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revision 1.1 by heimbach, Thu Aug 11 23:58:07 2005 UTC revision 1.11 by mlosch, Wed Mar 16 10:39:25 2011 UTC
# Line 5  Line 5 
5  <!-- CMIREDIR:package_obcs: -->  <!-- CMIREDIR:package_obcs: -->
6  \end{rawhtml}  \end{rawhtml}
7    
8  Authors: Martin Losch and Patrick Heimbach  Authors:
9    Alistair Adcroft, Patrick Heimbach, Samar Katiwala, Martin Losch
10    
11  \subsubsection{Introduction  \subsubsection{Introduction
12  \label{sec:pkg:obcs:intro}}  \label{sec:pkg:obcs:intro}}
13    
14    The OBCS-package is fundamental to regional ocean modelling with the
15    MITgcm, but there are so many details to be considered in
16    regional ocean modelling that this package cannot accomodate all
17    imaginable and possible options. Therefore, for a regional simulation
18    with very particular details, it is recommended to familiarize oneself
19    not only with the compile- and runtime-options of this package, but
20    also with the code itself. In many cases it will be necessary to adapt
21    the obcs-code (in particular \code{S/R OBCS\_CALC}) to the application
22    in question; in these cases the obcs-package (together with the
23    rbcs-package, section \ref{sec:pkg:rbcs}) is a very
24    useful infrastructure for implementing special regional models.
25    
26  %----------------------------------------------------------------------  %----------------------------------------------------------------------
27    
28  \subsubsection{OBCS configuration and compiling  \subsubsection{OBCS configuration and compiling
29  \label{sec:pkg:kpp:comp}}  \label{sec:pkg:obcs:comp}}
30    
31  As with all MITgcm packages, OBCS can be turned on or off  As with all MITgcm packages, OBCS can be turned on or off
32  at compile time  at compile time
# Line 23  at compile time Line 34  at compile time
34  \begin{itemize}  \begin{itemize}
35  %  %
36  \item  \item
37  using the \texttt{packages.conf} file by adding \texttt{obcs} to it,  using the \code{packages.conf} file by adding \code{obcs} to it,
38  %  %
39  \item  \item
40  or using \texttt{genmake2} adding  or using \code{genmake2} adding
41  \texttt{-enable=obcs} or \texttt{-disable=obcs} switches  \code{-enable=obcs} or \code{-disable=obcs} switches
42  %  %
43  \item  \item
44  \textit{Required packages and CPP options:} \\  \textit{Required packages and CPP options:} \\
# Line 36  To alternatives are available for prescr Line 47  To alternatives are available for prescr
47  which differ in the way how OB's are treated in time:  which differ in the way how OB's are treated in time:
48  A simple time-management (e.g. constant in time, or cyclic with  A simple time-management (e.g. constant in time, or cyclic with
49  fixed fequency) is provided through  fixed fequency) is provided through
50  S/R \texttt{obcs\_external\_fields\_load}.  S/R \code{obcs\_external\_fields\_load}.
51  More sophisticated ``real-time'' (i.e. calendar time) management is  More sophisticated ``real-time'' (i.e. calendar time) management is
52  available through \texttt{obcs\_prescribe\_read}.  available through \code{obcs\_prescribe\_read}.
53  The latter case requires  The latter case requires
54  packages \texttt{cal} and \texttt{exf} to be enabled.  packages \code{cal} and \code{exf} to be enabled.
55  %  %
56  \end{itemize}  \end{itemize}
57  (see also Section \ref{sect:buildingCode}).  (see also Section \ref{sec:buildingCode}).
58    
59  Parts of the OBCS code can be enabled or disabled at compile time  Parts of the OBCS code can be enabled or disabled at compile time
60  via CPP preprocessor flags. These options are set in  via CPP preprocessor flags. These options are set in
61  \texttt{OBCS\_OPTIONS.h}. Table \ref{tab:pkg:obcs:cpp} summarizes them.  \code{OBCS\_OPTIONS.h}. Table \ref{tab:pkg:obcs:cpp} summarizes them.
