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\subsubsection{Introduction |
\subsubsection{Introduction |
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\label{sec:pkg:obcs:intro}} |
\label{sec:pkg:obcs:intro}} |
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The OBCS-package is fundamental to regional ocean modelling with the |
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MITgcm, but because there are so many details to be considered in |
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regional ocean modelling that this package cannot accomodate all |
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imaginable and possible options. Therefore, for a regional simulation |
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with very particular details, it is recommended to familiarize oneself |
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not only with the compile- and runtime-options of this package, but |
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also with the code itself. In many cases it will be necessary to adapt |
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the obcs-code (in particular \code{S/R OBCS\_CALC}) to the application |
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in question; in these cases the obcs-package (together with the |
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rbcs-package, section \ref{sec:pkg:rbcs}) is a very |
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useful infrastructure for implementing special regional models. |
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%---------------------------------------------------------------------- |
%---------------------------------------------------------------------- |
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use prescribed boundary fields to compute Stevens boundary conditions. |
use prescribed boundary fields to compute Stevens boundary conditions. |
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\end{itemize} |
\end{itemize} |
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\paragraph{ORLANSKI:} ~ \\ |
\paragraph{ORLANSKI:} ~ \\ |
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% |
% |
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Orlanski radiation conditions \citep{orl:76}, examples can be found in |
Orlanski radiation conditions \citep{orl:76}, examples can be found in |
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\item If non-hydrostatic dynamics are used |
\item If non-hydrostatic dynamics are used |
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(\ref{sec:non-hydrostatic}), additional files |
(\ref{sec:non-hydrostatic}), additional files |
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\code{OB[N/S/E/W]wFile} for the vertical velocity $w$ with |
\code{OB[N/S/E/W]wFile} for the vertical velocity $w$ with |
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dimensions $(N_{x/y}\times N_r\times\mbox{time levels})$ may be |
dimensions $(N_{x/y}\times N_r\times\mbox{time levels})$ can be |
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specified. |
specified. |
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\item If \code{useSEAICE=.TRUE.} then additional files |
\item If \code{useSEAICE=.TRUE.} then additional files |
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\code{OB[N/S/E/W][a,h,sl,sn,uice,vice]} for sea ice area, thickness |
\code{OB[N/S/E/W][a,h,sl,sn,uice,vice]} for sea ice area, thickness |
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(\code{HEFF}), seaice salinity, snow and ice velocities |
(\code{HEFF}), seaice salinity, snow and ice velocities |
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$(N_{x/y}\times\mbox{time levels})$ may be specified. |
$(N_{x/y}\times\mbox{time levels})$ can be specified. |
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\end{itemize} |
\end{itemize} |
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When the \code{exf}-package is used, the time levels are controlled |
As in \code{S/R external\_fields\_load} or the \code{exf}-package, the |
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for each boundary separately in the same way as the \code{exf}-fields |
code reads two time levels for each variable, e.g.\ \code{OBNu0} and |
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in \code{data.exf}, namelist \code{EXF\_NML\_OBCS}. The runtime flags |
\code{OBNu1}, and interpolates linearly between these time levels to |
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obtain the value \code{OBNu} at the current model time (step). When the |
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\code{exf}-package is used, the time levels are controlled for each |
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boundary separately in the same way as the \code{exf}-fields in |
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\code{data.exf}, namelist \code{EXF\_NML\_OBCS}. The runtime flags |
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follow the above naming conventions, e.g. for the western boundary the |
follow the above naming conventions, e.g. for the western boundary the |
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corresponding flags are \code{OBCWstartdate1/2} and |
corresponding flags are \code{OBCWstartdate1/2} and |
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\code{OBCWperiod}. Sea-ice boundary values are controlled separately |
\code{OBCWperiod}. Sea-ice boundary values are controlled separately |
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with \code{siobWstartdate1/2} and \code{siobWperiod}. |
with \code{siobWstartdate1/2} and \code{siobWperiod}. When the |
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When the \code{exf}-package is not used, the time levels are |
\code{exf}-package is not used, the time levels are controlled by the |
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controlled by the runtime flags \code{externForcingPeriod} and |
