc\_apr} namelist)
742can be interpolated in time to the model time step, and even in space when the
743interpolation on-the-fly is used. When used to force the dynamics, the atmospheric
744pressure is further transformed into an equivalent inverse barometer sea surface height,
745$\eta_{ib}$, using:
746\begin{equation} \label{SBC_ssh_ib}
747   \eta_{ib} = -  \frac{1}{g\,\rho_o}  \left( P_{atm} - P_o \right)
748\end{equation}
749where $P_{atm}$ is the atmospheric pressure and $P_o$ a reference atmospheric pressure.
750A value of $101,000~N/m^2$ is used unless \np{ln\_ref\_apr} is set to true. In this case $P_o$ 
751is set to the value of $P_{atm}$ averaged over the ocean domain, $i.e.$ the mean value of
752$\eta_{ib}$ is kept to zero at all time step.
753
754The gradient of $\eta_{ib}$ is added to the RHS of the ocean momentum equation
755(see \mdl{dynspg} for the ocean). For sea-ice, the sea surface height, $\eta_m$,
756which is provided to the sea ice model is set to $\eta - \eta_{ib}$ (see \mdl{sbcssr} module).
757$\eta_{ib}$ can be set in the output. This can simplify altimetry data and model comparison
758as inverse barometer sea surface height is usually removed from these date prior to their distribution.
759
760When using time-splitting and BDY package for open boundaries conditions, the equivalent
761inverse barometer sea surface height $\eta_{ib}$ can be added to BDY ssh data:
762\np{ln\_apr\_obc}  might be set to true.
763
764% ================================================================
765%        Tidal Potential
766% ================================================================
767\section   [Tidal Potential (\textit{sbctide})]
768                        {Tidal Potential (\protect\mdl{sbctide})}
769\label{SBC_tide}
770
771%------------------------------------------nam_tide---------------------------------------
772\fortranfile{namelists/nam_tide}
773%-----------------------------------------------------------------------------------------
774
775A module is available to compute the tidal potential and use it in the momentum equation.
776This option is activated when \np{ln\_tide} is set to true in \ngn{nam\_tide}.
777
778Some parameters are available in namelist \ngn{nam\_tide}:
779
780- \np{ln\_tide\_load} activate the load potential forcing and \np{filetide\_load} is  the associated file
781
782- \np{ln\_tide\_pot} activate the tidal potential forcing
783
784- \np{nb\_harmo} is the number of constituent used
785
786- \np{clname} is the name of constituent
787
788The tide is generated by the forces of gravity ot the Earth-Moon and Earth-Sun sytem;
789they are expressed as the gradient of the astronomical potential ($\vec{\nabla}\Pi_{a}$). \\
790
791The potential astronomical expressed, for the three types of tidal frequencies
792following, by : \\
793Tide long period :
794\begin{equation}
795\Pi_{a}=gA_{k}(\frac{1}{2}-\frac{3}{2}sin^{2}\phi)cos(\omega_{k}t+V_{0k})
796\end{equation}
797diurnal Tide :
798\begin{equation}
799\Pi_{a}=gA_{k}(sin 2\phi)cos(\omega_{k}t+\lambda+V_{0k})
800\end{equation}
801Semi-diurnal tide:
802\begin{equation}
803\Pi_{a}=gA_{k}(cos^{2}\phi)cos(\omega_{k}t+2\lambda+V_{0k})
804\end{equation}
805
806
807$A_{k}$ is the amplitude of the wave k, $\omega_{k}$ the pulsation of the wave k, $V_{0k}$ the astronomical phase of the wave
808$k$ to Greenwich.
809
810We make corrections to the astronomical potential.
811We obtain :
812\begin{equation}
813\Pi-g\delta = (1+k-h) \Pi_{A}(\lambda,\phi)
814\end{equation}
815with $k$ a number of Love estimated to 0.6 which parameterised the astronomical tidal land,
816and $h$ a number of Love to 0.3 which parameterised the parameterisation due to the astronomical tidal land.
817
818A description of load potential can be found in  \citet{Arbic2010}
819
820% ================================================================
821%        River runoffs
822% ================================================================
823\section   [River runoffs (\textit{sbcrnf})]
824         {River runoffs (\protect\mdl{sbcrnf})}
825\label{SBC_rnf}
826%------------------------------------------namsbc_rnf----------------------------------------------------
827\fortranfile{namelists/namsbc_rnf} 
828%-------------------------------------------------------------------------------------------------------------
829
830%River runoff generally enters the ocean at a nonzero depth rather than through the surface.
831%Many models, however, have traditionally inserted river runoff to the top model cell.
832%This was the case in \NEMO prior to the version 3.3. The switch toward a input of runoff
833%throughout a nonzero depth has been motivated by the numerical and physical problems
834%that arise when the top grid cells are of the order of one meter. This situation is common in
835%coastal modelling and becomes more and more often open ocean and climate modelling
836%\footnote{At least a top cells thickness of 1~meter and a 3 hours forcing frequency are
837%required to properly represent the diurnal cycle \citep{Bernie_al_JC05}. see also \S\ref{SBC_dcy}.}.