62    
63  \begin{table}[h!]  \begin{table}[!ht]
64  \centering  \centering
65    \label{tab:pkg:obcs:cpp}    \label{tab:pkg:obcs:cpp}
66    {\footnotesize    {\footnotesize
# Line 57  via CPP preprocessor flags. These option Line 68  via CPP preprocessor flags. These option
68        \hline        \hline
69        \textbf{CPP option}  &  \textbf{Description}  \\        \textbf{CPP option}  &  \textbf{Description}  \\
70        \hline \hline        \hline \hline
71          \texttt{ALLOW\_OBCS\_NORTH} &          \code{ALLOW\_OBCS\_NORTH} &
72            enable Northern OB \\            enable Northern OB \\
73          \texttt{ALLOW\_OBCS\_SOUTH} &          \code{ALLOW\_OBCS\_SOUTH} &
74            enable Southern OB \\            enable Southern OB \\
75          \texttt{ALLOW\_OBCS\_EAST} &          \code{ALLOW\_OBCS\_EAST} &
76            enable Eastern OB \\            enable Eastern OB \\
77          \texttt{ALLOW\_OBCS\_WEST} &          \code{ALLOW\_OBCS\_WEST} &
78            enable Western OB \\            enable Western OB \\
79        \hline        \hline
80          \texttt{ALLOW\_OBCS\_PRESCRIBE} &          \code{ALLOW\_OBCS\_PRESCRIBE} &
81            enable code for prescribing OB's \\            enable code for prescribing OB's \\
82          \texttt{ALLOW\_OBCS\_SPONGE} &          \code{ALLOW\_OBCS\_SPONGE} &
83            enable sponge layer code \\            enable sponge layer code \\
84          \texttt{ALLOW\_OBCS\_BALANCE} &          \code{ALLOW\_OBCS\_BALANCE} &
85            enable code for balancing transports through OB's \\            enable code for balancing transports through OB's \\
86          \texttt{ALLOW\_ORLANSKI} &          \code{ALLOW\_ORLANSKI} &
87            enable Orlanski radiation conditions at OB's \\            enable Orlanski radiation conditions at OB's \\
88            \code{ALLOW\_OBCS\_STEVENS} &
89              enable Stevens (1990) boundary conditions at OB's \\
90            & (currently only implemented for eastern and western \\
91            &  boundaries and NOT for ptracers) \\
92        \hline        \hline
93      \end{tabular}      \end{tabular}
94    }    }
# Line 87  via CPP preprocessor flags. These option Line 102  via CPP preprocessor flags. These option
102  \label{sec:pkg:obcs:runtime}}  \label{sec:pkg:obcs:runtime}}
103    
104  Run-time parameters are set in files  Run-time parameters are set in files
105  \texttt{data.pkg}, \texttt{data.obcs}, and \texttt{data.exf}  \code{data.pkg}, \code{data.obcs}, and \code{data.exf}
106  if ``real-time'' prescription is requested  if ``real-time'' prescription is requested
107  (i.e. package \texttt{exf} enabled).  (i.e. package \code{exf} enabled).
108  These parameter files are read in S/R  vThese parameter files are read in S/R
109  \texttt{packages\_readparms.F}, \texttt{obcs\_readparms.F}, and  \code{packages\_readparms.F}, \code{obcs\_readparms.F}, and
110  \texttt{exf\_readparms.F}, respectively.  \code{exf\_readparms.F}, respectively.
111  Run-time parameters may be broken into 3 categories:  Run-time parameters may be broken into 3 categories:
112  (i) switching on/off the package at runtime,  (i) switching on/off the package at runtime,
113  (ii) OBCS package flags and parameters,  (ii) OBCS package flags and parameters,
114  (iii) additional timing flags in \texttt{data.exf}, if selected.  (iii) additional timing flags in \code{data.exf}, if selected.
