chap_SBC.tex

\documentclass[../main/NEMO_manual]{subfiles}

\begin{document}

\chapter{Surface Boundary Condition (SBC, SAS, ISF, ICB, TDE)}
\label{chap:SBC}

\chaptertoc

\paragraph{Changes record} ~\\

{\footnotesize
  \begin{tabularx}{\textwidth}{l||X|X}
    Release & Author(s) & Modifications \\
    \hline
    {\em  next} & {\em A. Moulin, E. Clementi} & {\em Update of \autoref{sec:SBC_wave}}\\[2mm]
    {\em  next} & {\em Simon M{\" u}ller} & {\em Update of \autoref{sec:SBC_TDE}; revision of \autoref{subsec:SBC_fwb}}\\[2mm]
    {\em  next} & {\em Pierre Mathiot} & {\em update of the ice shelf section (2019 developments)}\\[2mm]  
    {\em   4.0} & {\em ...} & {\em ...} \\
    {\em   3.6} & {\em ...} & {\em ...} \\
    {\em   3.4} & {\em ...} & {\em ...} \\
    {\em <=3.4} & {\em ...} & {\em ...}
  \end{tabularx}
}

\clearpage

\begin{listing}
  \nlst{namsbc}
  \caption{\forcode{&namsbc}}
  \label{lst:namsbc}
\end{listing}

The ocean needs seven fields as surface boundary condition:

\begin{itemize}
\item the two components of the surface ocean stress $\left( {\tau_u \;,\;\tau_v} \right)$
\item the incoming solar and non solar heat fluxes $\left( {Q_{ns} \;,\;Q_{sr} } \right)$
\item the surface freshwater budget $\left( {\textit{emp}} \right)$
\item the surface salt flux associated with freezing/melting of seawater $\left( {\textit{sfx}} \right)$
\item the atmospheric pressure at the ocean surface $\left( p_a \right)$
\end{itemize}

Five different ways are available to provide these fields to the ocean. They are controlled by
namelist \nam{sbc}{sbc} variables:

\begin{itemize}
\item a bulk formulation (\np[=.true.]{ln_blk}{ln\_blk}), featuring a selection of five bulk parameterization algorithms,
\item an atmospheric boundary layer model (\np[=.true.]{ln_abl}{ln\_abl}) associated with the bulk formulation,
\item a flux formulation (\np[=.true.]{ln_flx}{ln\_flx}),
\item a coupled or mixed forced/coupled formulation (exchanges with a atmospheric model via the OASIS coupler),
(\np{ln_cpl}{ln\_cpl} or \np[=.true.]{ln_mixcpl}{ln\_mixcpl}),
\item a user defined formulation (\np[=.true.]{ln_usr}{ln\_usr}).
\end{itemize}

The frequency at which the forcing fields have to be updated is given by the \np{nn_fsbc}{nn\_fsbc} namelist parameter.

When the fields are supplied from data files (bulk, abl, flux and mixed formulations),
the input fields do not need to be supplied on the model grid.
Instead, a file of coordinates and weights can be supplied to map the data from the input fields grid to
the model points (so called "Interpolation on the Fly", see \autoref{subsec:SBC_iof}).
If the "Interpolation on the Fly" option is used, input data belonging to land points (in the native grid)
should be masked or filled to avoid spurious results in proximity of the coasts, as
large sea-land gradients characterize most of the atmospheric variables.

In addition, the resulting fields can be further modified using several namelist options.
These options control:

\begin{itemize}
\item the rotation of vector components supplied relative to an east-north coordinate system onto
  the local grid directions in the model,
\item the use of a land/sea mask for input fields (\np[=.true.]{nn_lsm}{nn\_lsm}),
\item the addition of a surface restoring term to observed SST and/or SSS (\np[=.true.]{ln_ssr}{ln\_ssr}),
\item the modification of fluxes below ice-covered areas (using climatological ice-cover or a sea-ice model)
  (\np[=0..3]{nn_ice}{nn\_ice}),
\item the addition of river runoffs as surface freshwater fluxes or lateral inflow (\np[=.true.]{ln_rnf}{ln\_rnf}),
\item the addition of a freshwater flux adjustment in order to avoid a mean sea-level drift
  (\np[=0..2]{nn_fwb}{nn\_fwb}),
\item the transformation of the solar radiation (if provided as daily mean) into an analytical diurnal cycle
  (\np[=.true.]{ln_dm2dc}{ln\_dm2dc}),
\item the activation of wave effects from an external wave model  (\np[=.true.]{ln_wave}{ln\_wave}),
\item the light penetration in the ocean (\np[=.true.]{ln_traqsr}{ln\_traqsr} with \nam{tra_qsr}{tra\_qsr}),
\item the atmospheric surface pressure gradient effect on ocean and ice dynamics (\np[=.true.]{ln_apr_dyn}{ln\_apr\_dyn} with namelist \nam{sbc_apr}{sbc\_apr}),
\item the effect of sea-ice pressure on the ocean (\np[=.true.]{ln_ice_embd}{ln\_ice\_embd}).
\end{itemize}

In this chapter, we first discuss where the surface boundary conditions appear in the model equations.
Then we present the four ways of providing the surface boundary conditions,
followed by the description of the atmospheric pressure and the river runoff.
Next, the scheme for interpolation on the fly is described.
Finally, the different options that further modify the fluxes applied to the ocean are discussed.
One of these is modification by icebergs (see \autoref{sec:SBC_ICB_icebergs}),
which act as drifting sources of fresh water.

