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Chap_SBC.tex in branches/2015/dev_r5003_MERCATOR6_CRS/DOC/TexFiles/Chapters – NEMO

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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$\ $\newline    % force a new ligne
12%---------------------------------------namsbc--------------------------------------------------
13\namdisplay{namsbc}
14%--------------------------------------------------------------------------------------------------------------
15$\ $\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) (\np{nn\_isf}~=~2 or 3 and \np{ln\_isfcav}~=~false)
56or as fluxes applied at the land-ice ocean interface (\np{nn\_isf}~=~1 or 4 and \np{ln\_isfcav}~=~true) ;
57\item the addition of a freshwater flux adjustment in order to avoid a mean sea-level drift (\np{nn\_fwb}~=~0,~1~or~2) ;
58\item the transformation of the solar radiation (if provided as daily mean) into a diurnal cycle (\np{ln\_dm2dc}~=~true) ;
59and a neutral drag coefficient can be read from an external wave model (\np{ln\_cdgw}~=~true).
60\end{itemize}
61The latter option is possible only in case core or mfs bulk formulas are selected.
62
63In this chapter, we first discuss where the surface boundary condition appears in the
64model equations. Then we present the five ways of providing the surface boundary condition,
65followed by the description of the atmospheric pressure and the river runoff.
66Next the scheme for interpolation on the fly is described.
67Finally, the different options that further modify the fluxes applied to the ocean are discussed.
68One of these is modification by icebergs (see \S\ref{ICB_icebergs}), which act as drifting sources of fresh water.
69Another example of modification is that due to the ice shelf melting/freezing (see \S\ref{SBC_isf}),
70which provides additional sources of fresh water.
71
72
73% ================================================================
74% Surface boundary condition for the ocean
75% ================================================================
76\section{Surface boundary condition for the ocean}
77\label{SBC_general}
78
79The surface ocean stress is the stress exerted by the wind and the sea-ice
80on the ocean. It is applied in \mdl{dynzdf} module as a surface boundary condition of the
81computation of the momentum vertical mixing trend (see \eqref{Eq_dynzdf_sbc} in \S\ref{DYN_zdf}).
82As such, it has to be provided as a 2D vector interpolated
83onto the horizontal velocity ocean mesh, $i.e.$ resolved onto the model
84(\textbf{i},\textbf{j}) direction at $u$- and $v$-points.
85
86The surface heat flux is decomposed into two parts, a non solar and a solar heat
87flux, $Q_{ns}$ and $Q_{sr}$, respectively. The former is the non penetrative part
88of the heat flux ($i.e.$ the sum of sensible, latent and long wave heat fluxes
89plus the heat content of the mass exchange with the atmosphere and sea-ice).
90It is applied in \mdl{trasbc} module as a surface boundary condition trend of
91the first level temperature time evolution equation (see \eqref{Eq_tra_sbc} 
92and \eqref{Eq_tra_sbc_lin} in \S\ref{TRA_sbc}).
93The latter is the penetrative part of the heat flux. It is applied as a 3D
94trends of the temperature equation (\mdl{traqsr} module) when \np{ln\_traqsr}=\textit{true}.
95The way the light penetrates inside the water column is generally a sum of decreasing
96exponentials (see \S\ref{TRA_qsr}).
97
98The surface freshwater budget is provided by the \textit{emp} field.
99It represents the mass flux exchanged with the atmosphere (evaporation minus precipitation)
100and possibly with the sea-ice and ice shelves (freezing minus melting of ice).
101It affects both the ocean in two different ways:
102$(i)$   it changes the volume of the ocean and therefore appears in the sea surface height
103equation as a volume flux, and
104$(ii)$  it changes the surface temperature and salinity through the heat and salt contents
105of the mass exchanged with the atmosphere, the sea-ice and the ice shelves.
106
107
108%\colorbox{yellow}{Miss: }
109%
110%A extensive description of all namsbc namelist (parameter that have to be
111%created!)
112%
113%Especially the \np{nn\_fsbc}, the \mdl{sbc\_oce} module (fluxes + mean sst sss ssu
114%ssv) i.e. information required by flux computation or sea-ice
115%
116%\mdl{sbc\_oce} containt the definition in memory of the 7 fields (6+runoff), add
117%a word on runoff: included in surface bc or add as lateral obc{\ldots}.
118%
119%Sbcmod manage the ``providing'' (fourniture) to the ocean the 7 fields
120%
121%Fluxes update only each nn{\_}fsbc time step (namsbc) explain relation
122%between nn{\_}fsbc and nf{\_}ice, do we define nf{\_}blk??? ? only one
123%nn{\_}fsbc
124%
125%Explain here all the namlist namsbc variable{\ldots}.
126%
127% explain : use or not of surface currents
128%
129%\colorbox{yellow}{End Miss }
130
131The ocean model provides, at each time step, to the surface module (\mdl{sbcmod})
132the surface currents, temperature and salinity. 
133These variables are averaged over \np{nn\_fsbc} time-step (\ref{Tab_ssm}),
134and it is these averaged fields which are used to computes the surface fluxes
135at a frequency of \np{nn\_fsbc} time-step.
136
137
138%-------------------------------------------------TABLE---------------------------------------------------
139\begin{table}[tb]   \begin{center}   \begin{tabular}{|l|l|l|l|}
140\hline
141Variable description             & Model variable  & Units  & point \\  \hline
142i-component of the surface current  & ssu\_m & $m.s^{-1}$   & U \\   \hline
143j-component of the surface current  & ssv\_m & $m.s^{-1}$   & V \\   \hline
144Sea surface temperature          & sst\_m & \r{}$K$      & T \\   \hline
145Sea surface salinty              & sss\_m & $psu$        & T \\   \hline
146\end{tabular}
147\caption{  \label{Tab_ssm}   
148Ocean variables provided by the ocean to the surface module (SBC).
149The variable are averaged over nn{\_}fsbc time step, $i.e.$ the frequency of
150computation of surface fluxes.}
151\end{center}   \end{table}
152%--------------------------------------------------------------------------------------------------------------
153
154%\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
155
156
157% ================================================================
158%       Input Data
159% ================================================================
160\section{Input Data generic interface}
161\label{SBC_input}
162
163A generic interface has been introduced to manage the way input data (2D or 3D fields,
164like surface forcing or ocean T and S) are specify in \NEMO. This task is archieved by fldread.F90.
165The module was design with four main objectives in mind:
166\begin{enumerate} 
167\item optionally provide a time interpolation of the input data at model time-step,
168whatever their input frequency is, and according to the different calendars available in the model.
169\item optionally provide an on-the-fly space interpolation from the native input data grid to the model grid.
170\item make the run duration independent from the period cover by the input files.
171\item provide a simple user interface and a rather simple developer interface by limiting the
172 number of prerequisite information.
173\end{enumerate} 
174
175As a results the user have only to fill in for each variable a structure in the namelist file
176to defined the input data file and variable names, the frequency of the data (in hours or months),
177whether its is climatological data or not, the period covered by the input file (one year, month, week or day),
178and three additional parameters for on-the-fly interpolation. When adding a new input variable,
179the developer has to add the associated structure in the namelist, read this information
180by mirroring the namelist read in \rou{sbc\_blk\_init} for example, and simply call \rou{fld\_read} 
181to obtain the desired input field at the model time-step and grid points.
182
183The only constraints are that the input file is a NetCDF file, the file name follows a nomenclature
184(see \S\ref{SBC_fldread}), the period it cover is one year, month, week or day, and, if on-the-fly
185interpolation is used, a file of weights must be supplied (see \S\ref{SBC_iof}).
186
187Note that when an input data is archived on a disc which is accessible directly
188from the workspace where the code is executed, then the use can set the \np{cn\_dir} 
189to the pathway leading to the data. By default, the data are assumed to have been
190copied so that cn\_dir='./'.
191
192% -------------------------------------------------------------------------------------------------------------
193% Input Data specification (\mdl{fldread})
194% -------------------------------------------------------------------------------------------------------------
195\subsection{Input Data specification (\mdl{fldread})}
196\label{SBC_fldread}
197
198The structure associated with an input variable contains the following information:
199\begin{alltt}  {{\tiny   
200\begin{verbatim}
201!  file name  ! frequency (hours) ! variable  ! time interp. !  clim  ! 'yearly'/ ! weights  ! rotation ! land/sea mask !
202!             !  (if <0  months)  !   name    !   (logical)  !  (T/F) ! 'monthly' ! filename ! pairing  ! filename      !
203\end{verbatim}
204}}\end{alltt} 
205where
206\begin{description} 
207\item[File name]: the stem name of the NetCDF file to be open.
208This stem will be completed automatically by the model, with the addition of a '.nc' at its end
209and by date information and possibly a prefix (when using AGRIF).
210Tab.\ref{Tab_fldread} provides the resulting file name in all possible cases according to whether
211it is a climatological file or not, and to the open/close frequency (see below for definition).
212
213%--------------------------------------------------TABLE--------------------------------------------------
214\begin{table}[htbp]
215\begin{center}
216\begin{tabular}{|l|c|c|c|}
217\hline
218                         & daily or weekLLL          & monthly                   &   yearly          \\   \hline
219clim = false   & fn\_yYYYYmMMdDD  &   fn\_yYYYYmMM   &   fn\_yYYYY  \\   \hline
220clim = true       & not possible                  &  fn\_m??.nc             &   fn                \\   \hline
221\end{tabular}
222\end{center}
223\caption{ \label{Tab_fldread}   naming nomenclature for climatological or interannual input file,
224as a function of the Open/close frequency. The stem name is assumed to be 'fn'.