115    
116  \paragraph{Enabling the package}  \paragraph{Enabling the package}
117  ~ \\  ~ \\
118  %  %
119  The OBCS package is switched on at runtime by setting  The OBCS package is switched on at runtime by setting
120  \texttt{useOBCS = .TRUE.} in \texttt{data.pkg}.  \code{useOBCS = .TRUE.} in \code{data.pkg}.
121    
122  \paragraph{Package flags and parameters}  \paragraph{Package flags and parameters}
123  ~ \\  ~ \\
124  %  %
125  Table \ref{tab:pkg:obcs:runtime_flags} summarizes the  Table \ref{tab:pkg:obcs:runtime_flags} summarizes the
126  runtime flags that are set in \texttt{data.obcs}, and  runtime flags that are set in \code{data.obcs}, and
127  their default values.  their default values.
128    
129  \begin{table}[h!]  \begin{table}[!ht]
130  \centering  \centering
   \label{tab:pkg:obcs:runtime_flags}  
131    {\footnotesize    {\footnotesize
132      \begin{tabular}{|l|c|l|}      \begin{tabular}{|l|c|l|}
133        \hline        \hline
134        \textbf{Flag/parameter} & \textbf{default} &  \textbf{Description}  \\        \textbf{Flag/parameter} & \textbf{default} &  \textbf{Description}  \\
135        \hline \hline        \hline \hline
136           \multicolumn{3}{|c|}{\textit{basic flags \& parameters} } \\           \multicolumn{3}{|c|}{\textit{basic flags \& parameters} (OBCS\_PARM01) } \\
137           \hline           \hline
138          OB\_Jnorth & 0 &          OB\_Jnorth & 0 &
139             Nx-vector of J-indices (w.r.t. Ny) of Northern OB             Nx-vector of J-indices (w.r.t. Ny) of Northern OB
# Line 133  their default values. Line 147  their default values.
147          OB\_Iwest & 0 &          OB\_Iwest & 0 &
148             Ny-vector of I-indices (w.r.t. Nx) of Western OB             Ny-vector of I-indices (w.r.t. Nx) of Western OB
149             at each J-position (w.r.t. Ny) \\             at each J-position (w.r.t. Ny) \\
150          useOBCSprescribe & \texttt{.FALSE.} &          useOBCSprescribe & \code{.FALSE.} &
151             ~ \\             ~ \\
152          useOBCSsponge & \texttt{.FALSE.} &          useOBCSsponge & \code{.FALSE.} &
153             ~ \\             ~ \\
154          useOBCSbalance & \texttt{.FALSE.} &          useOBCSbalance & \code{.FALSE.} &