838
839
840%To do this we need to treat evaporation/precipitation fluxes and river runoff differently in the
841%\mdl{tra\_sbc} module.  We decided to separate them throughout the code, so that the variable
842%\textit{emp} represented solely evaporation minus precipitation fluxes, and a new 2d variable
843%rnf was added which represents the volume flux of river runoff (in kg/m2s to remain consistent with
844%emp).  This meant many uses of emp and emps needed to be changed, a list of all modules which use
845%emp or emps and the changes made are below:
846
847
848%Rachel:
849River runoff generally enters the ocean at a nonzero depth rather than through the surface.
850Many models, however, have traditionally inserted river runoff to the top model cell.
851This was the case in \NEMO prior to the version 3.3, and was combined with an option
852to increase vertical mixing near the river mouth.
853
854However, with this method numerical and physical problems arise when the top grid cells are
855of the order of one meter. This situation is common in coastal modelling and is becoming
856more common in open ocean and climate modelling
857\footnote{At least a top cells thickness of 1~meter and a 3 hours forcing frequency are
858required to properly represent the diurnal cycle \citep{Bernie_al_JC05}. see also \S\ref{SBC_dcy}.}.
859
860As such from V~3.3 onwards it is possible to add river runoff through a non-zero depth, and for the
861temperature and salinity of the river to effect the surrounding ocean.
862The user is able to specify, in a NetCDF input file, the temperature and salinity of the river, along with the   
863depth (in metres) which the river should be added to.
864
865Namelist variables in \ngn{namsbc\_rnf}, \np{ln\_rnf\_depth}, \np{ln\_rnf\_sal} and \np{ln\_rnf\_temp} control whether
866the river attributes (depth, salinity and temperature) are read in and used.  If these are set
867as false the river is added to the surface box only, assumed to be fresh (0~psu), and/or
868taken as surface temperature respectively.
869
870The runoff value and attributes are read in in sbcrnf. 
871For temperature -999 is taken as missing data and the river temperature is taken to be the
872surface temperatue at the river point.
873For the depth parameter a value of -1 means the river is added to the surface box only,
874and a value of -999 means the river is added through the entire water column.
875After being read in the temperature and salinity variables are multiplied by the amount of runoff (converted into m/s)
876to give the heat and salt content of the river runoff.
877After the user specified depth is read ini, the number of grid boxes this corresponds to is
878calculated and stored in the variable \np{nz\_rnf}.
879The variable \textit{h\_dep} is then calculated to be the depth (in metres) of the bottom of the
880lowest box the river water is being added to (i.e. the total depth that river water is being added to in the model).
881
882The mass/volume addition due to the river runoff is, at each relevant depth level, added to the horizontal divergence
883(\textit{hdivn}) in the subroutine \rou{sbc\_rnf\_div} (called from \mdl{divcur}).
884This increases the diffusion term in the vicinity of the river, thereby simulating a momentum flux.
885The sea surface height is calculated using the sum of the horizontal divergence terms, and so the
886river runoff indirectly forces an increase in sea surface height.
887
888The \textit{hdivn} terms are used in the tracer advection modules to force vertical velocities.
889This causes a mass of water, equal to the amount of runoff, to be moved into the box above.
890The heat and salt content of the river runoff is not included in this step, and so the tracer
891concentrations are diluted as water of ocean temperature and salinity is moved upward out of the box
892and replaced by the same volume of river water with no corresponding heat and salt addition.
893
894For the linear free surface case, at the surface box the tracer advection causes a flux of water
895(of equal volume to the runoff) through the sea surface out of the domain, which causes a salt and heat flux out of the model.
896As such the volume of water does not change, but the water is diluted.
897
898For the non-linear free surface case (\key{vvl}), no flux is allowed through the surface.
899Instead in the surface box (as well as water moving up from the boxes below) a volume of runoff water
900is added with no corresponding heat and salt addition and so as happens in the lower boxes there is a dilution effect.
901(The runoff addition to the top box along with the water being moved up through boxes below means the surface box has a large
902increase in volume, whilst all other boxes remain the same size)
903
904In trasbc the addition of heat and salt due to the river runoff is added.
905This is done in the same way for both vvl and non-vvl.
906The temperature and salinity are increased through the specified depth according to the heat and salt content of the river.
907
908In the non-linear free surface case (vvl), near the end of the time step the change in sea surface height is redistrubuted
909through the grid boxes, so that the original ratios of grid box heights are restored.
910In doing this water is moved into boxes below, throughout the water column, so the large volume addition to the surface box is spread between all the grid boxes.
911
912It is also possible for runnoff to be specified as a negative value for modelling flow through straits, i.e. modelling the Baltic flow in and out of the North Sea.
913When the flow is out of the domain there is no change in temperature and salinity, regardless of the namelist options used, as the ocean water leaving the domain removes heat and salt (at the same concentration) with it.