%% =================================================================================================
\section{Surface boundary condition for the ocean}
\label{sec:SBC_ocean}

The surface ocean stress is the stress exerted by the wind and the sea-ice on the ocean.
It is applied in \mdl{dynzdf} module as a surface boundary condition of the computation of
the momentum vertical mixing trend (see \autoref{eq:DYN_zdf_sbc} in \autoref{sec:DYN_zdf}).
As such, it has to be provided as a 2D vector interpolated onto the horizontal velocity ocean mesh,
\ie\ resolved onto the model (\textbf{i},\textbf{j}) direction at $u$- and $v$-points.

The surface heat flux is decomposed into two parts, a non solar and a solar heat flux,
$Q_{ns}$ and $Q_{sr}$, respectively.
The former is the non penetrative part of the heat flux
(\ie\ the sum of sensible, latent and long wave heat fluxes plus
the heat content of the mass exchange between the ocean and sea-ice).
It is applied in \mdl{trasbc} module as a surface boundary condition trend of
the first level temperature time evolution equation
(see \autoref{eq:TRA_sbc} and \autoref{eq:TRA_sbc_lin} in \autoref{subsec:TRA_sbc}).
The latter is the penetrative part of the heat flux.
It is applied as a 3D trend of the temperature equation (\mdl{traqsr} module) when
\np[=.true.]{ln_traqsr}{ln\_traqsr}.
The way the light penetrates inside the water column is generally a sum of decreasing exponentials
(see \autoref{subsec:TRA_qsr}).

The surface freshwater budget is provided by the \textit{emp} field.
It represents the mass flux exchanged with the atmosphere (evaporation minus precipitation) and
possibly with the sea-ice and ice shelves (freezing minus melting of ice).
It affects the ocean in two different ways:
$(i)$  it changes the volume of the ocean, and therefore appears in the sea surface height equation as		%GS: autoref ssh equation to be added
a volume flux, and
$(ii)$ it changes the surface temperature and salinity through the heat and salt contents of
the mass exchanged with atmosphere, sea-ice and ice shelves.

%\colorbox{yellow}{Miss: }
%A extensive description of all namsbc namelist (parameter that have to be
%created!)
%Especially the \np{nn_fsbc}{nn\_fsbc}, the \mdl{sbc\_oce} module (fluxes + mean sst sss ssu
%ssv) \ie\ information required by flux computation or sea-ice
%\mdl{sbc\_oce} containt the definition in memory of the 7 fields (6+runoff), add
%a word on runoff: included in surface bc or add as lateral obc{\ldots}.
%Sbcmod manage the ``providing'' (fourniture) to the ocean the 7 fields
%Fluxes update only each nf\_sbc time step (namsbc) explain relation
%between nf\_sbc and nf\_ice, do we define nf\_blk??? ? only one
%nf\_sbc
%Explain here all the namlist namsbc variable{\ldots}.
% explain : use or not of surface currents
%\colorbox{yellow}{End Miss }

The ocean model provides, at each time step, to the surface module (\mdl{sbcmod})
the surface currents, temperature and salinity.
These variables are averaged over \np{nn_fsbc}{nn\_fsbc} time-step (\autoref{tab:SBC_ssm}), and
these averaged fields are used to compute the surface fluxes at the frequency of \np{nn_fsbc}{nn\_fsbc} time-steps.

\begin{table}[tb]
  \centering
  \begin{tabular}{|l|l|l|l|}
    \hline
    Variable description			                  & Model variable	& Units	& point                 \\
    \hline
    i-component of the surface current	& ssu\_m	              & $m.s^{-1}$	    & U     \\
    \hline
    j-component of the surface current	& ssv\_m	              & $m.s^{-1}$	    & V     \\
    \hline
    Sea surface temperature			         & sst\_m	              & \r{}$K$	             & T     \\\hline
    Sea surface salinty			                  & sss\_m	              & $psu$		        & T     \\	\hline
  \end{tabular}
  \caption[Ocean variables provided to the surface module)]{
    Ocean variables provided to the surface module (\texttt{SBC}).
    The variable are averaged over \protect\np{nn_fsbc}{nn\_fsbc} time-step,
    \ie\ the frequency of computation of surface fluxes.}
  \label{tab:SBC_ssm}
\end{table}

%\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


%% =================================================================================================
\section{Input data generic interface}
\label{sec:SBC_input}

A generic interface has been introduced to manage the way input data
(2D or 3D fields, like surface forcing or ocean T and S) are specified in \NEMO.
This task is achieved by \mdl{fldread}.
The module is designed with four main objectives in mind:
\begin{enumerate}
\item optionally provide a time interpolation of the input data every specified model time-step, whatever their input frequency is,
  and according to the different calendars available in the model.
\item optionally provide an on-the-fly space interpolation from the native input data grid to the model grid.
\item make the run duration independent from the period cover by the input files.
\item provide a simple user interface and a rather simple developer interface by
  limiting the number of prerequisite informations.
\end{enumerate}

As a result, the user has only to fill in for each variable a structure in the namelist file to
define the input data file and variable names, the frequency of the data (in hours or months),
whether its is climatological data or not, the period covered by the input file (one year, month, week or day),
and three additional parameters for the on-the-fly interpolation.
When adding a new input variable, the developer has to add the associated structure in the namelist,
read this information by mirroring the namelist read in \rou{sbc\_blk\_init} for example,
and simply call \rou{fld\_read} to obtain the desired input field at the model time-step and grid points.

The only constraints are that the input file is a NetCDF file, the file name follows a nomenclature
(see \autoref{subsec:SBC_fldread}), the period it cover is one year, month, week or day, and,
if on-the-fly interpolation is used, a file of weights must be supplied (see \autoref{subsec:SBC_iof}).

Note that when an input data is archived on a disc which is accessible directly from the workspace where