
% ================================================================
% INTRODUCTION
% ================================================================

\chapter{Introduction}

The Nucleus for European Modelling of the Ocean (\NEMO) is a framework of ocean 
related engines, namely OPA\footnote{OPA = Oc\'{e}an PArall\'{e}lis\'{e}} for the 
ocean dynamics and thermodynamics, LIM\footnote{LIM= Louvain)la-neuve Ice 
Model} for the sea-ice dynamics and thermodynamics, TOP\footnote{TOP = Tracer 
in the Ocean Paradigm} for the biogeochemistry (both transport (TRP) and sources 
minus sinks (LOBSTER, PISCES)\footnote{Both LOBSTER and PISCES are not 
acronyms just name}. It is intended to be a flexible tool for studying the ocean and 
its interactions with the other components of the earth climate system (atmosphere, 
sea-ice, biogeochemical tracers, ...) over a wide range of space and time scales. 
This documentation provides information about the physics represented by the ocean 
component of \NEMO and the rationale for the choice of numerical schemes and 
the model design. More specific information about running the model on different 
computers, or how to set up a configuration, are found on the \NEMO web site 
(www.locean-ipsl.upmc.fr/NEMO). 

The ocean component of \NEMO has been developed from the OPA model, 
release 8.2, described in \citet{Madec1998}. This model has been used for a wide 
range of applications, both regional or global, as a forced ocean model and as a 
model coupled with the atmosphere. A complete list of references is found on the 
\NEMO web site. 

This manual is organised in as follows. Chapter~\ref{PE} presents the model basics, 
$i.e.$ the equations and their assumptions, the vertical coordinates used, and the 
subgrid scale physics. This part deals with the continuous equations of the model 
(primitive equations, with potential temperature, salinity and an equation of state). 
The equations are written in a curvilinear coordinate system, with a choice of vertical 
coordinates ($z$ or $s$, with the rescaled height coordinate formulation \textit{z*}, or  
\textit{s*}). Momentum equations are formulated in the vector invariant form or in the 
flux form. Dimensional units in the meter, kilogram, second (MKS) international system 
are used throughout.

The following chapters deal with the discrete equations. Chapter~\ref{STP} presents the 
time domain. The model time stepping environment is a three level scheme in which 
the tendency terms of the equations are evaluated either centered  in time, or forward, 
or backward depending of the nature of the term.
Chapter~\ref{DOM} presents the space domain. The model is discretised on a staggered 
grid (Arakawa C grid) with masking of land areas. Vertical discretisation used depends 
on both how the bottom topography is represented and whether the free surface is linear or not. 
Full step or partial step $z$-coordinate or $s$- (terrain-following) coordinate is used 
with linear free surface (level position are then fixed in time). In non-linear free surface, 
the corresponding rescaled height coordinate formulation (\textit{z*} or \textit{s*}) is used 
(the level position then vary in time as a function of the sea surface heigh). 
The following two chapters (\ref{TRA} and \ref{DYN}) describe the discretisation of the 
prognostic equations for the active tracers and the momentum. Explicit, split-explicit 
and filtered free surface formulations are implemented. 
A number of numerical schemes are available for momentum advection, for the computation 
of the pressure gradients, as well as for the advection of tracers (second or higher 
order advection schemes, including positive ones).

Surface boundary conditions (chapter~\ref{SBC}) can be implemented as prescribed
fluxes, or bulk formulations for the surface fluxes (wind stress, heat, freshwater). The 
model allows penetration of solar radiation  There is an optional geothermal heating at 
the ocean bottom. Within the \NEMO system the ocean model is interactively coupled 
with a sea ice model (LIM) and with biogeochemistry models (PISCES, LOBSTER). 
Interactive coupling to Atmospheric models is possible via the OASIS coupler 
\citep{OASIS2006}. Two-way nesting is also available through an interface to the
AGRIF package (Adaptative Grid Refinement in \textsc{Fortran}) \citep{Debreu_al_CG2008}.
The interface code for coupling to an alternative sea ice model (CICE, \citet{Hunke2008}) is now 
available although this is currently only designed for global domains, without the use of AGRIF.

Other model characteristics are the lateral boundary conditions (chapter~\ref{LBC}).  
Global configurations of the model make use of the ORCA tripolar grid, with special north 
fold boundary condition. Free-slip or no-slip boundary conditions are allowed at land 
boundaries. Closed basin geometries as well as periodic domains and open boundary 
conditions are possible. 

Physical parameterisations are described in chapters~\ref{LDF} and \ref{ZDF}. The 
model includes an implicit treatment of vertical viscosity and diffusivity. The lateral 
Laplacian and biharmonic viscosity and diffusion can be rotated following a geopotential 
or neutral direction. There is an optional eddy induced velocity \citep{Gent1990} with a 
space and time variable coefficient \citet{Treguier1997}. The model has vertical harmonic 
viscosity and diffusion with a space and time variable coefficient, with options to compute 
the coefficients with \citet{Blanke1993}, \citet{Large_al_RG94}, \citet{Pacanowski_Philander_JPO81}, 
or \citet{Umlauf_Burchard_JMS03} mixing schemes.