914
915
916%\colorbox{yellow}{Nevertheless, Pb of vertical resolution and 3D input : increase vertical mixing near river mouths to mimic a 3D river
917
918%All river runoff and emp fluxes are assumed to be fresh water (zero salinity) and at the same temperature as the sea surface.}
919
920%\colorbox{yellow}{river mouths{\ldots}}
921
922%IF( ln_rnf ) THEN                                     ! increase diffusivity at rivers mouths
923%        DO jk = 2, nkrnf   ;   avt(:,:,jk) = avt(:,:,jk) + rn_avt_rnf * rnfmsk(:,:)   ;   END DO
924%ENDIF
925
926%\gmcomment{  word doc of runoffs:
927%
928%In the current \NEMO setup river runoff is added to emp fluxes, these are then applied at just the sea surface as a volume change (in the variable volume case this is a literal volume change, and in the linear free surface case the free surface is moved) and a salt flux due to the concentration/dilution effect.  There is also an option to increase vertical mixing near river mouths; this gives the effect of having a 3d river.  All river runoff and emp fluxes are assumed to be fresh water (zero salinity) and at the same temperature as the sea surface.
929%Our aim was to code the option to specify the temperature and salinity of river runoff, (as well as the amount), along with the depth that the river water will affect.  This would make it possible to model low salinity outflow, such as the Baltic, and would allow the ocean temperature to be affected by river runoff. 
930
931%The depth option makes it possible to have the river water affecting just the surface layer, throughout depth, or some specified point in between.
932
933%To do this we need to treat evaporation/precipitation fluxes and river runoff differently in the tra_sbc module.  We decided to separate them throughout the code, so that the variable emp represented solely evaporation minus precipitation fluxes, and a new 2d variable rnf was added which represents the volume flux of river runoff (in kg/m2s to remain consistent with emp).  This meant many uses of emp and emps needed to be changed, a list of all modules which use emp or emps and the changes made are below:
934
935%}
936% ================================================================
937%        Ice shelf melting
938% ================================================================
939\section   [Ice shelf melting (\textit{sbcisf})]
940                        {Ice shelf melting (\protect\mdl{sbcisf})}
941\label{SBC_isf}
942%------------------------------------------namsbc_isf----------------------------------------------------
943\fortranfile{namelists/namsbc_isf}
944%--------------------------------------------------------------------------------------------------------
945Namelist variable in \ngn{namsbc}, \np{nn\_isf}, controls the ice shelf representation used.
946\begin{description}
947\item[\np{nn\_isf}~=~1]
948The ice shelf cavity is represented (\np{ln\_isfcav}~=~true needed). The fwf and heat flux are computed.
949Two different bulk formula are available:
950   \begin{description}
951   \item[\np{nn\_isfblk}~=~1]
952   The bulk formula used to compute the melt is based the one described in \citet{Hunter2006}.
953        This formulation is based on a balance between the upward ocean heat flux and the latent heat flux at the ice shelf base.
954
955   \item[\np{nn\_isfblk}~=~2] 
956   The bulk formula used to compute the melt is based the one described in \citet{Jenkins1991}.
957        This formulation is based on a 3 equations formulation (a heat flux budget, a salt flux budget
958         and a linearised freezing point temperature equation).
959   \end{description}
960
961For this 2 bulk formulations, there are 3 different ways to compute the exchange coeficient:
962   \begin{description}
963        \item[\np{nn\_gammablk~=~0~}]
964   The salt and heat exchange coefficients are constant and defined by \np{rn\_gammas0} and \np{rn\_gammat0}
965
966   \item[\np{nn\_gammablk~=~1~}]
967   The salt and heat exchange coefficients are velocity dependent and defined as $\np{rn\_gammas0} \times u_{*}$ and $\np{rn\_gammat0} \times u_{*}$
968        where $u_{*}$ is the friction velocity in the top boundary layer (ie first \np{rn\_hisf\_tbl} meters).
969        See \citet{Jenkins2010} for all the details on this formulation.
970   
971   \item[\np{nn\_gammablk~=~2~}]
972   The salt and heat exchange coefficients are velocity and stability dependent and defined as
973        $\gamma_{T,S} = \frac{u_{*}}{\Gamma_{Turb} + \Gamma^{T,S}_{Mole}}$
974        where $u_{*}$ is the friction velocity in the top boundary layer (ie first \np{rn\_hisf\_tbl} meters),
975        $\Gamma_{Turb}$ the contribution of the ocean stability and
976        $\Gamma^{T,S}_{Mole}$ the contribution of the molecular diffusion.
977        See \citet{Holland1999} for all the details on this formulation.
978        \end{description}
979
980\item[\np{nn\_isf}~=~2]
981A parameterisation of isf is used. The ice shelf cavity is not represented.
982The fwf is distributed along the ice shelf edge between the depth of the average grounding line (GL)
983(\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).
984Furthermore the fwf and heat flux are computed using the \citet{Beckmann2003} parameterisation of isf melting.
985The effective melting length (\np{sn\_Leff\_isf}) is read from a file.
986
987\item[\np{nn\_isf}~=~3]
988A simple parameterisation of isf is used. The ice shelf cavity is not represented.
989The fwf (\np{sn\_rnfisf}) is prescribed and distributed along the ice shelf edge between the depth of the average grounding line (GL)
990(\np{sn\_depmax\_isf}) and the base of the ice shelf along the calving front (\np{sn\_depmin\_isf}).
991The heat flux ($Q_h$) is computed as $Q_h = fwf \times L_f$.