225For 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
226actual year/month/day, always coded with 4 or 2 digits. Note that (1) in mpp, if the file is split
227over each subdomain, the suffix '.nc' is replaced by '\_PPPP.nc', where 'PPPP' is the
228process number coded with 4 digits; (2) when using AGRIF, the prefix
229'\_N' is added to files,
230where 'N'  is the child grid number.}
231\end{table}
232%--------------------------------------------------------------------------------------------------------------
233 
234
235\item[Record frequency]: the frequency of the records contained in the input file.
236Its unit is in hours if it is positive (for example 24 for daily forcing) or in months if negative
237(for example -1 for monthly forcing or -12 for annual forcing).
238Note that this frequency must really be an integer and not a real.
239On some computers, seting it to '24.' can be interpreted as 240!
240
241\item[Variable name]: the name of the variable to be read in the input NetCDF file.
242
243\item[Time interpolation]: a logical to activate, or not, the time interpolation. If set to 'false',
244the forcing will have a steplike shape remaining constant during each forcing period.
245For example, when using a daily forcing without time interpolation, the forcing remaining
246constant from 00h00'00'' to 23h59'59". If set to 'true', the forcing will have a broken line shape.
247Records are assumed to be dated the middle of the forcing period.
248For example, when using a daily forcing with time interpolation, linear interpolation will
249be performed between mid-day of two consecutive days.
250
251\item[Climatological forcing]: a logical to specify if a input file contains climatological forcing
252which can be cycle in time, or an interannual forcing which will requires additional files
253if the period covered by the simulation exceed the one of the file. See the above the file
254naming strategy which impacts the expected name of the file to be opened.
255
256\item[Open/close frequency]: the frequency at which forcing files must be opened/closed.
257Four cases are coded: 'daily', 'weekLLL' (with 'LLL' the first 3 letters of the first day of the week),
258'monthly' and 'yearly' which means the forcing files will contain data for one day, one week,
259one month or one year. Files are assumed to contain data from the beginning of the open/close period.
260For example, the first record of a yearly file containing daily data is Jan 1st even if the experiment
261is not starting at the beginning of the year.
262
263\item[Others]: 'weights filename', 'pairing rotation' and 'land/sea mask' are associted with on-the-fly interpolation
264which is described in \S\ref{SBC_iof}.
265
266\end{description}
267
268Additional remarks:\\
269(1) The time interpolation is a simple linear interpolation between two consecutive records of
270the input data. The only tricky point is therefore to specify the date at which we need to do
271the interpolation and the date of the records read in the input files.
272Following \citet{Leclair_Madec_OM09}, the date of a time step is set at the middle of the
273time step. For example, for an experiment starting at 0h00'00" with a one hour time-step,
274a time interpolation will be performed at the following time: 0h30'00", 1h30'00", 2h30'00", etc.
275However, for forcing data related to the surface module, values are not needed at every
276time-step but at every \np{nn\_fsbc} time-step. For example with \np{nn\_fsbc}~=~3,
277the surface module will be called at time-steps 1, 4, 7, etc. The date used for the time interpolation
278is thus redefined to be at the middle of \np{nn\_fsbc} time-step period. In the previous example,
279this leads to: 1h30'00", 4h30'00", 7h30'00", etc. \\ 
280(2) For code readablility and maintenance issues, we don't take into account the NetCDF input file
281calendar. The calendar associated with the forcing field is build according to the information
282provided by user in the record frequency, the open/close frequency and the type of temporal interpolation.
283For example, the first record of a yearly file containing daily data that will be interpolated in time
284is assumed to be start Jan 1st at 12h00'00" and end Dec 31st at 12h00'00". \\
285(3) If a time interpolation is requested, the code will pick up the needed data in the previous (next) file
286when interpolating data with the first (last) record of the open/close period.
287For example, if the input file specifications are ''yearly, containing daily data to be interpolated in time'',
288the values given by the code between 00h00'00" and 11h59'59" on Jan 1st will be interpolated values
289between Dec 31st 12h00'00" and Jan 1st 12h00'00". If the forcing is climatological, Dec and Jan will
290be keep-up from the same year. However, if the forcing is not climatological, at the end of the
291open/close period the code will automatically close the current file and open the next one.
292Note that, if the experiment is starting (ending) at the beginning (end) of an open/close period
293we do accept that the previous (next) file is not existing. In this case, the time interpolation
294will be performed between two identical values. For example, when starting an experiment on
295Jan 1st of year Y with yearly files and daily data to be interpolated, we do accept that the file
296related to year Y-1 is not existing. The value of Jan 1st will be used as the missing one for
297Dec 31st of year Y-1. If the file of year Y-1 exists, the code will read its last record.
298Therefore, this file can contain only one record corresponding to Dec 31st, a useful feature for
299user considering that it is too heavy to manipulate the complete file for year Y-1.
300
301
302% -------------------------------------------------------------------------------------------------------------
303% Interpolation on the Fly
304% -------------------------------------------------------------------------------------------------------------
305\subsection [Interpolation on-the-Fly] {Interpolation on-the-Fly}
306\label{SBC_iof}
307
308Interpolation on the Fly allows the user to supply input files required
309for the surface forcing on grids other than the model grid.
310To do this he or she must supply, in addition to the source data file,
311a file of weights to be used to interpolate from the data grid to the model grid.
312The original development of this code used the SCRIP package (freely available
313\href{http://climate.lanl.gov/Software/SCRIP}{here} under a copyright agreement).
314In principle, any package can be used to generate the weights, but the
315variables in the input weights file must have the same names and meanings as
316assumed by the model.
317Two methods are currently available: bilinear and bicubic interpolation.
318Prior to the interpolation, providing a land/sea mask file, the user can decide to
319 remove land points from the input file and substitute the corresponding values
320with the average of the 8 neighbouring points in the native external grid.
321 Only "sea points" are considered for the averaging. The land/sea mask file must
322be provided in the structure associated with the input variable.
323 The netcdf land/sea mask variable name must be 'LSM' it must have the same
324horizontal and vertical dimensions of the associated variable and should
325be equal to 1 over land and 0 elsewhere.
326The procedure can be recursively applied setting nn\_lsm > 1 in namsbc namelist.
327Note that nn\_lsm=0 forces the code to not apply the procedure even if a file for land/sea mask is supplied.
328
329\subsubsection{Bilinear Interpolation}
330\label{SBC_iof_bilinear}
331
332The input weights file in this case has two sets of variables: src01, src02,
333src03, src04 and wgt01, wgt02, wgt03, wgt04.
334The "src" variables correspond to the point in the input grid to which the weight
335"wgt" is to be applied. Each src value is an integer corresponding to the index of a
336point in the input grid when written as a one dimensional array.  For example, for an input grid
337of size 5x10, point (3,2) is referenced as point 8, since (2-1)*5+3=8.
338There are four of each variable because bilinear interpolation uses the four points defining
339the grid box containing the point to be interpolated.
340All of these arrays are on the model grid, so that values src01(i,j) and
341wgt01(i,j) are used to generate a value for point (i,j) in the model.
342
343Symbolically, the algorithm used is:
344
345\begin{equation}
346f_{m}(i,j) = f_{m}(i,j) + \sum_{k=1}^{4} {wgt(k)f(idx(src(k)))}
347\end{equation}
348where function idx() transforms a one dimensional index src(k) into a two dimensional index,
349and wgt(1) corresponds to variable "wgt01" for example.
350
351\subsubsection{Bicubic Interpolation}
352\label{SBC_iof_bicubic}
353
354Again there are two sets of variables: "src" and "wgt".
355But in this case there are 16 of each.
356The symbolic algorithm used to calculate values on the model grid is now:
357
358\begin{equation*} \begin{split}
359f_{m}(i,j) =  f_{m}(i,j) +& \sum_{k=1}^{4} {wgt(k)f(idx(src(k)))}     
360              +   \sum_{k=5}^{8} {wgt(k)\left.\frac{\partial f}{\partial i}\right| _{idx(src(k))} }    \\
361              +& \sum_{k=9}^{12} {wgt(k)\left.\frac{\partial f}{\partial j}\right| _{idx(src(k))} }   
362              +   \sum_{k=13}^{16} {wgt(k)\left.\frac{\partial ^2 f}{\partial i \partial j}\right| _{idx(src(k))} }
363\end{split}
364\end{equation*}
365The gradients here are taken with respect to the horizontal indices and not distances since the spatial dependency has been absorbed into the weights.
366
367\subsubsection{Implementation}
368\label{SBC_iof_imp}
369
370To activate this option, a non-empty string should be supplied in the weights filename column
371of the relevant namelist; if this is left as an empty string no action is taken.
372In the model, weights files are read in and stored in a structured type (WGT) in the fldread
373module, as and when they are first required.
374This initialisation procedure determines whether the input data grid should be treated
375as cyclical or not by inspecting a global attribute stored in the weights input file.
376This attribute must be called "ew\_wrap" and be of integer type.
377If it is negative, the input non-model grid is assumed not to be cyclic.
378If zero or greater, then the value represents the number of columns that overlap.
379$E.g.$ if the input grid has columns at longitudes 0, 1, 2, .... , 359, then ew\_wrap should be set to 0;
380if longitudes are 0.5, 2.5, .... , 358.5, 360.5, 362.5, ew\_wrap should be 2.
381If the model does not find attribute ew\_wrap, then a value of -999 is assumed.
382In this case the \rou{fld\_read} routine defaults ew\_wrap to value 0 and therefore the grid
383is assumed to be cyclic with no overlapping columns.
384(In fact this only matters when bicubic interpolation is required.)
385Note that no testing is done to check the validity in the model, since there is no way
386of knowing the name used for the longitude variable,
387so it is up to the user to make sure his or her data is correctly represented.
388
389Next the routine reads in the weights.
