Changeset 2195 for branches/DEV_r1826_DOC/DOC/TexFiles/Chapters/Chap_SBC.tex

Timestamp:

2010-10-09T17:55:02+02:00 (14 years ago)

Author:

gm

Message:

ticket:#658 update TRA DYN & SBC with sbc, qsr, nxt and rnf considerations

File:

• branches/DEV_r1826_DOC/DOC/TexFiles/Chapters/Chap_SBC.tex (modified) (4 diffs)

branches/DEV_r1826_DOC/DOC/TexFiles/Chapters/Chap_SBC.tex

@@ -103 +103 @@
 In the real ocean, $\textit{emp}=\textit{emp}_S$ and the ocean salt content is conserved, 
 but it exist several numerical reasons why this equality should be broken. 
-For example:
-
-When the rigid-lid assumption is made, the ocean volume becomes constant and 
-thus, $\textit{emp}=0$, not $\textit{emp}_S$.
-
-When the ocean is coupled to a sea-ice model, the water exchanged between ice and 
-ocean is slightly salty (mean sea-ice salinity is $\sim $\textit{4 psu}). In this case, 
+For example, when the ocean is coupled to a sea-ice model, the water exchanged between 
+ice and ocean is slightly salty (mean sea-ice salinity is $\sim $\textit{4 psu}). In this case, 
 $\textit{emp}_{S}$ take into account both concentration/dilution effect associated with 
 freezing/melting and the salt flux between ice and ocean, while \textit{emp} is 
@@ -359 +354 @@
 
 % ================================================================
-%        Addition of river runoffs
+%        River runoffs
 % ================================================================
 \section   [river runoffs (\textit{sbcrnf})]
@@ -375 +370 @@
 coastal modelling and becomes more and more often open ocean and climate modelling 
 \footnote{At least a top cells thickness of 1~meter and a 3 hours forcing frequency are
-required to properly represent the diurnal cycle \citep{Bernie_al_OM05}.}.
-
-
- 
+required to properly represent the diurnal cycle \citep{Bernie_al_JC05}. see also \S\ref{SBC_dcy}.}.
+
+
+To do this we need to treat evaporation/precipitation fluxes and river runoff differently in the \mdl{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:
+
+
+Rachel:
+
 It is convenient to introduce the river runoff in the model as a surface 
-fresh water flux. 
-
-
-%Griffies:  River runoff generally enters the ocean at a nonzero depth rather than through the surface. Many global models, however, have traditionally inserted river runoff to the top model cell. Such can become problematic numerically and physically when the top grid cells are reÞned to levels common in coastal modelling. Hence, more applications are now considering the input of runoff throughout a nonzero depth. Likewise, sea ice can melt at depth, thus necessitating a mass transport to occur within the ocean between the liquid and solid water masses.
-
-\colorbox{yellow}{Nevertheless, Pb of vertical resolution and increase of Kz in vicinity of }
+fresh water flux. This is the defualt option within NEMO, and there is then
+ the option for the user to increase vertical mixing in the vicinity of the rivermouth.
+
+However, this method is not very appropriate for coastal modelling.  As such its now possible
+ to specify, in a netcdf input file, the temperature and salinity of the river, along with the 
+depth (in metres) which the river should be added to.  This enables to river to be correctly 
+added through the water column, instead of as a surface flux, and also means the temperature 
+or salinity (for low salinity outflow) of the river impacts the surrounding ocean.
+
+For temperature -999 is taken as missing data and the river temperature is taken to be the 
+surface temperatue at the river point.  For the depth parameter a value of -1 means the 
+river is added to the surface box only, and a value of -999 means the river is added through 
+the entire water column.
+
+Namelist options, \np{ln\_rnf\_depth}, \np{ln\_rnf\_sal} and \np{ln\_rnf\_temp} control whether 
+the river attributes (depth, salinity and temperature) are read in and used.  If these are set 
+as false the river is added to the surface box only, assumed to be fresh (0~psu), and/or 
+taken as surface temperature respectively.
+
+It 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.  When the flow is out of the 
+domain there is no change in temperature and salinity, regardless of the namelist options used.
+
+The runoff value and attributes are read in in sbcrnf.  The mass/volume addition is added to the 
+divergence term in \rou{sbc\_rnf\_div}.  The dilution effect of the river is automatically applied through 
+the vertical tracer advection, and the direct flux of tracers into the domain is done in trasbc.
+
+
+\colorbox{yellow}{Nevertheless, Pb of vertical resolution and 3D input : increase vertical mixing near river mouths to mimic 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.}
 
 \colorbox{yellow}{river mouths{\ldots}}
@@ -396 +420 @@
 
 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.
-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.  The depth option makes it possible to have the river water affecting just the surface layer, throughout depth, or some specified point in between.
+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.  
+
+The depth option makes it possible to have the river water affecting just the surface layer, throughout depth, or some specified point in between.
 
 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:
 
 }
+
+
+% ================================================================
+%        Diurnal cycle
+% ================================================================
+\section   [Diurnal  cycle (\textit{sbcdcy})]
+         {Diurnal cycle (\mdl{sbcdcy})}
+\label{SBC_dcy}
+%------------------------------------------namsbc_rnf----------------------------------------------------
+%\namdisplay{namsbc} 
+%-------------------------------------------------------------------------------------------------------------
+
+\cite{Bernie_al_JC05} have shown that to capture 90$\%$ of the diurnal variability of 
+SST requires a vertical resolution in upper ocean of 1~m or better and a temporal resolution 
+of the surface fluxes of 3~h or less. Unfortunately high frequency forcing fields are rare, 
+not to say inexistent. Nevertheless, it is possible to obtain a reasonable diurnal cycle 
+of the SST knowning only short wave flux (SWF) at high frequency \citep{Bernie_al_CD07}.
+Furthermore, only the knowledge of daily mean value of SWF is needed, 
+as higher frequency variations can be reconstructed from them, assuming that 
+the diurnal cycle of SWF is a scaling of the top of the atmosphere diurnal cycle 
+of incident SWF. The \cite{Bernie_al_CD07} reconstruction algorithm is available
+in \NEMO by setting \np{ln\_dm2dc}=true (a \textit{namsbc} namelist parameter) when using 
+CORE bulk formulea (\np{ln\_blk\_core}=true) or the flux formulation (\np{ln\_flx}=true). 
+The algorithm used is detailed in the appendix~A of \cite{Bernie_al_CD07} 
+and illustrated on Fig.\ref{Fig_SBC_diurnal}. 
+
+Note that the reconstruction can change the daily mean value of SWF by a few tenth of 
+W/m$^2$ ($\pm 0.5$~W/m$^2$ with a 1~h sampling) but this is not a systematic bias. 
+Note also that the setting a diurnal cycle in SWF is highly recommended  when 
+the top layer thickness approach 1~m or less, otherwise large error in SST can 
+appear due to an inconsistency between the scale of the vertical resolution 
+and the forcing acting on that scale.
+
+%>>>>>>>>>>>>>>>>>>>>>>>>>>>>
+\begin{figure}[!t] \label{Fig_SBC_diurnal}  \begin{center}
+\includegraphics[width=1.0\textwidth]{./TexFiles/Figures/Fig_SBC_diurnal.pdf}
+\caption{Example of recontruction of the diurnal cycle variation of short wave flux  
+from daily mean values. From \citet{Bernie_al_CD07}.}
+\end{center}   \end{figure}
+%>>>>>>>>>>>>>>>>>>>>>>>>>>>>
 
 % ================================================================