992
993\item[\np{nn\_isf}~=~4]
994The ice shelf cavity is opened (\np{ln\_isfcav}~=~true needed). However, the fwf is not computed but specified from file \np{sn\_fwfisf}).
995The heat flux ($Q_h$) is computed as $Q_h = fwf \times L_f$.\\
996\end{description}
997
998
999$\bullet$ \np{nn\_isf}~=~1 and \np{nn\_isf}~=~2 compute a melt rate based on the water mass properties, ocean velocities and depth.
1000 This flux is thus highly dependent of the model resolution (horizontal and vertical), realism of the water masses onto the shelf ...\\
1001
1002
1003$\bullet$ \np{nn\_isf}~=~3 and \np{nn\_isf}~=~4 read the melt rate from a file. You have total control of the fwf forcing.
1004This can be usefull if the water masses on the shelf are not realistic or the resolution (horizontal/vertical) are too
1005coarse to have realistic melting or for studies where you need to control your heat and fw input.\\ 
1006
1007A namelist parameters control over how many meters the heat and fw fluxes are spread.
1008\np{rn\_hisf\_tbl}] is the top boundary layer thickness as defined in \citet{Losch2008}.
1009This parameter is only used if \np{nn\_isf}~=~1 or \np{nn\_isf}~=~4
1010
1011If \np{rn\_hisf\_tbl} = 0., the fluxes are put in the top level whatever is its tickness.
1012
1013If \np{rn\_hisf\_tbl} $>$ 0., the fluxes are spread over the first \np{rn\_hisf\_tbl} m (ie over one or several cells).\\
1014
1015The ice shelf melt is implemented as a volume flux with in the same way as for the runoff.
1016The fw addition due to the ice shelf melting is, at each relevant depth level, added to the horizontal divergence
1017(\textit{hdivn}) in the subroutine \rou{sbc\_isf\_div}, called from \mdl{divcur}.
1018See the runoff section \ref{SBC_rnf} for all the details about the divergence correction.
1019
1020
1021\section{ Ice sheet coupling}
1022\label{SBC_iscpl}
1023%------------------------------------------namsbc_iscpl----------------------------------------------------
1024\fortranfile{namelists/namsbc_iscpl}
1025%--------------------------------------------------------------------------------------------------------
1026Ice sheet/ocean coupling is done through file exchange at the restart step. NEMO, at each restart step,
1027read the bathymetry and ice shelf draft variable in a netcdf file.
1028If \np{ln\_iscpl = ~true}, the isf draft is assume to be different at each restart step
1029with potentially some new wet/dry cells due to the ice sheet dynamics/thermodynamics.
1030The wetting and drying scheme applied on the restart is very simple and described below for the 6 different cases:
1031\begin{description}
1032\item[Thin a cell down:]
1033   T/S/ssh are unchanged and U/V in the top cell are corrected to keep the barotropic transport (bt) constant ($bt_b=bt_n$).
1034\item[Enlarge  a cell:]
1035   See case "Thin a cell down"
1036\item[Dry a cell:]
1037   mask, T/S, U/V and ssh are set to 0. Furthermore, U/V into the water column are modified to satisfy ($bt_b=bt_n$).
1038\item[Wet a cell:] 
1039   mask is set to 1, T/S is extrapolated from neighbours, $ssh_n = ssh_b$ and U/V set to 0. If no neighbours along i,j and k, T/S/U/V and mask are set to 0.
1040\item[Dry a column:]
1041   mask, T/S, U/V are set to 0 everywhere in the column and ssh set to 0.
1042\item[Wet a column:]
1043   set mask to 1, T/S is extrapolated from neighbours, ssh is extrapolated from neighbours and U/V set to 0. If no neighbour, T/S/U/V and mask set to 0.
1044\end{description}
1045The extrapolation is call \np{nn\_drown} times. It means that if the grounding line retreat by more than \np{nn\_drown} cells between 2 coupling steps,
1046 the code will be unable to fill all the new wet cells properly. The default number is set up for the MISOMIP idealised experiments.\\
1047This coupling procedure is able to take into account grounding line and calving front migration. However, it is a non-conservative processe.
1048This could lead to a trend in heat/salt content and volume. In order to remove the trend and keep the conservation level as close to 0 as possible,
1049 a simple conservation scheme is available with \np{ln\_hsb = ~true}. The heat/salt/vol. gain/loss is diagnose, as well as the location.
1050Based on what is done on sbcrnf to prescribed a source of heat/salt/vol., the heat/salt/vol. gain/loss is removed/added,
1051 over a period of \np{rn\_fiscpl} time step, into the system.
1052So after \np{rn\_fiscpl} time step, all the heat/salt/vol. gain/loss due to extrapolation process is canceled.\\
1053
1054As the before and now fields are not compatible (modification of the geometry), the restart time step is prescribed to be an euler time step instead of a leap frog and $fields_b = fields_n$.
1055%
1056% ================================================================
1057%        Handling of icebergs
1058% ================================================================
1059\section{Handling of icebergs (ICB)}
1060\label{ICB_icebergs}
1061%------------------------------------------namberg----------------------------------------------------
1062\fortranfile{namelists/namberg}
1063%-------------------------------------------------------------------------------------------------------------
1064
1065Icebergs are modelled as lagrangian particles in NEMO \citep{Marsh_GMD2015}.