390Bicubic interpolation is assumed if it finds a variable with name "src05", otherwise
391bilinear interpolation is used. The WGT structure includes dynamic arrays both for
392the storage of the weights (on the model grid), and when required, for reading in
393the variable to be interpolated (on the input data grid).
394The size of the input data array is determined by examining the values in the "src"
395arrays to find the minimum and maximum i and j values required.
396Since bicubic interpolation requires the calculation of gradients at each point on the grid,
397the corresponding arrays are dimensioned with a halo of width one grid point all the way around.
398When the array of points from the data file is adjacent to an edge of the data grid,
399the halo is either a copy of the row/column next to it (non-cyclical case), or is a copy
400of one from the first few columns on the opposite side of the grid (cyclical case).
401
402\subsubsection{Limitations}
403\label{SBC_iof_lim}
404
405\begin{enumerate} 
406\item  The case where input data grids are not logically rectangular has not been tested.
407\item  This code is not guaranteed to produce positive definite answers from positive definite inputs
408          when a bicubic interpolation method is used.
409\item  The cyclic condition is only applied on left and right columns, and not to top and bottom rows.
410\item  The gradients across the ends of a cyclical grid assume that the grid spacing between
411          the two columns involved are consistent with the weights used.
412\item  Neither interpolation scheme is conservative. (There is a conservative scheme available
413          in SCRIP, but this has not been implemented.)
414\end{enumerate}
415
416\subsubsection{Utilities}
417\label{SBC_iof_util}
418
419% to be completed
420A set of utilities to create a weights file for a rectilinear input grid is available
421(see the directory NEMOGCM/TOOLS/WEIGHTS).
422
423% -------------------------------------------------------------------------------------------------------------
424% Standalone Surface Boundary Condition Scheme
425% -------------------------------------------------------------------------------------------------------------
426\subsection [Standalone Surface Boundary Condition Scheme] {Standalone Surface Boundary Condition Scheme}
427\label{SAS_iof}
428
429%---------------------------------------namsbc_ana--------------------------------------------------
430\namdisplay{namsbc_sas}
431%--------------------------------------------------------------------------------------------------------------
432
433In some circumstances it may be useful to avoid calculating the 3D temperature, salinity and velocity fields
434and simply read them in from a previous run or receive them from OASIS. 
435For example:
436
437\begin{itemize}
438\item  Multiple runs of the model are required in code development to see the effect of different algorithms in
439       the bulk formulae.
440\item  The effect of different parameter sets in the ice model is to be examined.
441\item  Development of sea-ice algorithms or parameterizations.
442\item  spinup of the iceberg floats
443\item  ocean/sea-ice simulation with both media running in parallel (\np{ln\_mixcpl}~=~\textit{true})
444\end{itemize}
445
446The StandAlone Surface scheme provides this utility.
447Its options are defined through the \ngn{namsbc\_sas} namelist variables.
448A new copy of the model has to be compiled with a configuration based on ORCA2\_SAS\_LIM.
449However no namelist parameters need be changed from the settings of the previous run (except perhaps nn{\_}date0)
450In this configuration, a few routines in the standard model are overriden by new versions.
451Routines replaced are:
452
453\begin{itemize}
454\item \mdl{nemogcm} : This routine initialises the rest of the model and repeatedly calls the stp time stepping routine (step.F90)
455       Since the ocean state is not calculated all associated initialisations have been removed.
456\item  \mdl{step} : The main time stepping routine now only needs to call the sbc routine (and a few utility functions).
457\item  \mdl{sbcmod} : This has been cut down and now only calculates surface forcing and the ice model required.  New surface modules
458       that can function when only the surface level of the ocean state is defined can also be added (e.g. icebergs).
459\item  \mdl{daymod} : No ocean restarts are read or written (though the ice model restarts are retained), so calls to restart functions
460       have been removed.  This also means that the calendar cannot be controlled by time in a restart file, so the user
461       must make sure that nn{\_}date0 in the model namelist is correct for his or her purposes.
462\item  \mdl{stpctl} : Since there is no free surface solver, references to it have been removed from \rou{stp\_ctl} module.
463\item  \mdl{diawri} : All 3D data have been removed from the output.  The surface temperature, salinity and velocity components (which
464       have been read in) are written along with relevant forcing and ice data.
465\end{itemize}
466
467One new routine has been added:
468
469\begin{itemize}
470\item  \mdl{sbcsas} : This module initialises the input files needed for reading temperature, salinity and velocity arrays at the surface.
471       These filenames are supplied in namelist namsbc{\_}sas.  Unfortunately because of limitations with the \mdl{iom} module,
472       the full 3D fields from the mean files have to be read in and interpolated in time, before using just the top level.
473       Since fldread is used to read in the data, Interpolation on the Fly may be used to change input data resolution.
474\end{itemize}
475
476
477% Missing the description of the 2 following variables:
478%   ln_3d_uve   = .true.    !  specify whether we are supplying a 3D u,v and e3 field
479%   ln_read_frq = .false.    !  specify whether we must read frq or not
480
481
482
483% ================================================================
484% Analytical formulation (sbcana module)
485% ================================================================
486\section  [Analytical formulation (\textit{sbcana}) ]
487      {Analytical formulation (\mdl{sbcana} module) }
488\label{SBC_ana}
489
490%---------------------------------------namsbc_ana--------------------------------------------------
491\namdisplay{namsbc_ana}
492%--------------------------------------------------------------------------------------------------------------
493
494The analytical formulation of the surface boundary condition is the default scheme.
495In this case, all the six fluxes needed by the ocean are assumed to
496be uniform in space. They take constant values given in the namelist
497\ngn{namsbc{\_}ana} by the variables \np{rn\_utau0}, \np{rn\_vtau0}, \np{rn\_qns0},
498\np{rn\_qsr0}, and \np{rn\_emp0} ($\textit{emp}=\textit{emp}_S$). The runoff is set to zero.
499In addition, the wind is allowed to reach its nominal value within a given number
500of time steps (\np{nn\_tau000}).
501
502If a user wants to apply a different analytical forcing, the \mdl{sbcana} 
503module can be modified to use another scheme. As an example,
504the \mdl{sbc\_ana\_gyre} routine provides the analytical forcing for the
505GYRE configuration (see GYRE configuration manual, in preparation).
506
507
508% ================================================================
509% Flux formulation
510% ================================================================
511\section  [Flux formulation (\textit{sbcflx}) ]
512      {Flux formulation (\mdl{sbcflx} module) }
513\label{SBC_flx}
514%------------------------------------------namsbc_flx----------------------------------------------------
515\namdisplay{namsbc_flx} 
516%-------------------------------------------------------------------------------------------------------------
517
518In the flux formulation (\np{ln\_flx}=true), the surface boundary
519condition fields are directly read from input files. The user has to define
520in the namelist \ngn{namsbc{\_}flx} the name of the file, the name of the variable
521read in the file, the time frequency at which it is given (in hours), and a logical
522setting whether a time interpolation to the model time step is required
523for this field. See \S\ref{SBC_fldread} for a more detailed description of the parameters.
524
525Note that in general, a flux formulation is used in associated with a
526restoring term to observed SST and/or SSS. See \S\ref{SBC_ssr} for its
527specification.
528
529
530% ================================================================
531% Bulk formulation
532% ================================================================
533\section  [Bulk formulation (\textit{sbcblk\_core}, \textit{sbcblk\_clio} or \textit{sbcblk\_mfs}) ]
534      {Bulk formulation \small{(\mdl{sbcblk\_core} \mdl{sbcblk\_clio} \mdl{sbcblk\_mfs} modules)} }
535\label{SBC_blk}
536
537In the bulk formulation, the surface boundary condition fields are computed
538using bulk formulae and atmospheric fields and ocean (and ice) variables.
539
540The atmospheric fields used depend on the bulk formulae used. Three bulk formulations
541are available : the CORE, the CLIO and the MFS bulk formulea. The choice is made by setting to true
542one of the following namelist variable : \np{ln\_core} ; \np{ln\_clio} or  \np{ln\_mfs}.
543
544Note : in forced mode, when a sea-ice model is used, a bulk formulation (CLIO or CORE) have to be used.
545Therefore the two bulk (CLIO and CORE) formulea include the computation of the fluxes over both
546an ocean and an ice surface.
547
548% -------------------------------------------------------------------------------------------------------------
549%        CORE Bulk formulea
550% -------------------------------------------------------------------------------------------------------------
551\subsection    [CORE Bulk formulea (\np{ln\_core}=true)]
552            {CORE Bulk formulea (\np{ln\_core}=true, \mdl{sbcblk\_core})}
553\label{SBC_blk_core}
554%------------------------------------------namsbc_core----------------------------------------------------
555\namdisplay{namsbc_core} 
556%-------------------------------------------------------------------------------------------------------------
557
558The CORE bulk formulae have been developed by \citet{Large_Yeager_Rep04}.
559They have been designed to handle the CORE forcing, a mixture of NCEP
560reanalysis and satellite data. They use an inertial dissipative method to compute
561the turbulent transfer coefficients (momentum, sensible heat and evaporation)
562from the 10 meters wind speed, air temperature and specific humidity.
563This \citet{Large_Yeager_Rep04} dataset is available through the
564\href{http://nomads.gfdl.noaa.gov/nomads/forms/mom4/CORE.html}{GFDL web site}.
565
566Note that substituting ERA40 to NCEP reanalysis fields
567does not require changes in the bulk formulea themself.
568This is the so-called DRAKKAR Forcing Set (DFS) \citep{Brodeau_al_OM09}.
569
570Options are defined through the  \ngn{namsbc\_core} namelist variables.