155             ~ \\             ~ \\
156               OBCS\_balanceFacN/S/E/W & 1 & factor(s) determining the details
157               of the balaning code \\
158            useOrlanskiNorth/South/EastWest & \code{.FALSE.} &
159               turn on Orlanski boundary conditions for individual boundary\\
160            useStevensNorth/South/EastWest & \code{.FALSE.} &
161               turn on Stevens boundary conditions for individual boundary\\
162          OB\textbf{X}\textbf{y}File & ~ &          OB\textbf{X}\textbf{y}File & ~ &
163             file name of OB field \\             file name of OB field \\
164          ~ & ~ &          ~ & ~ &
# Line 146  their default values. Line 166  their default values.
166                         \textbf{E}(ast), \textbf{W}(est) \\                         \textbf{E}(ast), \textbf{W}(est) \\
167          ~ & ~ &          ~ & ~ &
168             \textbf{y}: \textbf{t}(emperature), \textbf{s}(salinity),             \textbf{y}: \textbf{t}(emperature), \textbf{s}(salinity),
169             \textbf{u}(-velocity), \textbf{v}(-velocity) \\             \textbf{u}(-velocity), \textbf{v}(-velocity), \\
170            ~ & ~ &
171               \textbf{w}(-velocity), \textbf{eta}(sea surface height)\\
172            ~ & ~ &
173               \textbf{a}(sea ice area), \textbf{h}(sea ice thickness),
174               \textbf{sn}(snow thickness), \textbf{sl}(sea ice salinity)\\
175        \hline        \hline
176        \multicolumn{3}{|c|}{\textit{Orlanski parameters} } \\        \multicolumn{3}{|c|}{\textit{Orlanski parameters} (OBCS\_PARM02) } \\
177        \hline        \hline
178          cvelTimeScale & 2000 sec &          cvelTimeScale & 2000 sec &
179             averaging period for phase speed \\             averaging period for phase speed \\
# Line 156  their default values. Line 181  their default values.
181             maximum allowable phase speed-CFL for AB-II \\             maximum allowable phase speed-CFL for AB-II \\
182          CFIX & 0.8 m/s &          CFIX & 0.8 m/s &
183             fixed boundary phase speed \\             fixed boundary phase speed \\
184          useFixedCEast & .FALSE. &          useFixedCEast & \code{.FALSE.} &
185             ~ \\             ~ \\
186          useFixedCWest & .FALSE. &          useFixedCWest & \code{.FALSE.} &
187             ~ \\             ~ \\
188        \hline        \hline
189        \multicolumn{3}{|c|}{\textit{Sponge-layer parameters} } \\        \multicolumn{3}{|c|}{\textit{Sponge-layer parameters} (OBCS\_PARM03)} \\
190        \hline        \hline
191          spongeThickness & 0 &          spongeThickness & 0 &
192             sponge layer thickness (in \# grid points) \\             sponge layer thickness (in \# grid points) \\
# Line 177  their default values. Line 202  their default values.
202          Vrelaxobcsbound & 0 sec &          Vrelaxobcsbound & 0 sec &
203             relaxation time scale at the             relaxation time scale at the
204             outermost sponge layer point of a zonal OB \\             outermost sponge layer point of a zonal OB \\
205          \hline
206          \multicolumn{3}{|c|}{\textit{Stevens parameters} (OBCS\_PARM04) } \\
207          \hline
208            T/SrelaxStevens & 0~sec & relaxation time scale for
209               temperature/salinity \\
210            useStevensPhaseVel & \code{.TRUE.} & \\
211            useStevensAdvection & \code{.TRUE.} & \\
212           \hline           \hline
213        \hline        \hline
214      \end{tabular}      \end{tabular}
215    }    }
216    \caption{~}    \caption{pkg OBCS run-time parameters}
217      \label{tab:pkg:obcs:runtime_flags}
218  \end{table}  \end{table}
219    
220    
221    
222  %----------------------------------------------------------------------  %----------------------------------------------------------------------
223    
224    \subsubsection{Defining open boundary positions
225    \label{sec:pkg:obcs:defining}}
226    
227    There are four open boundaries (OBs), a
228    Northern, Southern, Eastern, and Western.
229    All OB locations are specified by their absolute
230    meridional (Northern/Southern) or zonal (Eastern/Western) indices.
231    Thus, for each zonal position $i=1,\ldots,N_x$ a meridional index
232    $j$ specifies the Northern/Southern OB position,
233    and for each meridional position $j=1,\ldots,N_y$, a zonal index
234    $i$ specifies the Eastern/Western OB position.
235    For Northern/Southern OB this defines an $N_x$-dimensional
236    ``row'' array $\tt OB\_Jnorth(Ny)$ / $\tt OB\_Jsouth(Ny)$,
237    and an $N_y$-dimenisonal
238    ``column'' array $\tt OB\_Ieast(Nx)$ / $\tt OB\_Iwest(Nx)$.