1066Their physical behaviour is controlled by equations as described in \citet{Martin_Adcroft_OM10} ).
1067(Note that the authors kindly provided a copy of their code to act as a basis for implementation in NEMO).
1068Icebergs are initially spawned into one of ten classes which have specific mass and thickness as described
1069in the \ngn{namberg} namelist:
1070\np{rn\_initial\_mass} and \np{rn\_initial\_thickness}.
1071Each class has an associated scaling (\np{rn\_mass\_scaling}), which is an integer representing how many icebergs
1072of this class are being described as one lagrangian point (this reduces the numerical problem of tracking every single iceberg).
1073They are enabled by setting \np{ln\_icebergs}~=~true.
1074
1075Two initialisation schemes are possible.
1076\begin{description}
1077\item[\np{nn\_test\_icebergs}~$>$~0]
1078In this scheme, the value of \np{nn\_test\_icebergs} represents the class of iceberg to generate
1079(so between 1 and 10), and \np{nn\_test\_icebergs} provides a lon/lat box in the domain at each
1080grid point of which an iceberg is generated at the beginning of the run.
1081(Note that this happens each time the timestep equals \np{nn\_nit000}.)
1082\np{nn\_test\_icebergs} is defined by four numbers in \np{nn\_test\_box} representing the corners
1083of the geographical box: lonmin,lonmax,latmin,latmax
1084\item[\np{nn\_test\_icebergs}~=~-1]
1085In this scheme the model reads a calving file supplied in the \np{sn\_icb} parameter.
1086This should be a file with a field on the configuration grid (typically ORCA) representing ice accumulation rate at each model point.
1087These should be ocean points adjacent to land where icebergs are known to calve.
1088Most points in this input grid are going to have value zero.
1089When the model runs, ice is accumulated at each grid point which has a non-zero source term.
1090At each time step, a test is performed to see if there is enough ice mass to calve an iceberg of each class in order (1 to 10).
1091Note that this is the initial mass multiplied by the number each particle represents ($i.e.$ the scaling).
1092If there is enough ice, a new iceberg is spawned and the total available ice reduced accordingly.
1093\end{description}
1094
1095Icebergs are influenced by wind, waves and currents, bottom melt and erosion.
1096The latter act to disintegrate the iceberg. This is either all melted freshwater, or
1097(if \np{rn\_bits\_erosion\_fraction}~$>$~0) into melt and additionally small ice bits
1098which are assumed to propagate with their larger parent and thus delay fluxing into the ocean.
1099Melt water (and other variables on the configuration grid) are written into the main NEMO model output files.
1100
1101Extensive diagnostics can be produced.
1102Separate output files are maintained for human-readable iceberg information.
1103A separate file is produced for each processor (independent of \np{ln\_ctl}).
1104The amount of information is controlled by two integer parameters:
1105\begin{description}
1106\item[\np{nn\_verbose\_level}]  takes a value between one and four and represents
1107an increasing number of points in the code at which variables are written, and an
1108increasing level of obscurity.
1109\item[\np{nn\_verbose\_write}] is the number of timesteps between writes
1110\end{description}
1111
1112Iceberg trajectories can also be written out and this is enabled by setting \np{nn\_sample\_rate}~$>$~0.
1113A non-zero value represents how many timesteps between writes of information into the output file.
1114These output files are in NETCDF format.
1115When \key{mpp\_mpi} is defined, each output file contains only those icebergs in the corresponding processor.
1116Trajectory points are written out in the order of their parent iceberg in the model's "linked list" of icebergs.
1117So care is needed to recreate data for individual icebergs, since its trajectory data may be spread across
1118multiple files.
1119
1120
1121% ================================================================
1122% Miscellanea options
1123% ================================================================
1124\section{Miscellaneous options}
1125\label{SBC_misc}
1126
1127% -------------------------------------------------------------------------------------------------------------
1128%        Diurnal cycle
1129% -------------------------------------------------------------------------------------------------------------
1130\subsection   [Diurnal  cycle (\textit{sbcdcy})]
1131         {Diurnal cycle (\protect\mdl{sbcdcy})}
1132\label{SBC_dcy}
1133%------------------------------------------namsbc_rnf----------------------------------------------------
1134%\fortranfile{namelists/namsbc}
1135%-------------------------------------------------------------------------------------------------------------
1136
1137%>>>>>>>>>>>>>>>>>>>>>>>>>>>>
1138\begin{figure}[!t]    \begin{center}
1139\includegraphics[width=0.8\textwidth]{Fig_SBC_diurnal}
1140\caption{ \protect\label{Fig_SBC_diurnal}   
1141Example of recontruction of the diurnal cycle variation of short wave flux 
1142from daily mean values. The reconstructed diurnal cycle (black line) is chosen
1143as the mean value of the analytical cycle (blue line) over a time step, not
1144as the mid time step value of the analytically cycle (red square). From \citet{Bernie_al_CD07}.}
1145\end{center}   \end{figure}
1146%>>>>>>>>>>>>>>>>>>>>>>>>>>>>
1147
1148\cite{Bernie_al_JC05} have shown that to capture 90$\%$ of the diurnal variability of
1149SST requires a vertical resolution in upper ocean of 1~m or better and a temporal resolution
1150of the surface fluxes of 3~h or less. Unfortunately high frequency forcing fields are rare,
1151not to say inexistent. Nevertheless, it is possible to obtain a reasonable diurnal cycle
1152of the SST knowning only short wave flux (SWF) at high frequency \citep{Bernie_al_CD07}.