571The required 8 input fields are:
572
573%--------------------------------------------------TABLE--------------------------------------------------
574\begin{table}[htbp]   \label{Tab_CORE}
575\begin{center}
576\begin{tabular}{|l|c|c|c|}
577\hline
578Variable desciption              & Model variable  & Units   & point \\    \hline
579i-component of the 10m air velocity & utau      & $m.s^{-1}$         & T  \\  \hline
580j-component of the 10m air velocity & vtau      & $m.s^{-1}$         & T  \\  \hline
58110m air temperature              & tair      & \r{}$K$            & T   \\ \hline
582Specific humidity             & humi      & \%              & T \\      \hline
583Incoming long wave radiation     & qlw    & $W.m^{-2}$         & T \\      \hline
584Incoming short wave radiation    & qsr    & $W.m^{-2}$         & T \\      \hline
585Total precipitation (liquid + solid)   & precip & $Kg.m^{-2}.s^{-1}$ & T \\   \hline
586Solid precipitation              & snow      & $Kg.m^{-2}.s^{-1}$ & T \\   \hline
587\end{tabular}
588\end{center}
589\end{table}
590%--------------------------------------------------------------------------------------------------------------
591
592Note that the air velocity is provided at a tracer ocean point, not at a velocity ocean
593point ($u$- and $v$-points). It is simpler and faster (less fields to be read),
594but it is not the recommended method when the ocean grid size is the same
595or larger than the one of the input atmospheric fields.
596
597The  \np{sn\_wndi}, \np{sn\_wndj}, \np{sn\_qsr}, \np{sn\_qlw}, \np{sn\_tair},\np{sn\_humi},\np{sn\_prec}, \np{sn\_snow}, \np{sn\_tdif} parameters describe the fields and the way they have to be used (spatial and temporal interpolations).
598
599\np{cn\_dir} is the directory of location of bulk files
600\np{ln\_taudif} is the flag to specify if we use Hight Frequency (HF) tau information (.true.) or not (.false.)
601\np{rn\_zqt}: is the height of humidity and temperature measurements (m)
602\np{rn\_zu}: is the height of wind measurements (m)
603The multiplicative factors to activate (value is 1) or deactivate (value is 0) :
604\np{rn\_pfac} for precipitations (total and snow)
605\np{rn\_efac} for evaporation
606\np{rn\_vfac} for for ice/ocean velocities in the calculation of wind stress 
607
608% -------------------------------------------------------------------------------------------------------------
609%        CLIO Bulk formulea
610% -------------------------------------------------------------------------------------------------------------
611\subsection    [CLIO Bulk formulea (\np{ln\_clio}=true)]
612            {CLIO Bulk formulea (\np{ln\_clio}=true, \mdl{sbcblk\_clio})}
613\label{SBC_blk_clio}
614%------------------------------------------namsbc_clio----------------------------------------------------
615\namdisplay{namsbc_clio} 
616%-------------------------------------------------------------------------------------------------------------
617
618The CLIO bulk formulae were developed several years ago for the
619Louvain-la-neuve coupled ice-ocean model (CLIO, \cite{Goosse_al_JGR99}).
620They are simpler bulk formulae. They assume the stress to be known and
621compute the radiative fluxes from a climatological cloud cover.
622
623Options are defined through the  \ngn{namsbc\_clio} namelist variables.
624The required 7 input fields are:
625
626%--------------------------------------------------TABLE--------------------------------------------------
627\begin{table}[htbp]   \label{Tab_CLIO}
628\begin{center}
629\begin{tabular}{|l|l|l|l|}
630\hline
631Variable desciption           & Model variable  & Units           & point \\  \hline
632i-component of the ocean stress     & utau         & $N.m^{-2}$         & U \\   \hline
633j-component of the ocean stress     & vtau         & $N.m^{-2}$         & V \\   \hline
634Wind speed module             & vatm         & $m.s^{-1}$         & T \\   \hline
63510m air temperature              & tair         & \r{}$K$            & T \\   \hline
636Specific humidity                & humi         & \%              & T \\   \hline
637Cloud cover                   &           & \%              & T \\   \hline
638Total precipitation (liquid + solid)   & precip    & $Kg.m^{-2}.s^{-1}$ & T \\   \hline
639Solid precipitation              & snow         & $Kg.m^{-2}.s^{-1}$ & T \\   \hline
640\end{tabular}
641\end{center}
642\end{table}
643%--------------------------------------------------------------------------------------------------------------
644
645As for the flux formulation, information about the input data required by the
646model is provided in the namsbc\_blk\_core or namsbc\_blk\_clio
647namelist (see \S\ref{SBC_fldread}).
648
649% -------------------------------------------------------------------------------------------------------------
650%        MFS Bulk formulae
651% -------------------------------------------------------------------------------------------------------------
652\subsection    [MFS Bulk formulea (\np{ln\_mfs}=true)]
653            {MFS Bulk formulea (\np{ln\_mfs}=true, \mdl{sbcblk\_mfs})}
654\label{SBC_blk_mfs}
655%------------------------------------------namsbc_mfs----------------------------------------------------
656\namdisplay{namsbc_mfs} 
657%----------------------------------------------------------------------------------------------------------
658
659The MFS (Mediterranean Forecasting System) bulk formulae have been developed by
660 \citet{Castellari_al_JMS1998}.
661They have been designed to handle the ECMWF operational data and are currently
662in use in the MFS operational system \citep{Tonani_al_OS08}, \citep{Oddo_al_OS09}.
663The wind stress computation uses a drag coefficient computed according to \citet{Hellerman_Rosenstein_JPO83}.
664The surface boundary condition for temperature involves the balance between surface solar radiation,
665net long-wave radiation, the latent and sensible heat fluxes.
666Solar radiation is dependent on cloud cover and is computed by means of
667an astronomical formula \citep{Reed_JPO77}. Albedo monthly values are from \citet{Payne_JAS72} 
668as means of the values at $40^{o}N$ and $30^{o}N$ for the Atlantic Ocean (hence the same latitudinal
669band of the Mediterranean Sea). The net long-wave radiation flux
670\citep{Bignami_al_JGR95} is a function of
671air temperature, sea-surface temperature, cloud cover and relative humidity.
672Sensible heat and latent heat fluxes are computed by classical
673bulk formulae parameterised according to \citet{Kondo1975}.
674Details on the bulk formulae used can be found in \citet{Maggiore_al_PCE98} and \citet{Castellari_al_JMS1998}.
675
676Options are defined through the  \ngn{namsbc\_mfs} namelist variables.
677The required 7 input fields must be provided on the model Grid-T and  are:
678\begin{itemize}
679\item          Zonal Component of the 10m wind ($ms^{-1}$)  (\np{sn\_windi})
680\item          Meridional Component of the 10m wind ($ms^{-1}$)  (\np{sn\_windj})
681\item          Total Claud Cover (\%)  (\np{sn\_clc})
682\item          2m Air Temperature ($K$) (\np{sn\_tair})
683\item          2m Dew Point Temperature ($K$)  (\np{sn\_rhm})
684\item          Total Precipitation ${Kg} m^{-2} s^{-1}$ (\np{sn\_prec})
685\item          Mean Sea Level Pressure (${Pa}$) (\np{sn\_msl})
686\end{itemize}
687% -------------------------------------------------------------------------------------------------------------
688% ================================================================
689% Coupled formulation
690% ================================================================
691\section  [Coupled formulation (\textit{sbccpl}) ]
692      {Coupled formulation (\mdl{sbccpl} module)}
693\label{SBC_cpl}
694%------------------------------------------namsbc_cpl----------------------------------------------------
695\namdisplay{namsbc_cpl} 
696%-------------------------------------------------------------------------------------------------------------
697
698In the coupled formulation of the surface boundary condition, the fluxes are
699provided by the OASIS coupler at a frequency which is defined in the OASIS coupler,
700while sea and ice surface temperature, ocean and ice albedo, and ocean currents
701are sent to the atmospheric component.
702
703A generalised coupled interface has been developed.
704It is currently interfaced with OASIS-3-MCT (\key{oasis3}).
705It has been successfully used to interface \NEMO to most of the European atmospheric
706GCM (ARPEGE, ECHAM, ECMWF, HadAM, HadGAM, LMDz),
707as well as to \href{http://wrf-model.org/}{WRF} (Weather Research and Forecasting Model).
708
709Note that in addition to the setting of \np{ln\_cpl} to true, the \key{coupled} have to be defined.
710The CPP key is mainly used in sea-ice to ensure that the atmospheric fluxes are
711actually recieved by the ice-ocean system (no calculation of ice sublimation in coupled mode).
712When PISCES biogeochemical model (\key{top} and \key{pisces}) is also used in the coupled system,
713the whole carbon cycle is computed by defining \key{cpl\_carbon\_cycle}. In this case,
714CO$_2$ fluxes will be exchanged between the atmosphere and the ice-ocean system (and need to be activated in \ngn{namsbc{\_}cpl} ).
715
716The namelist above allows control of various aspects of the coupling fields (particularly for
717vectors) and now allows for any coupling fields to have multiple sea ice categories (as required by LIM3
718and CICE).  When indicating a multi-category coupling field in namsbc{\_}cpl the number of categories will be
719determined by the number used in the sea ice model.  In some limited cases it may be possible to specify
720single category coupling fields even when the sea ice model is running with multiple categories - in this
721case the user should examine the code to be sure the assumptions made are satisfactory.  In cases where
722this is definitely not possible the model should abort with an error message.  The new code has been tested using
723ECHAM with LIM2, and HadGAM3 with CICE but although it will compile with \key{lim3} additional minor code changes
724may be required to run using LIM3.