239    Positions determined in this way allows Northern/Southern
240    OBs to be at variable $j$ (or $y$) positions, and Eastern/Western
241    OBs at variable $i$ (or $x$) positions.
242    Here, indices refer to tracer points on the C-grid.
243    A zero (0) element in $\tt OB\_I\ldots$, $\tt OB\_J\ldots$
244    means there is no corresponding OB in that column/row.
245    For a Northern/Southern OB, the OB V point is to the South/North.
246    For an Eastern/Western OB, the OB U point is to the West/East.
247    For example,
248    \begin{tabbing}
249      \code{OB\_Jnorth(3)=34} \=  means that:  \= \\
250      \> \code{T(3,34)} \> is a an OB point  \\
251      \> \code{U(3,34)} \> is a an OB point \\
252      \> \code{V(3,34)} \> is a an OB point \\
253      \code{OB\_Jsouth(3)=1} \> means that: \\
254      \> \code{T(3,1)} \> is a an OB point \\
255      \> \code{U(3,1)} \> is a an OB point \\
256      \> \code{V(3,2)} \> is a an OB point \\
257      \code{OB\_Ieast(10)=69} \>  means that:  \>  \\
258      \> \code{T(69,10)} \> is a an OB point \\
259      \> \code{U(69,10)} \> is a an OB point \\
260      \> \code{V(69,10)} \> is a an OB point \\
261      \code{OB\_Iwest(10)=1} \>  means that:  \>  \\
262      \> \code{T(1,10)} \> is a an OB point \\
263      \> \code{U(2,10)} \> is a an OB point \\
264      \> \code{V(1,10)} \> is a an OB point
265    \end{tabbing}
266    For convenience, negative values for \code{Jnorth}/\code{Ieast} refer to
267    points relative to the Northern/Eastern edges of the model
268    eg. $\tt OB\_Jnorth(3)=-1$  means that the point $\tt (3,Ny)$
269    is a northern OB.
270    
271    \noindent
272    \textsf{Add special comments for case \#define NONLIN\_FRSURF,
273    see obcs\_ini\_fixed.F}
274    
275    %----------------------------------------------------------------------
276    
277  \subsubsection{Equations and key routines  \subsubsection{Equations and key routines
278  \label{sec:pkg:obcs:equations}}  \label{sec:pkg:obcs:equations}}
279    
280  \paragraph{OBCS\_READPARMS:} Setting OB positions \\  \paragraph{OBCS\_READPARMS:} ~ \\
281  ~  Set OB positions through arrays
282    {\tt OB\_Jnorth(Ny), OB\_Jsouth(Ny), OB\_Ieast(Nx), OB\_Iwest(Nx)},
283    and runtime flags (see Table \ref{tab:pkg:obcs:runtime_flags}).
284    
285  \paragraph{OBCS\_CALC:} ~ \\  \paragraph{OBCS\_CALC:} ~ \\
 ~  
   
 \paragraph{OBCS\_APPLY\_*:} ~ \\  
 ~  
   
 \paragraph{OBCS\_PRESCRIBE\_READ} Setting OB fields and updates \\  
286  %  %
287  ~  Top-level routine for filling values to be applied at OB for
288    $T,S,U,V,\eta$ into corresponding
289  \paragraph{OBCS\_SPONGE} Setting sponge layer characteristics \\  ``slice'' arrays $(x,z)$, $(y,z)$ for each OB:
290    $\tt OB[N/S/E/W][t/s/u/v]$; e.g. for salinity array at
291    Southern OB, array name is $\tt OBSt$.
292    Values filled are either
293  %  %
294  ~  \begin{itemize}
295    %
296    \item
297    constant vertical $T,S$ profiles as specified in file
298    {\tt data} ({\tt tRef(Nr), sRef(Nr)}) with zero velocities $U,V$,
299    %
300    \item
301    $T,S,U,V$ values determined via Orlanski radiation conditions
302    (see below),
303    %
304    \item
305    prescribed time-constant or time-varying fields (see below).