1153Furthermore, only the knowledge of daily mean value of SWF is needed,
1154as higher frequency variations can be reconstructed from them, assuming that
1155the diurnal cycle of SWF is a scaling of the top of the atmosphere diurnal cycle
1156of incident SWF. The \cite{Bernie_al_CD07} reconstruction algorithm is available
1157in \NEMO by setting \np{ln\_dm2dc}~=~true (a \textit{\ngn{namsbc}} namelist variable) when using
1158CORE bulk formulea (\np{ln\_blk\_core}~=~true) or the flux formulation (\np{ln\_flx}~=~true).
1159The reconstruction is performed in the \mdl{sbcdcy} module. The detail of the algoritm used
1160can be found in the appendix~A of \cite{Bernie_al_CD07}. The algorithm preserve the daily
1161mean incomming SWF as the reconstructed SWF at a given time step is the mean value
1162of the analytical cycle over this time step (Fig.\ref{Fig_SBC_diurnal}).
1163The use of diurnal cycle reconstruction requires the input SWF to be daily
1164($i.e.$ a frequency of 24 and a time interpolation set to true in \np{sn\_qsr} namelist parameter).
1165Furthermore, it is recommended to have a least 8 surface module time step per day,
1166that is  $\rdt \ \np{nn\_fsbc} < 10,800~s = 3~h$. An example of recontructed SWF
1167is given in Fig.\ref{Fig_SBC_dcy} for a 12 reconstructed diurnal cycle, one every 2~hours
1168(from 1am to 11pm).
1169
1170%>>>>>>>>>>>>>>>>>>>>>>>>>>>>
1171\begin{figure}[!t]  \begin{center}
1172\includegraphics[width=0.7\textwidth]{Fig_SBC_dcy}
1173\caption{ \protect\label{Fig_SBC_dcy}   
1174Example of recontruction of the diurnal cycle variation of short wave flux 
1175from daily mean values on an ORCA2 grid with a time sampling of 2~hours (from 1am to 11pm).
1176The display is on (i,j) plane. }
1177\end{center}   \end{figure}
1178%>>>>>>>>>>>>>>>>>>>>>>>>>>>>
1179
1180Note also that the setting a diurnal cycle in SWF is highly recommended  when
1181the top layer thickness approach 1~m or less, otherwise large error in SST can
1182appear due to an inconsistency between the scale of the vertical resolution
1183and the forcing acting on that scale.
1184
1185% -------------------------------------------------------------------------------------------------------------
1186%        Rotation of vector pairs onto the model grid directions
1187% -------------------------------------------------------------------------------------------------------------
1188\subsection{Rotation of vector pairs onto the model grid directions}
1189\label{SBC_rotation}
1190
1191When using a flux (\np{ln\_flx}=true) or bulk (\np{ln\_clio}=true or \np{ln\_core}=true) formulation,
1192pairs of vector components can be rotated from east-north directions onto the local grid directions. 
1193This is particularly useful when interpolation on the fly is used since here any vectors are likely to be defined
1194relative to a rectilinear grid.
1195To activate this option a non-empty string is supplied in the rotation pair column of the relevant namelist.
1196The eastward component must start with "U" and the northward component with "V". 
1197The remaining characters in the strings are used to identify which pair of components go together.
1198So for example, strings "U1" and "V1" next to "utau" and "vtau" would pair the wind stress components together
1199and rotate them on to the model grid directions; "U2" and "V2" could be used against a second pair of components,
1200and so on.
1201The extra characters used in the strings are arbitrary.
1202The rot\_rep routine from the \mdl{geo2ocean} module is used to perform the rotation.
1203
1204% -------------------------------------------------------------------------------------------------------------
1205%        Surface restoring to observed SST and/or SSS
1206% -------------------------------------------------------------------------------------------------------------
1207\subsection    [Surface restoring to observed SST and/or SSS (\textit{sbcssr})]
1208         {Surface restoring to observed SST and/or SSS (\protect\mdl{sbcssr})}
1209\label{SBC_ssr}
1210%------------------------------------------namsbc_ssr----------------------------------------------------
1211\fortranfile{namelists/namsbc_ssr} 
1212%-------------------------------------------------------------------------------------------------------------
1213
1214IOptions are defined through the  \ngn{namsbc\_ssr} namelist variables.
1215n forced mode using a flux formulation (\np{ln\_flx}~=~true), a
1216feedback term \emph{must} be added to the surface heat flux $Q_{ns}^o$:
1217\begin{equation} \label{Eq_sbc_dmp_q}
1218Q_{ns} = Q_{ns}^o + \frac{dQ}{dT} \left( \left. T \right|_{k=1} - SST_{Obs} \right)
1219\end{equation}
1220where SST is a sea surface temperature field (observed or climatological), $T$ is
1221the model surface layer temperature and $\frac{dQ}{dT}$ is a negative feedback
1222coefficient usually taken equal to $-40~W/m^2/K$. For a $50~m$ 
1223mixed-layer depth, this value corresponds to a relaxation time scale of two months.