725
726
727% ================================================================
728%        Atmospheric pressure
729% ================================================================
730\section   [Atmospheric pressure (\textit{sbcapr})]
731         {Atmospheric pressure (\mdl{sbcapr})}
732\label{SBC_apr}
733%------------------------------------------namsbc_apr----------------------------------------------------
734\namdisplay{namsbc_apr} 
735%-------------------------------------------------------------------------------------------------------------
736
737The optional atmospheric pressure can be used to force ocean and ice dynamics
738(\np{ln\_apr\_dyn}~=~true, \textit{\ngn{namsbc}} namelist ).
739The input atmospheric forcing defined via \np{sn\_apr} structure (\textit{namsbc\_apr} namelist)
740can be interpolated in time to the model time step, and even in space when the
741interpolation on-the-fly is used. When used to force the dynamics, the atmospheric
742pressure is further transformed into an equivalent inverse barometer sea surface height,
743$\eta_{ib}$, using:
744\begin{equation} \label{SBC_ssh_ib}
745   \eta_{ib} = -  \frac{1}{g\,\rho_o}  \left( P_{atm} - P_o \right)
746\end{equation}
747where $P_{atm}$ is the atmospheric pressure and $P_o$ a reference atmospheric pressure.
748A value of $101,000~N/m^2$ is used unless \np{ln\_ref\_apr} is set to true. In this case $P_o$ 
749is set to the value of $P_{atm}$ averaged over the ocean domain, $i.e.$ the mean value of
750$\eta_{ib}$ is kept to zero at all time step.
751
752The gradient of $\eta_{ib}$ is added to the RHS of the ocean momentum equation
753(see \mdl{dynspg} for the ocean). For sea-ice, the sea surface height, $\eta_m$,
754which is provided to the sea ice model is set to $\eta - \eta_{ib}$ (see \mdl{sbcssr} module).
755$\eta_{ib}$ can be set in the output. This can simplify altimetry data and model comparison
756as inverse barometer sea surface height is usually removed from these date prior to their distribution.
757
758When using time-splitting and BDY package for open boundaries conditions, the equivalent
759inverse barometer sea surface height $\eta_{ib}$ can be added to BDY ssh data:
760\np{ln\_apr\_obc}  might be set to true.
761
762% ================================================================
763%        Tidal Potential
764% ================================================================
765\section   [Tidal Potential (\textit{sbctide})]
766                        {Tidal Potential (\mdl{sbctide})}
767\label{SBC_tide}
768
769%------------------------------------------nam_tide---------------------------------------
770\namdisplay{nam_tide}
771%-----------------------------------------------------------------------------------------
772
773A module is available to compute the tidal potential and use it in the momentum equation.
774This option is activated when \key{tide} is defined.
775
776Some parameters are available in namelist \ngn{nam\_tide}:
777
778- \np{ln\_tide\_pot} activate the tidal potential forcing
779
780- \np{nb\_harmo} is the number of constituent used
781
782- \np{clname} is the name of constituent
783
784The tide is generated by the forces of gravity ot the Earth-Moon and Earth-Sun sytem;
785they are expressed as the gradient of the astronomical potential ($\vec{\nabla}\Pi_{a}$). \\
786
787The potential astronomical expressed, for the three types of tidal frequencies
788following, by : \\
789Tide long period :
790\begin{equation}
791\Pi_{a}=gA_{k}(\frac{1}{2}-\frac{3}{2}sin^{2}\phi)cos(\omega_{k}t+V_{0k})
792\end{equation}
793diurnal Tide :
794\begin{equation}
795\Pi_{a}=gA_{k}(sin 2\phi)cos(\omega_{k}t+\lambda+V_{0k})
796\end{equation}
797Semi-diurnal tide:
798\begin{equation}
799\Pi_{a}=gA_{k}(cos^{2}\phi)cos(\omega_{k}t+2\lambda+V_{0k})
800\end{equation}
801
802
803$A_{k}$ is the amplitude of the wave k, $\omega_{k}$ the pulsation of the wave k, $V_{0k}$ the astronomical phase of the wave
804$k$ to Greenwich.
805
806We make corrections to the astronomical potential.
807We obtain :
808\begin{equation}
809\Pi-g\delta = (1+k-h) \Pi_{A}(\lambda,\phi)
810\end{equation}
811with $k$ a number of Love estimated to 0.6 which parameterised the astronomical tidal land,
812and $h$ a number of Love to 0.3 which parameterised the parameterisation due to the astronomical tidal land.
813
814% ================================================================
815%        River runoffs
816% ================================================================
817\section   [River runoffs (\textit{sbcrnf})]
818         {River runoffs (\mdl{sbcrnf})}
819\label{SBC_rnf}
820%------------------------------------------namsbc_rnf----------------------------------------------------
821\namdisplay{namsbc_rnf} 
822%-------------------------------------------------------------------------------------------------------------
823
824%River runoff generally enters the ocean at a nonzero depth rather than through the surface.
825%Many models, however, have traditionally inserted river runoff to the top model cell.
826%This was the case in \NEMO prior to the version 3.3. The switch toward a input of runoff
827%throughout a nonzero depth has been motivated by the numerical and physical problems
828%that arise when the top grid cells are of the order of one meter. This situation is common in
829%coastal modelling and becomes more and more often open ocean and climate modelling
830%\footnote{At least a top cells thickness of 1~meter and a 3 hours forcing frequency are
831%required to properly represent the diurnal cycle \citep{Bernie_al_JC05}. see also \S\ref{SBC_dcy}.}.
832
833
834%To do this we need to treat evaporation/precipitation fluxes and river runoff differently in the
835%\mdl{tra\_sbc} module.  We decided to separate them throughout the code, so that the variable
836%\textit{emp} represented solely evaporation minus precipitation fluxes, and a new 2d variable
837%rnf was added which represents the volume flux of river runoff (in kg/m2s to remain consistent with
838%emp).  This meant many uses of emp and emps needed to be changed, a list of all modules which use
839%emp or emps and the changes made are below:
840
841
842%Rachel:
843River runoff generally enters the ocean at a nonzero depth rather than through the surface.
844Many models, however, have traditionally inserted river runoff to the top model cell.
845This was the case in \NEMO prior to the version 3.3, and was combined with an option
846to increase vertical mixing near the river mouth.
847
848However, with this method numerical and physical problems arise when the top grid cells are
849of the order of one meter. This situation is common in coastal modelling and is becoming
850more common in open ocean and climate modelling
851\footnote{At least a top cells thickness of 1~meter and a 3 hours forcing frequency are
852required to properly represent the diurnal cycle \citep{Bernie_al_JC05}. see also \S\ref{SBC_dcy}.}.
853
854As such from V~3.3 onwards it is possible to add river runoff through a non-zero depth, and for the
855temperature and salinity of the river to effect the surrounding ocean.
856The user is able to specify, in a NetCDF input file, the temperature and salinity of the river, along with the   
857depth (in metres) which the river should be added to.
858
859Namelist variables in \ngn{namsbc\_rnf}, \np{ln\_rnf\_depth}, \np{ln\_rnf\_sal} and \np{ln\_rnf\_temp} control whether
860the river attributes (depth, salinity and temperature) are read in and used.  If these are set
861as false the river is added to the surface box only, assumed to be fresh (0~psu), and/or
862taken as surface temperature respectively.
863
864The runoff value and attributes are read in in sbcrnf. 
865For temperature -999 is taken as missing data and the river temperature is taken to be the
866surface temperatue at the river point.
867For the depth parameter a value of -1 means the river is added to the surface box only,
868and a value of -999 means the river is added through the entire water column.
869After being read in the temperature and salinity variables are multiplied by the amount of runoff (converted into m/s)
870to give the heat and salt content of the river runoff.
871After the user specified depth is read ini, the number of grid boxes this corresponds to is
872calculated and stored in the variable \np{nz\_rnf}.
873The variable \textit{h\_dep} is then calculated to be the depth (in metres) of the bottom of the
874lowest box the river water is being added to (i.e. the total depth that river water is being added to in the model).
875
876%Christian:
877If the depth information is not provide in the NetCDF file, it can be estimate from the runoff input file at the initial time-step, by setting the namelist parameter \np{ln\_rnf\_depth\_ini} to true.
878
879This estimation is a simple linear relation between the runoff and a given depth :
880\begin{equation} 
881h\_dep  = \frac{rn\_dep\_max} {rn\_rnf\_max}  rnf
882\end{equation}
883where  \np{rn\_dep\_max} is the given maximum depth over which the runoffs is spread,
884 \np{rn\_rnf\_max} is the maximum value of the runoff climatologie over the global domain
885and rnf is the maximum value in time of the runoff climatology at each grid cell (computed online).
886
887The estimated depth array can be output if needed in a NetCDF file by setting the namelist parameter \np{nn\_rnf\_depth\_file} to 1.
888
889The mass/volume addition due to the river runoff is, at each relevant depth level, added to the horizontal divergence
890(\textit{hdivn}) in the subroutine \rou{sbc\_rnf\_div} (called from \mdl{divcur}).
891This increases the diffusion term in the vicinity of the river, thereby simulating a momentum flux.
892The sea surface height is calculated using the sum of the horizontal divergence terms, and so the
893river runoff indirectly forces an increase in sea surface height.
894
895The \textit{hdivn} terms are used in the tracer advection modules to force vertical velocities.
896This causes a mass of water, equal to the amount of runoff, to be moved into the box above.
897The heat and salt content of the river runoff is not included in this step, and so the tracer
898concentrations are diluted as water of ocean temperature and salinity is moved upward out of the box
899and replaced by the same volume of river water with no corresponding heat and salt addition.
900
901For the linear free surface case, at the surface box the tracer advection causes a flux of water
902(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.
903As such the volume of water does not change, but the water is diluted.
904
905For the non-linear free surface case (\key{vvl}), no flux is allowed through the surface.