Model outputs management and specific online diagnostics are described in chapters~\ref{DIA}.
The diagnostics includes the output of all the tendencies of the momentum and tracers equations, 
the output of tracers tendencies averaged over the time evolving mixed layer, the output of 
the tendencies of the barotropic vorticity equation, the computation of on-line floats trajectories... 
Chapter~\ref{OBS} describes a tool which reads in observation files (profile temperature 
and salinity, sea surface temperature, sea level anomaly and sea ice concentration) 
and calculates an interpolated model equivalent value at the observation location 
and nearest model timestep. Originally developed of data assimilation, it is a fantastic 
tool for model and data comparison. Chapter~\ref{ASM} describes how increments 
produced by data assimilation may be applied to the model equations.
Finally, Chapter~\ref{CFG} provides a brief introduction to the pre-defined model 
configurations (water column model, ORCA and GYRE families of configurations).

The model is implemented in \textsc{Fortran 90}, with preprocessing (C-pre-processor). 
It runs under UNIX. It is optimized for vector computers and parallelised by domain  
decomposition with MPI. All input and output is done in NetCDF (Network Common Data 
Format) with a optional direct access format for output. To ensure the clarity and 
readability of the code it is necessary to follow coding rules. The coding rules for OPA 
include conventions for naming variables, with different starting letters for different types 
of variables (real, integer, parameter\ldots). Those rules are briefly presented in 
Appendix~\ref{Apdx_D} and a more complete document is available on the \NEMO web site.

The model is organized with a high internal modularity based on physics. For example, 
each trend ($i.e.$, a term in the RHS of the prognostic equation) for momentum and 
tracers is computed in a dedicated module.  To make it easier for the user to find his way 
around the code, the module names follow a three-letter rule. For example, \mdl{traldf} 
is a module related to the TRAcers equation, computing the Lateral DiFfussion. 
%The complete list of module names is presented in Appendix~\ref{Apdx_D}.      %====>>>> to be done !
Furthermore, modules are organized in a few directories that correspond to their category, 
as indicated by the first three letters of their name (Tab.~\ref{Tab_chap}). 

The manual mirrors the organization of the model. 
After the presentation of the continuous equations (Chapter \ref{PE}), the following chapters 
refer to specific terms of the equations each associated with a group of modules (Tab.~\ref{Tab_chap}).


%--------------------------------------------------TABLE--------------------------------------------------
\begin{table}[!t]	
%\begin{center} \begin{tabular}{|p{143pt}|l|l|}	\hline
\caption{ \label{Tab_chap}   Organization of Chapters mimicking the one of the model directories. }
\begin{center} 	\begin{tabular}{|l|l|l|}	\hline
Chapter \ref{STP}	& -   		        & model time STePping environment \\		\hline
Chapter \ref{DOM}	& DOM		& model DOMain \\		\hline
Chapter \ref{TRA}	& TRA		& TRAcer equations (potential temperature and salinity) \\   \hline
Chapter \ref{DYN}	& DYN		& DYNamic equations (momentum) \\		\hline
Chapter \ref{SBC} 	& SBC		& Surface Boundary Conditions \\			\hline
Chapter \ref{LBC}	& LBC		& Lateral Boundary Conditions (also OBC and BDY)  \\ 		\hline
Chapter \ref{LDF}	& LDF		& Lateral DiFfusion (parameterisations) \\	\hline
Chapter \ref{ZDF}	& ZDF		& vertical (Z) DiFfusion (parameterisations)  \\		\hline
Chapter \ref{DIA}	& DIA		& I/O and DIAgnostics (also IOM, FLO and TRD) \\		\hline
Chapter \ref{OBS}	& OBS		& OBServation and model comparison  \\		\hline
Chapter \ref{ASM}	& ASM		& ASsiMilation increment  \\		\hline
Chapter \ref{MISC}	& SOL		& Miscellaneous  topics (including solvers)  \\       \hline
Chapter \ref{CFG}	&  - 			& predefined configurations (including C1D) \\		\hline
\end{tabular} 	
\end{center}	\end{table}
%--------------------------------------------------------------------------------------------------------------


\subsubsection{Changes between releases}
NEMO/OPA, like all research tools, is in perpetual evolution. The present document describes 
the OPA version include in the release 3.3 of NEMO.  This release differs significantly
from version 8, documented in \citet{Madec1998}.\\