1224This term ensures that if $T$ perfectly matches the supplied SST, then $Q$ is
1225equal to $Q_o$.
1226
1227In the fresh water budget, a feedback term can also be added. Converted into an
1228equivalent freshwater flux, it takes the following expression :
1229
1230\begin{equation} \label{Eq_sbc_dmp_emp}
1231\textit{emp} = \textit{emp}_o + \gamma_s^{-1} e_{3t}  \frac{  \left(\left.S\right|_{k=1}-SSS_{Obs}\right)}
1232                                             {\left.S\right|_{k=1}}
1233\end{equation}
1234
1235where $\textit{emp}_{o }$ is a net surface fresh water flux (observed, climatological or an
1236atmospheric model product), \textit{SSS}$_{Obs}$ is a sea surface salinity (usually a time
1237interpolation of the monthly mean Polar Hydrographic Climatology \citep{Steele2001}),
1238$\left.S\right|_{k=1}$ is the model surface layer salinity and $\gamma_s$ is a negative
1239feedback coefficient which is provided as a namelist parameter. Unlike heat flux, there is no
1240physical justification for the feedback term in \ref{Eq_sbc_dmp_emp} as the atmosphere
1241does not care about ocean surface salinity \citep{Madec1997}. The SSS restoring
1242term should be viewed as a flux correction on freshwater fluxes to reduce the
1243uncertainties we have on the observed freshwater budget.
1244
1245% -------------------------------------------------------------------------------------------------------------
1246%        Handling of ice-covered area
1247% -------------------------------------------------------------------------------------------------------------
1248\subsection{Handling of ice-covered area  (\textit{sbcice\_...})}
1249\label{SBC_ice-cover}
1250
1251The presence at the sea surface of an ice covered area modifies all the fluxes
1252transmitted to the ocean. There are several way to handle sea-ice in the system
1253depending on the value of the \np{nn\_ice} namelist parameter found in \ngn{namsbc} namelist. 
1254\begin{description}
1255\item[nn{\_}ice = 0]  there will never be sea-ice in the computational domain.
1256This is a typical namelist value used for tropical ocean domain. The surface fluxes
1257are simply specified for an ice-free ocean. No specific things is done for sea-ice.
1258\item[nn{\_}ice = 1]  sea-ice can exist in the computational domain, but no sea-ice model
1259is used. An observed ice covered area is read in a file. Below this area, the SST is
1260restored to the freezing point and the heat fluxes are set to $-4~W/m^2$ ($-2~W/m^2$)
1261in the northern (southern) hemisphere. The associated modification of the freshwater
1262fluxes are done in such a way that the change in buoyancy fluxes remains zero.
1263This prevents deep convection to occur when trying to reach the freezing point
1264(and so ice covered area condition) while the SSS is too large. This manner of
1265managing sea-ice area, just by using si IF case, is usually referred as the \textit{ice-if} 
1266model. It can be found in the \mdl{sbcice{\_}if} module.
1267\item[nn{\_}ice = 2 or more]  A full sea ice model is used. This model computes the
1268ice-ocean fluxes, that are combined with the air-sea fluxes using the ice fraction of
1269each model cell to provide the surface ocean fluxes. Note that the activation of a
1270sea-ice model is is done by defining a CPP key (\key{lim3} or \key{cice}).
1271The activation automatically overwrites the read value of nn{\_}ice to its appropriate
1272value ($i.e.$ $2$ for LIM-3 or $3$ for CICE).
1273\end{description}
1274
1275% {Description of Ice-ocean interface to be added here or in LIM 2 and 3 doc ?}
1276
1277\subsection   [Interface to CICE (\textit{sbcice\_cice})]
1278         {Interface to CICE (\protect\mdl{sbcice\_cice})}
1279\label{SBC_cice}
1280
1281It is now possible to couple a regional or global NEMO configuration (without AGRIF) to the CICE sea-ice
1282model by using \key{cice}.  The CICE code can be obtained from
1283\href{http://oceans11.lanl.gov/trac/CICE/}{LANL} and the additional 'hadgem3' drivers will be required,
1284even with the latest code release.  Input grid files consistent with those used in NEMO will also be needed,
1285and CICE CPP keys \textbf{ORCA\_GRID}, \textbf{CICE\_IN\_NEMO} and \textbf{coupled} should be used (seek advice from UKMO
1286if necessary).  Currently the code is only designed to work when using the CORE forcing option for NEMO (with
1287\textit{calc\_strair~=~true} and \textit{calc\_Tsfc~=~true} in the CICE name-list), or alternatively when NEMO
1288is coupled to the HadGAM3 atmosphere model (with \textit{calc\_strair~=~false} and \textit{calc\_Tsfc~=~false}).
1289The code is intended to be used with \np{nn\_fsbc} set to 1 (although coupling ocean and ice less frequently
1290should work, it is possible the calculation of some of the ocean-ice fluxes needs to be modified slightly - the
1291user should check that results are not significantly different to the standard case).