906Instead in the surface box (as well as water moving up from the boxes below) a volume of runoff water
907is added with no corresponding heat and salt addition and so as happens in the lower boxes there is a dilution effect.
908(The runoff addition to the top box along with the water being moved up through boxes below means the surface box has a large
909increase in volume, whilst all other boxes remain the same size)
910
911In trasbc the addition of heat and salt due to the river runoff is added.
912This is done in the same way for both vvl and non-vvl.
913The temperature and salinity are increased through the specified depth according to the heat and salt content of the river.
914
915In the non-linear free surface case (vvl), near the end of the time step the change in sea surface height is redistrubuted
916through the grid boxes, so that the original ratios of grid box heights are restored.
917In 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.
918
919It 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.
920When 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.
921
922
923%\colorbox{yellow}{Nevertheless, Pb of vertical resolution and 3D input : increase vertical mixing near river mouths to mimic a 3D river
924
925%All river runoff and emp fluxes are assumed to be fresh water (zero salinity) and at the same temperature as the sea surface.}
926
927%\colorbox{yellow}{river mouths{\ldots}}
928
929%IF( ln_rnf ) THEN                                     ! increase diffusivity at rivers mouths
930%        DO jk = 2, nkrnf   ;   avt(:,:,jk) = avt(:,:,jk) + rn_avt_rnf * rnfmsk(:,:)   ;   END DO
931%ENDIF
932
933%\gmcomment{  word doc of runoffs:
934%
935%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.
936%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. 
937
938%The depth option makes it possible to have the river water affecting just the surface layer, throughout depth, or some specified point in between.
939
940%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:
941
942%}
943% ================================================================
944%        Ice shelf melting
945% ================================================================
946\section   [Ice shelf melting (\textit{sbcisf})]
947                        {Ice shelf melting (\mdl{sbcisf})}
948\label{SBC_isf}
949%------------------------------------------namsbc_isf----------------------------------------------------
950\namdisplay{namsbc_isf}
951%--------------------------------------------------------------------------------------------------------
952Namelist variable in \ngn{namsbc}, \np{nn\_isf}, controls the ice shelf representation used (Fig. \ref{Fig_SBC_isf}):
953
954%>>>>>>>>>>>>>>>>>>>>>>>>>>>>
955\begin{figure}[!h]    \begin{center}
956\includegraphics[width=0.8\textwidth]{Fig_SBC_isf}
957\caption{ \label{Fig_SBC_isf}
958Schematic for all the options available trough \np{nn\_isf}.}
959\end{center}   \end{figure}
960%>>>>>>>>>>>>>>>>>>>>>>>>>>>>
961
962\begin{description}
963\item[\np{nn\_isf}~=~0]
964The ice shelf routines are not used. The ice shelf melting is not computed or prescribed, the cavity have to be closed.
965If needed, the ice shelf melting should be added to the runoff or the precipitation file.
966
967\item[\np{nn\_isf}~=~1]
968The ice shelf cavity is represented. The fwf and heat flux are computed. Two different bulk formula are available:
969   \begin{description}
970   \item[\np{nn\_isfblk}~=~1]
971   The bulk formula used to compute the melt is based the one described in \citet{Hunter2006}.
972        This formulation is based on a balance between the upward ocean heat flux and the latent heat flux at the ice shelf base.
973
974   \item[\np{nn\_isfblk}~=~2] 
975   The bulk formula used to compute the melt is based the one described in \citet{Jenkins1991}.
976        This formulation is based on a 3 equations formulation (a heat flux budget, a salt flux budget and a linearised freezing point temperature equation).
977   \end{description}
978
979For this 2 bulk formulations, there are 3 different ways to compute the exchange coeficient:
980   \begin{description}
981        \item[\np{nn\_gammablk~=~0~}]
982   The salt and heat exchange coefficients are constant and defined by \np{rn\_gammas0} and \np{rn\_gammat0}
983
984   \item[\np{nn\_gammablk~=~1~}]
985   The salt and heat exchange coefficients are velocity dependent and defined as $\np{rn\_gammas0} \times u_{*}$ and $\np{rn\_gammat0} \times u_{*}$
986        where $u_{*}$ is the friction velocity in the top boundary layer (ie first \np{rn\_hisf\_tbl} meters).
987        See \citet{Jenkins2010} for all the details on this formulation.
988   
989   \item[\np{nn\_gammablk~=~2~}]
990   The salt and heat exchange coefficients are velocity and stability dependent and defined as
991        $\gamma_{T,S} = \frac{u_{*}}{\Gamma_{Turb} + \Gamma^{T,S}_{Mole}}$
992        where $u_{*}$ is the friction velocity in the top boundary layer (ie first \np{rn\_hisf\_tbl} meters),
993        $\Gamma_{Turb}$ the contribution of the ocean stability and
994        $\Gamma^{T,S}_{Mole}$ the contribution of the molecular diffusion.
995        See \citet{Holland1999} for all the details on this formulation.
996        \end{description}
997
998\item[\np{nn\_isf}~=~2]
999A parameterisation of isf is used. The ice shelf cavity is not represented.
1000The fwf is distributed along the ice shelf edge between the depth of the average grounding line (GL)
1001(\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).
1002Furthermore the fwf and heat flux are computed using the \citet{Beckmann2003} parameterisation of isf melting.
1003The effective melting length (\np{sn\_Leff\_isf}) is read from a file.
1004
1005\item[\np{nn\_isf}~=~3]
1006A simple parameterisation of isf is used. The ice shelf cavity is not represented.
1007The fwf (\np{sn\_rnfisf}) is prescribed and distributed along the ice shelf edge between the depth of the average grounding line (GL)
1008(\np{sn\_depmax\_isf}) and the base of the ice shelf along the calving front (\np{sn\_depmin\_isf}).
1009The heat flux ($Q_h$) is computed as $Q_h = fwf \times L_f$.
1010
1011\item[\np{nn\_isf}~=~4]
1012The ice shelf cavity is opened. However, the fwf is not computed but specified from file \np{sn\_fwfisf}).
1013The heat flux ($Q_h$) is computed as $Q_h = fwf \times L_f$.\\
1014\end{description}
1015
1016
1017$\bullet$ \np{nn\_isf}~=~1 and \np{nn\_isf}~=~2 compute a melt rate based on the water mass properties, ocean velocities and depth.
1018 This flux is thus highly dependent of the model resolution (horizontal and vertical), realism of the water masses onto the shelf ...\\
1019
1020$\bullet$ \np{nn\_isf}~=~3 and \np{nn\_isf}~=~4 read the melt rate from a file. You have total control of the fwf forcing.
1021This can be usefull if the water masses on the shelf are not realistic or the resolution (horizontal/vertical) are too
1022coarse to have realistic melting or for studies where you need to control your heat and fw input.\\ 
1023
1024Two namelist parameters control how the heat and fw fluxes are passed to NEMO: \np{rn\_hisf\_tbl} and \np{ln\_divisf}
1025\begin{description}
1026\item[\np{rn\_hisf\_tbl}] is the top boundary layer thickness as defined in \citet{Losch2008}.
1027This parameter is only used if \np{nn\_isf}~=~1 or \np{nn\_isf}~=~4
1028It allows you to control over which depth you want to spread the heat and fw fluxes.
1029
1030If \np{rn\_hisf\_tbl} = 0.0, the fluxes are put in the top level whatever is its tickness.
1031
1032If \np{rn\_hisf\_tbl} $>$ 0.0, the fluxes are spread over the first \np{rn\_hisf\_tbl} m (ie over one or several cells).
1033
1034\item[\np{ln\_divisf}] is a flag to apply the fw flux as a volume flux or as a salt flux.
1035
1036\np{ln\_divisf}~=~true applies the fwf as a volume flux. This volume flux is implemented with in the same way as for the runoff.
1037The fw addition due to the ice shelf melting is, at each relevant depth level, added to the horizontal divergence
1038(\textit{hdivn}) in the subroutine \rou{sbc\_isf\_div}, called from \mdl{divcur}.
1039See the runoff section \ref{SBC_rnf} for all the details about the divergence correction.
1040
1041\np{ln\_divisf}~=~false applies the fwf and heat flux directly on the salinity and temperature tendancy.
1042
1043\item[\np{ln\_conserve}] is a flag for \np{nn\_isf}~=~1. A conservative boundary layer scheme as described in \citet{Jenkins2001} 
1044is used if \np{ln\_conserve}=true. It takes into account the fact that the melt water is at freezing T and needs to be warm up to ocean temperature.
1045It is only relevant for \np{ln\_divisf}~=~false.
1046If \np{ln\_divisf}~=~true, \np{ln\_conserve} has to be set to false to avoid a double counting of the contribution.
1047 
1048\end{description}
1049%
1050% ================================================================
1051%        Handling of icebergs
1052% ================================================================
1053\section{Handling of icebergs (ICB)}
1054\label{ICB_icebergs}
1055%------------------------------------------namberg----------------------------------------------------
1056\namdisplay{namberg}
1057%-------------------------------------------------------------------------------------------------------------
1058
1059Icebergs are modelled as lagrangian particles in NEMO \citep{Marsh_GMD2015}.
1060Their physical behaviour is controlled by equations as described in \citet{Martin_Adcroft_OM10} ).
1061(Note that the authors kindly provided a copy of their code to act as a basis for implementation in NEMO).
1062Icebergs are initially spawned into one of ten classes which have specific mass and thickness as described
1063in the \ngn{namberg} namelist:
1064\np{rn\_initial\_mass} and \np{rn\_initial\_thickness}.
1065Each class has an associated scaling (\np{rn\_mass\_scaling}), which is an integer representing how many icebergs
1066of this class are being described as one lagrangian point (this reduces the numerical problem of tracking every single iceberg).