$\bullet$ The main modifications from OPA v8 and NEMO/OPA v3.2 are :\\
\begin{enumerate}
\item transition to full native \textsc{Fortran} 90, deep code restructuring and drastic 
reduction of CPP keys; 
\item introduction of partial step representation of bottom topography \citep{Barnier_al_OD06, Le_Sommer_al_OM09, Penduff_al_OS07}; 
\item partial reactivation of a terrain-following vertical coordinate ($s$- and hybrid $s$-$z$) 
with the addition of several options for pressure gradient computation \footnote{Partial 
support of $s$-coordinate: there is presently no support for neutral physics in $s$-
coordinate and for the new options for horizontal pressure gradient computation with 
a non-linear equation of state.}; 
\item more choices for the treatment of the free surface: full explicit, split-explicit or filtered 
schemes, and suppression of the rigid-lid option;
\item non linear free surface associated with the rescaled height coordinate  
\textit{z*} or  \textit{s}; 
\item additional schemes for vector and flux forms of the momentum  advection; 
\item additional advection schemes for tracers; 
\item implementation of the AGRIF package (Adaptative Grid Refinement in \textsc{Fortran}) \citep{Debreu_al_CG2008}; 
\item online diagnostics : tracers trend in the mixed layer and vorticity balance;
\item rewriting of the I/O management with the use of an I/O server; 
\item generalized ocean-ice-atmosphere-CO2 coupling interface, interfaced with OASIS 3 coupler ;
\item surface module (SBC) that simplify the way the ocean is forced and include two
bulk formulea (CLIO and CORE) and which includes an on-the-fly interpolation of input forcing fields ;
\item RGB light penetration and optional use of ocean color 
\item major changes in the TKE schemes: it now includes a Langmuir cell parameterization  \citep{Axell_JGR02}, 
the \citet{Mellor_Blumberg_JPO04} surface wave breaking parameterization, and has a time discretization 
which is energetically consistent with the ocean model equations \citep{Burchard_OM02, Marsaleix_al_OM08}; 
\item tidal mixing parametrisation (bottom intensification) + Indonesian specific tidal mixing \citep{Koch-Larrouy_al_GRL07}; 
\item introduction of LIM-3, the new Louvain-la-Neuve sea-ice model (C-grid rheology and
new thermodynamics including bulk ice salinity) \citep{Vancoppenolle_al_OM09a, Vancoppenolle_al_OM09b}
\end{enumerate}

 \vspace{1cm}
$\bullet$ The main modifications from NEMO/OPA v3.2 and  v3.3 are :\\
\begin{enumerate}
\item introduction of a modified leapfrog-Asselin filter time stepping scheme \citep{Leclair_Madec_OM09}; 
\item additional scheme for iso-neutral mixing \citep{Griffies_al_JPO98}, although it is still a "work in progress"; 
\item a rewriting of the bottom boundary layer scheme, following \citet{Campin_Goosse_Tel99}; 
\item addition of a Generic Length Scale vertical mixing scheme, following \citet{Umlauf_Burchard_JMS03}; 
\item addition of the atmospheric pressure as an external forcing on both ocean and sea-ice dynamics; 
\item addition of a diurnal cycle on solar radiation \citep{Bernie_al_CD07}; 
\item river runoffs added through a non-zero depth, and having its own temperature and salinity; 
\item CORE II normal year forcing set as the default forcing of ORCA2-LIM configuration ; 
\item generalisation of the use of \mdl{fldread} for all input fields (ocean climatology, sea-ice damping...) ; 
\item addition of an on-line observation and model comparison (thanks to NEMOVAR project); 
\item optional application of an assimilation increment (thanks to NEMOVAR project); 
\item coupling interface adjusted for WRF atmospheric model; 
\item C-grid ice rheology now available fro both LIM-2 and LIM-3 \citep{Bouillon_al_OM09}; 
\item LIM-3 ice-ocean momentum coupling applied to LIM-2 ; 
\item a deep re-writting and simplification of the off-line tracer component (OFF\_SRC) ; 
\item the merge of passive and active advection and diffusion modules ;
\item  Use of the Flexible Configuration Manager (FCM) to build configurations, generate the Makefile and produce the executable ;
\item Linear-tangent and Adjoint component (TAM) added, phased with v3.0
\end{enumerate}
 \vspace{1cm}
In addition, several minor modifications in the coding have been introduced with the constant 
concern of improving the model performance. 

 \vspace{1cm}
$\bullet$ The main modifications from NEMO/OPA v3.3 and  v3.4 are :\\
\begin{enumerate}
\item finalisation of above iso-neutral mixing \citep{Griffies_al_JPO98}"; 
\item "Neptune effect" parametrisation;
\item horizontal pressure gradient suitable for s-coordinate; 
\item semi-implicit bottom friction;
\item finalisation of the merge of passive and active tracers advection-diffusion modules; 
\item a new bulk formulae (so-called MFS);
\item use fldread for the off-line tracer component (OFF\_SRC) ; 
\item use MPI point to point communications  for north fold;
\item diagnostic of transport ; 
\end{enumerate}