1292
1293There are two options for the technical coupling between NEMO and CICE.  The standard version allows
1294complete flexibility for the domain decompositions in the individual models, but this is at the expense of global
1295gather and scatter operations in the coupling which become very expensive on larger numbers of processors. The
1296alternative option (using \key{nemocice\_decomp} for both NEMO and CICE) ensures that the domain decomposition is
1297identical in both models (provided domain parameters are set appropriately, and
1298\textit{processor\_shape~=~square-ice} and \textit{distribution\_wght~=~block} in the CICE name-list) and allows
1299much more efficient direct coupling on individual processors.  This solution scales much better although it is at
1300the expense of having more idle CICE processors in areas where there is no sea ice.
1301
1302% -------------------------------------------------------------------------------------------------------------
1303%        Freshwater budget control
1304% -------------------------------------------------------------------------------------------------------------
1305\subsection   [Freshwater budget control (\textit{sbcfwb})]
1306         {Freshwater budget control (\protect\mdl{sbcfwb})}
1307\label{SBC_fwb}
1308
1309For global ocean simulation it can be useful to introduce a control of the mean sea
1310level in order to prevent unrealistic drift of the sea surface height due to inaccuracy
1311in the freshwater fluxes. In \NEMO, two way of controlling the the freshwater budget.
1312\begin{description}
1313\item[\np{nn\_fwb}=0]  no control at all. The mean sea level is free to drift, and will
1314certainly do so.
1315\item[\np{nn\_fwb}=1]  global mean \textit{emp} set to zero at each model time step.
1316%Note that with a sea-ice model, this technique only control the mean sea level with linear free surface (\key{vvl} not defined) and no mass flux between ocean and ice (as it is implemented in the current ice-ocean coupling).
1317\item[\np{nn\_fwb}=2]  freshwater budget is adjusted from the previous year annual
1318mean budget which is read in the \textit{EMPave\_old.dat} file. As the model uses the
1319Boussinesq approximation, the annual mean fresh water budget is simply evaluated
1320from the change in the mean sea level at January the first and saved in the
1321\textit{EMPav.dat} file.
1322\end{description}
1323
1324% -------------------------------------------------------------------------------------------------------------
1325%        Neutral Drag Coefficient from external wave model
1326% -------------------------------------------------------------------------------------------------------------
1327\subsection   [Neutral drag coefficient from external wave model (\textit{sbcwave})]
1328              {Neutral drag coefficient from external wave model (\protect\mdl{sbcwave})}
1329\label{SBC_wave}
1330%------------------------------------------namwave----------------------------------------------------
1331\fortranfile{namelists/namsbc_wave}
1332%-------------------------------------------------------------------------------------------------------------
1333
1334In order to read a neutral drag coeff, from an external data source ($i.e.$ a wave model), the
1335logical variable \np{ln\_cdgw} in \ngn{namsbc} namelist must be set to \textit{true}.
1336The \mdl{sbcwave} module containing the routine \np{sbc\_wave} reads the
1337namelist \ngn{namsbc\_wave} (for external data names, locations, frequency, interpolation and all
1338the miscellanous options allowed by Input Data generic Interface see \S\ref{SBC_input})
1339and a 2D field of neutral drag coefficient.
1340Then using the routine TURB\_CORE\_1Z or TURB\_CORE\_2Z, and starting from the neutral drag coefficent provided,
1341the drag coefficient is computed according to stable/unstable conditions of the air-sea interface following \citet{Large_Yeager_Rep04}.
1342
1343
1344% Griffies doc:
1345% When running ocean-ice simulations, we are not explicitly representing land processes,
1346% such as rivers, catchment areas, snow accumulation, etc. However, to reduce model drift,
1347% it is important to balance the hydrological cycle in ocean-ice models.
1348% We thus need to prescribe some form of global normalization to the precipitation minus evaporation plus river runoff.
1349% The result of the normalization should be a global integrated zero net water input to the ocean-ice system over
1350% a chosen time scale.
1351%How often the normalization is done is a matter of choice. In mom4p1, we choose to do so at each model time step,
1352% so that there is always a zero net input of water to the ocean-ice system.
1353% Others choose to normalize over an annual cycle, in which case the net imbalance over an annual cycle is used
1354% to alter the subsequent year�s water budget in an attempt to damp the annual water imbalance.
1355% Note that the annual budget approach may be inappropriate with interannually varying precipitation forcing.
1356% When running ocean-ice coupled models, it is incorrect to include the water transport between the ocean
1357% and ice models when aiming to balance the hydrological cycle.
1358% 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,
1359% not the water in any one sub-component. As an extreme example to illustrate the issue,
1360% consider an ocean-ice model with zero initial sea ice. As the ocean-ice model spins up,
1361% there should be a net accumulation of water in the growing sea ice, and thus a net loss of water from the ocean.
1362% The total water contained in the ocean plus ice system is constant, but there is an exchange of water between
1363% the subcomponents. This exchange should not be part of the normalization used to balance the hydrological cycle
1364% in ocean-ice models.
1365
1366
1367\end{document}
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