1067They are enabled by setting \np{ln\_icebergs}~=~true.
1068
1069Two initialisation schemes are possible.
1070\begin{description}
1071\item[\np{nn\_test\_icebergs}~$>$~0]
1072In this scheme, the value of \np{nn\_test\_icebergs} represents the class of iceberg to generate
1073(so between 1 and 10), and \np{nn\_test\_icebergs} provides a lon/lat box in the domain at each
1074grid point of which an iceberg is generated at the beginning of the run.
1075(Note that this happens each time the timestep equals \np{nn\_nit000}.)
1076\np{nn\_test\_icebergs} is defined by four numbers in \np{nn\_test\_box} representing the corners
1077of the geographical box: lonmin,lonmax,latmin,latmax
1078\item[\np{nn\_test\_icebergs}~=~-1]
1079In this scheme the model reads a calving file supplied in the \np{sn\_icb} parameter.
1080This should be a file with a field on the configuration grid (typically ORCA) representing ice accumulation rate at each model point.
1081These should be ocean points adjacent to land where icebergs are known to calve.
1082Most points in this input grid are going to have value zero.
1083When the model runs, ice is accumulated at each grid point which has a non-zero source term.
1084At 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).
1085Note that this is the initial mass multiplied by the number each particle represents ($i.e.$ the scaling).
1086If there is enough ice, a new iceberg is spawned and the total available ice reduced accordingly.
1087\end{description}
1088
1089Icebergs are influenced by wind, waves and currents, bottom melt and erosion.
1090The latter act to disintegrate the iceberg. This is either all melted freshwater, or
1091(if \np{rn\_bits\_erosion\_fraction}~$>$~0) into melt and additionally small ice bits
1092which are assumed to propagate with their larger parent and thus delay fluxing into the ocean.
1093Melt water (and other variables on the configuration grid) are written into the main NEMO model output files.
1094
1095Extensive diagnostics can be produced.
1096Separate output files are maintained for human-readable iceberg information.
1097A separate file is produced for each processor (independent of \np{ln\_ctl}).
1098The amount of information is controlled by two integer parameters:
1099\begin{description}
1100\item[\np{nn\_verbose\_level}]  takes a value between one and four and represents
1101an increasing number of points in the code at which variables are written, and an
1102increasing level of obscurity.
1103\item[\np{nn\_verbose\_write}] is the number of timesteps between writes
1104\end{description}
1105
1106Iceberg trajectories can also be written out and this is enabled by setting \np{nn\_sample\_rate}~$>$~0.
1107A non-zero value represents how many timesteps between writes of information into the output file.
1108These output files are in NETCDF format.
1109When \key{mpp\_mpi} is defined, each output file contains only those icebergs in the corresponding processor.
1110Trajectory points are written out in the order of their parent iceberg in the model's "linked list" of icebergs.
1111So care is needed to recreate data for individual icebergs, since its trajectory data may be spread across
1112multiple files.
1113
1114
1115% ================================================================
1116% Miscellanea options
1117% ================================================================
1118\section{Miscellaneous options}
1119\label{SBC_misc}
1120
1121% -------------------------------------------------------------------------------------------------------------
1122%        Diurnal cycle
1123% -------------------------------------------------------------------------------------------------------------
1124\subsection   [Diurnal  cycle (\textit{sbcdcy})]
1125         {Diurnal cycle (\mdl{sbcdcy})}
1126\label{SBC_dcy}
1127%------------------------------------------namsbc_rnf----------------------------------------------------
1128%\namdisplay{namsbc}
1129%-------------------------------------------------------------------------------------------------------------
1130
1131%>>>>>>>>>>>>>>>>>>>>>>>>>>>>
1132\begin{figure}[!t]    \begin{center}
1133\includegraphics[width=0.8\textwidth]{Fig_SBC_diurnal}
1134\caption{ \label{Fig_SBC_diurnal}   
1135Example of recontruction of the diurnal cycle variation of short wave flux 
1136from daily mean values. The reconstructed diurnal cycle (black line) is chosen
1137as the mean value of the analytical cycle (blue line) over a time step, not
1138as the mid time step value of the analytically cycle (red square). From \citet{Bernie_al_CD07}.}
1139\end{center}   \end{figure}
1140%>>>>>>>>>>>>>>>>>>>>>>>>>>>>
1141
1142\cite{Bernie_al_JC05} have shown that to capture 90$\%$ of the diurnal variability of
1143SST requires a vertical resolution in upper ocean of 1~m or better and a temporal resolution
1144of the surface fluxes of 3~h or less. Unfortunately high frequency forcing fields are rare,
1145not to say inexistent. Nevertheless, it is possible to obtain a reasonable diurnal cycle
1146of the SST knowning only short wave flux (SWF) at high frequency \citep{Bernie_al_CD07}.
1147Furthermore, only the knowledge of daily mean value of SWF is needed,
1148as higher frequency variations can be reconstructed from them, assuming that
1149the diurnal cycle of SWF is a scaling of the top of the atmosphere diurnal cycle
1150of incident SWF. The \cite{Bernie_al_CD07} reconstruction algorithm is available
1151in \NEMO by setting \np{ln\_dm2dc}~=~true (a \textit{\ngn{namsbc}} namelist variable) when using
1152CORE bulk formulea (\np{ln\_blk\_core}~=~true) or the flux formulation (\np{ln\_flx}~=~true).
1153The reconstruction is performed in the \mdl{sbcdcy} module. The detail of the algoritm used
1154can be found in the appendix~A of \cite{Bernie_al_CD07}. The algorithm preserve the daily
1155mean incomming SWF as the reconstructed SWF at a given time step is the mean value
1156of the analytical cycle over this time step (Fig.\ref{Fig_SBC_diurnal}).
1157The use of diurnal cycle reconstruction requires the input SWF to be daily
1158($i.e.$ a frequency of 24 and a time interpolation set to true in \np{sn\_qsr} namelist parameter).
1159Furthermore, it is recommended to have a least 8 surface module time step per day,
1160that is  $\rdt \ \np{nn\_fsbc} < 10,800~s = 3~h$. An example of recontructed SWF
1161is given in Fig.\ref{Fig_SBC_dcy} for a 12 reconstructed diurnal cycle, one every 2~hours
1162(from 1am to 11pm).
1163
1164%>>>>>>>>>>>>>>>>>>>>>>>>>>>>
1165\begin{figure}[!t]  \begin{center}
1166\includegraphics[width=0.7\textwidth]{Fig_SBC_dcy}
1167\caption{ \label{Fig_SBC_dcy}   
1168Example of recontruction of the diurnal cycle variation of short wave flux 
1169from daily mean values on an ORCA2 grid with a time sampling of 2~hours (from 1am to 11pm).
1170The display is on (i,j) plane. }
1171\end{center}   \end{figure}
1172%>>>>>>>>>>>>>>>>>>>>>>>>>>>>
1173
1174Note also that the setting a diurnal cycle in SWF is highly recommended  when
1175the top layer thickness approach 1~m or less, otherwise large error in SST can
1176appear due to an inconsistency between the scale of the vertical resolution
1177and the forcing acting on that scale.
1178
1179% -------------------------------------------------------------------------------------------------------------
1180%        Rotation of vector pairs onto the model grid directions
1181% -------------------------------------------------------------------------------------------------------------
1182\subsection{Rotation of vector pairs onto the model grid directions}
1183\label{SBC_rotation}
1184
1185When using a flux (\np{ln\_flx}=true) or bulk (\np{ln\_clio}=true or \np{ln\_core}=true) formulation,
1186pairs of vector components can be rotated from east-north directions onto the local grid directions. 
1187This is particularly useful when interpolation on the fly is used since here any vectors are likely to be defined
1188relative to a rectilinear grid.
1189To activate this option a non-empty string is supplied in the rotation pair column of the relevant namelist.
1190The eastward component must start with "U" and the northward component with "V". 
1191The remaining characters in the strings are used to identify which pair of components go together.
1192So for example, strings "U1" and "V1" next to "utau" and "vtau" would pair the wind stress components together
1193and rotate them on to the model grid directions; "U2" and "V2" could be used against a second pair of components,
1194and so on.
1195The extra characters used in the strings are arbitrary.
1196The rot\_rep routine from the \mdl{geo2ocean} module is used to perform the rotation.
1197
1198% -------------------------------------------------------------------------------------------------------------
1199%        Surface restoring to observed SST and/or SSS
1200% -------------------------------------------------------------------------------------------------------------
1201\subsection    [Surface restoring to observed SST and/or SSS (\textit{sbcssr})]
1202         {Surface restoring to observed SST and/or SSS (\mdl{sbcssr})}
1203\label{SBC_ssr}
1204%------------------------------------------namsbc_ssr----------------------------------------------------
1205\namdisplay{namsbc_ssr} 
1206%-------------------------------------------------------------------------------------------------------------
1207
1208IOptions are defined through the  \ngn{namsbc\_ssr} namelist variables.
1209n forced mode using a flux formulation (\np{ln\_flx}~=~true), a
1210feedback term \emph{must} be added to the surface heat flux $Q_{ns}^o$:
1211\begin{equation} \label{Eq_sbc_dmp_q}
1212Q_{ns} = Q_{ns}^o + \frac{dQ}{dT} \left( \left. T \right|_{k=1} - SST_{Obs} \right)
1213\end{equation}
1214where SST is a sea surface temperature field (observed or climatological), $T$ is
1215the model surface layer temperature and $\frac{dQ}{dT}$ is a negative feedback
1216coefficient usually taken equal to $-40~W/m^2/K$. For a $50~m$ 
1217mixed-layer depth, this value corresponds to a relaxation time scale of two months.