runtime flags \code{externForcingPeriod} and \code{externForcingCycle} |
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\code{externForcingCycle} in \code{data}, see \code{verification/exp4} |
in \code{data}, see \code{verification/exp4} for an example. |
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for an example. |
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\paragraph{OBCS\_CALC\_STEVENS:} ~ \\ |
\paragraph{OBCS\_CALC\_STEVENS:} ~ \\ |
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(THE IMPLEMENTATION OF THESE BOUNDARY CONDITIONS IS NOT COMPLETE. SO |
(THE IMPLEMENTATION OF THESE BOUNDARY CONDITIONS IS NOT COMPLETE. SO |
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\texttt{useStevensPhaseVel} and \texttt{useStevensAdvection} to |
\texttt{useStevensPhaseVel} and \texttt{useStevensAdvection} to |
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\texttt{.FALSE.}.\end{itemize} See \citet{stevens:90} for details. |
\texttt{.FALSE.}.\end{itemize} See \citet{stevens:90} for details. |
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\paragraph{OBCS\_BALANCE} ~ \\ |
\paragraph{OBCS\_BALANCE:} ~ \\ |
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% |
% |
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~ |
This is not (yet) a separate routine in the code, but it may become |
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one to make this code more transparent. The code is part of |
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\code{S/R~OBCS\_CALC}. When turned on (\code{ALLOW\_OBCS\_BALANCE} |
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defined in \code{OBCS\_OPTIONS.h} and \code{useOBCSbalance=.true.} in |
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\code{data.obcs/OBCS\_PARM01}), the normal velocities across each of |
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the four boundaries are modified separately, so that the net volume |
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transport across \emph{each} boundary is zero. For example, for the |
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western boundary at $i=i_{b}$, the modified velocity is: |
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\[ |
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u(y,z) - \int_{\mbox{western boundary}}u\,dy\,dz \approx OBNu(j,k) - \sum_{j,k} |
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OBNu(j,k) h_{w}(i_{b},j,k)\Delta{y_G(i_{b},j)}\Delta{z(k)}. |
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\] |
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This also ensures a net total inflow of zero through all boundaries to |
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make it a useful flag to prevent infinite sea-level change within the |
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domain, but the flag is \emph{not} useful if you want to simulate, |
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say, a sector of the Southern Ocean with a strong ACC entering through |
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the western and leaving through the eastern boundary, because this |
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flag will make sure that the strong inflow is removed. It is |
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recommended that this part of the code is adapted to the particular |
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needs of the simulation in question. |
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\paragraph{OBCS\_APPLY\_*:} ~ \\ |
\paragraph{OBCS\_APPLY\_*:} ~ \\ |
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~ |
~ |
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%---------------------------------------------------------------------- |
%---------------------------------------------------------------------- |
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\subsubsection{Reference experiments} |
\subsubsection{Reference experiments} |
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In the directory \code{verifcation}, the following experiments use |
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\code{obcs}: |
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\begin{itemize} |
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\item \code{exp4}: box with 4 open boundaries, simulating flow over a |
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Gaussian bump based on \citet{adcroft:97}, also tests |
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Stevens-boundary conditions; |
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\item \code{dome}: based on ``Denmark Strait Overflow Model |
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Experiment'', use Orlanski-BCs; |
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\item \code{internal\_wave}: uses a heavily modified \code{S/R~OBCS\_CALC} |
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\item \code{seaice\_obcs}: simple example who to use the sea-ice |
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related code, based on \code{lab\_sea}; |
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\item \code{tutorial\_plume\_on\_slope}: uses Orlanski-BCs, see also |
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section~\ref{sec:eg-gravityplume}. |
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\end{itemize} |
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\label{sec:pkg:obcs:experiments} |
\label{sec:pkg:obcs:experiments} |
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\begin{itemize} |
\begin{itemize} |
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\item{Ocean experiment in exp4 verification directory. } |
\item \code{tutorial\_plume\_on\_slope} (section~\ref{sec:eg-gravityplume}) |
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\end{itemize} |
\end{itemize} |
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