1218This term ensures that if $T$ perfectly matches the supplied SST, then $Q$ is
1219equal to $Q_o$.
1220
1221In the fresh water budget, a feedback term can also be added. Converted into an
1222equivalent freshwater flux, it takes the following expression :
1223
1224\begin{equation} \label{Eq_sbc_dmp_emp}
1225\textit{emp} = \textit{emp}_o + \gamma_s^{-1} e_{3t}  \frac{  \left(\left.S\right|_{k=1}-SSS_{Obs}\right)}
1226                                             {\left.S\right|_{k=1}}
1227\end{equation}
1228
1229where $\textit{emp}_{o }$ is a net surface fresh water flux (observed, climatological or an
1230atmospheric model product), \textit{SSS}$_{Obs}$ is a sea surface salinity (usually a time
1231interpolation of the monthly mean Polar Hydrographic Climatology \citep{Steele2001}),
1232$\left.S\right|_{k=1}$ is the model surface layer salinity and $\gamma_s$ is a negative
1233feedback coefficient which is provided as a namelist parameter. Unlike heat flux, there is no
1234physical justification for the feedback term in \ref{Eq_sbc_dmp_emp} as the atmosphere
1235does not care about ocean surface salinity \citep{Madec1997}. The SSS restoring
1236term should be viewed as a flux correction on freshwater fluxes to reduce the
1237uncertainties we have on the observed freshwater budget.
1238
1239% -------------------------------------------------------------------------------------------------------------
1240%        Handling of ice-covered area
1241% -------------------------------------------------------------------------------------------------------------
1242\subsection{Handling of ice-covered area  (\textit{sbcice\_...})}
1243\label{SBC_ice-cover}
1244
1245The presence at the sea surface of an ice covered area modifies all the fluxes
1246transmitted to the ocean. There are several way to handle sea-ice in the system
1247depending on the value of the \np{nn\_ice} namelist parameter found in \ngn{namsbc} namelist. 
1248\begin{description}
1249\item[nn{\_}ice = 0]  there will never be sea-ice in the computational domain.
1250This is a typical namelist value used for tropical ocean domain. The surface fluxes
1251are simply specified for an ice-free ocean. No specific things is done for sea-ice.
1252\item[nn{\_}ice = 1]  sea-ice can exist in the computational domain, but no sea-ice model
1253is used. An observed ice covered area is read in a file. Below this area, the SST is
1254restored to the freezing point and the heat fluxes are set to $-4~W/m^2$ ($-2~W/m^2$)
1255in the northern (southern) hemisphere. The associated modification of the freshwater
1256fluxes are done in such a way that the change in buoyancy fluxes remains zero.
1257This prevents deep convection to occur when trying to reach the freezing point
1258(and so ice covered area condition) while the SSS is too large. This manner of
1259managing sea-ice area, just by using si IF case, is usually referred as the \textit{ice-if} 
1260model. It can be found in the \mdl{sbcice{\_}if} module.
1261\item[nn{\_}ice = 2 or more]  A full sea ice model is used. This model computes the
1262ice-ocean fluxes, that are combined with the air-sea fluxes using the ice fraction of
1263each model cell to provide the surface ocean fluxes. Note that the activation of a
1264sea-ice model is is done by defining a CPP key (\key{lim2}, \key{lim3} or \key{cice}).
1265The activation automatically overwrites the read value of nn{\_}ice to its appropriate
1266value ($i.e.$ $2$ for LIM-2, $3$ for LIM-3 or $4$ for CICE).
1267\end{description}
1268
1269% {Description of Ice-ocean interface to be added here or in LIM 2 and 3 doc ?}
1270
1271\subsection   [Interface to CICE (\textit{sbcice\_cice})]
1272         {Interface to CICE (\mdl{sbcice\_cice})}
1273\label{SBC_cice}
1274
1275It is now possible to couple a regional or global NEMO configuration (without AGRIF) to the CICE sea-ice
1276model by using \key{cice}.  The CICE code can be obtained from
1277\href{http://oceans11.lanl.gov/trac/CICE/}{LANL} and the additional 'hadgem3' drivers will be required,
1278even with the latest code release.  Input grid files consistent with those used in NEMO will also be needed,
1279and CICE CPP keys \textbf{ORCA\_GRID}, \textbf{CICE\_IN\_NEMO} and \textbf{coupled} should be used (seek advice from UKMO
1280if necessary).  Currently the code is only designed to work when using the CORE forcing option for NEMO (with
1281\textit{calc\_strair~=~true} and \textit{calc\_Tsfc~=~true} in the CICE name-list), or alternatively when NEMO
1282is coupled to the HadGAM3 atmosphere model (with \textit{calc\_strair~=~false} and \textit{calc\_Tsfc~=~false}).
1283The code is intended to be used with \np{nn\_fsbc} set to 1 (although coupling ocean and ice less frequently
1284should work, it is possible the calculation of some of the ocean-ice fluxes needs to be modified slightly - the
1285user should check that results are not significantly different to the standard case).
1286
1287There are two options for the technical coupling between NEMO and CICE.  The standard version allows
1288complete flexibility for the domain decompositions in the individual models, but this is at the expense of global
1289gather and scatter operations in the coupling which become very expensive on larger numbers of processors. The
1290alternative option (using \key{nemocice\_decomp} for both NEMO and CICE) ensures that the domain decomposition is
1291identical in both models (provided domain parameters are set appropriately, and
1292\textit{processor\_shape~=~square-ice} and \textit{distribution\_wght~=~block} in the CICE name-list) and allows
1293much more efficient direct coupling on individual processors.  This solution scales much better although it is at
1294the expense of having more idle CICE processors in areas where there is no sea ice.
1295
1296% -------------------------------------------------------------------------------------------------------------
1297%        Freshwater budget control
1298% -------------------------------------------------------------------------------------------------------------
1299\subsection   [Freshwater budget control (\textit{sbcfwb})]
1300         {Freshwater budget control (\mdl{sbcfwb})}
1301\label{SBC_fwb}
1302
1303For global ocean simulation it can be useful to introduce a control of the mean sea
1304level in order to prevent unrealistic drift of the sea surface height due to inaccuracy
1305in the freshwater fluxes. In \NEMO, two way of controlling the the freshwater budget.
1306\begin{description}
1307\item[\np{nn\_fwb}=0]  no control at all. The mean sea level is free to drift, and will
1308certainly do so.
1309\item[\np{nn\_fwb}=1]  global mean \textit{emp} set to zero at each model time step.
1310%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).
1311\item[\np{nn\_fwb}=2]  freshwater budget is adjusted from the previous year annual
1312mean budget which is read in the \textit{EMPave\_old.dat} file. As the model uses the
1313Boussinesq approximation, the annual mean fresh water budget is simply evaluated
1314from the change in the mean sea level at January the first and saved in the
1315\textit{EMPav.dat} file.
1316\end{description}
1317
1318% -------------------------------------------------------------------------------------------------------------
1319%        Neutral Drag Coefficient from external wave model
1320% -------------------------------------------------------------------------------------------------------------
1321\subsection   [Neutral drag coefficient from external wave model (\textit{sbcwave})]
1322              {Neutral drag coefficient from external wave model (\mdl{sbcwave})}
1323\label{SBC_wave}
1324%------------------------------------------namwave----------------------------------------------------
1325\namdisplay{namsbc_wave}
1326%-------------------------------------------------------------------------------------------------------------
1327
1328In order to read a neutral drag coeff, from an external data source ($i.e.$ a wave model), the
1329logical variable \np{ln\_cdgw} in \ngn{namsbc} namelist must be set to \textit{true}.
1330The \mdl{sbcwave} module containing the routine \np{sbc\_wave} reads the
1331namelist \ngn{namsbc\_wave} (for external data names, locations, frequency, interpolation and all
1332the miscellanous options allowed by Input Data generic Interface see \S\ref{SBC_input})
1333and a 2D field of neutral drag coefficient.
1334Then using the routine TURB\_CORE\_1Z or TURB\_CORE\_2Z, and starting from the neutral drag coefficent provided,
1335the drag coefficient is computed according to stable/unstable conditions of the air-sea interface following \citet{Large_Yeager_Rep04}.
1336
1337
1338% Griffies doc:
1339% When running ocean-ice simulations, we are not explicitly representing land processes,
1340% such as rivers, catchment areas, snow accumulation, etc. However, to reduce model drift,
1341% it is important to balance the hydrological cycle in ocean-ice models.
1342% We thus need to prescribe some form of global normalization to the precipitation minus evaporation plus river runoff.
1343% The result of the normalization should be a global integrated zero net water input to the ocean-ice system over
1344% a chosen time scale.
1345%How often the normalization is done is a matter of choice. In mom4p1, we choose to do so at each model time step,
1346% so that there is always a zero net input of water to the ocean-ice system.
1347% Others choose to normalize over an annual cycle, in which case the net imbalance over an annual cycle is used
1348% to alter the subsequent year�s water budget in an attempt to damp the annual water imbalance.
1349% Note that the annual budget approach may be inappropriate with interannually varying precipitation forcing.
1350% When running ocean-ice coupled models, it is incorrect to include the water transport between the ocean
1351% and ice models when aiming to balance the hydrological cycle.
1352% 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,
1353% not the water in any one sub-component. As an extreme example to illustrate the issue,
1354% consider an ocean-ice model with zero initial sea ice. As the ocean-ice model spins up,
1355% there should be a net accumulation of water in the growing sea ice, and thus a net loss of water from the ocean.
1356% The total water contained in the ocean plus ice system is constant, but there is an exchange of water between
1357% the subcomponents. This exchange should not be part of the normalization used to balance the hydrological cycle
1358% in ocean-ice models.
1359
1360
1361\end{document}
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