306    %
307    \item
308    use prescribed boundary fields to compute Stevens boundary conditions.
309    \end{itemize}
310    
311  \paragraph{OBCS\_BALANCE} ~ \\  \paragraph{ORLANSKI:} ~ \\
312  %  %
313    Orlanski radiation conditions \citep{orl:76}, examples can be found in
314    \code{verification/dome} and
315    \code{verification/tutorial\_plume\_on\_slope}
316    (\ref{sec:eg-gravityplume}).
317    
318    \paragraph{OBCS\_PRESCRIBE\_READ:} ~ \\
319    %
320    When \code{useOBCSprescribe = .TRUE.} the model tries to read
321    temperature, salinity, u- and v-velocities from files specified in the
322    runtime parameters \code{OB[N/S/E/W][t/s/u/v]File}. These files are
323    the usual IEEE, big-endian files with dimensions of a section along an
324    open boundary:
325    \begin{itemize}
326    \item For North/South boundary files the dimensions are
327      $(N_x\times N_r\times\mbox{time levels})$, for East/West boundary
328      files the dimensions are $(N_y\times N_r\times\mbox{time levels})$.
329    \item If a non-linear free surface is used
330      (\ref{sec:nonlinear-freesurface}), additional files
331      \code{OB[N/S/E/W]etaFile} for the sea surface height $\eta$ with
332      dimension $(N_{x/y}\times\mbox{time levels})$ may be specified.
333    \item If non-hydrostatic dynamics are used
334      (\ref{sec:non-hydrostatic}), additional files
335      \code{OB[N/S/E/W]wFile} for the vertical velocity $w$ with
336      dimensions $(N_{x/y}\times N_r\times\mbox{time levels})$ can be
337      specified.
338    \item If \code{useSEAICE=.TRUE.} then additional files
339      \code{OB[N/S/E/W][a,h,sl,sn,uice,vice]} for sea ice area, thickness
340      (\code{HEFF}), seaice salinity, snow and ice velocities
341      $(N_{x/y}\times\mbox{time levels})$ can be specified.
342    \end{itemize}
343    As in \code{S/R external\_fields\_load} or the \code{exf}-package, the
344    code reads two time levels for each variable, e.g.\ \code{OBNu0} and
345    \code{OBNu1}, and interpolates linearly between these time levels to
346    obtain the value \code{OBNu} at the current model time (step). When the
347    \code{exf}-package is used, the time levels are controlled for each
348    boundary separately in the same way as the \code{exf}-fields in
349    \code{data.exf}, namelist \code{EXF\_NML\_OBCS}. The runtime flags
350    follow the above naming conventions, e.g. for the western boundary the
351    corresponding flags are \code{OBCWstartdate1/2} and
352    \code{OBCWperiod}. Sea-ice boundary values are controlled separately
353    with \code{siobWstartdate1/2} and \code{siobWperiod}.  When the
354    \code{exf}-package is not used, the time levels are controlled by the
355    runtime flags \code{externForcingPeriod} and \code{externForcingCycle}
356    in \code{data}, see \code{verification/exp4} for an example.
357    
358    \paragraph{OBCS\_CALC\_STEVENS:} ~ \\
359    (THE IMPLEMENTATION OF THESE BOUNDARY CONDITIONS IS NOT COMPLETE. SO
360    FAR ONLY EASTERN AND WESTERN BOUNDARIES ARE SUPPORTED.) \\
361    The boundary conditions following \citet{stevens:90} require the
362    vertically averaged normal velocity (originally specified as a stream
363    function along the open boundary) $\bar{u}_{ob}$ and the tracer fields
364    $\chi_{ob}$ (note: passive tracers are currently not implemented and
365    the code stops when package \code{ptracers} is used together with this
366    option). Currently, the code vertically averages the normal velocity
367    as specified. From these prescribed values the code computes the
368    boundary values for the next timestep $n+1$ as follows (as an
369    example, we use the notation for an eastern or western boundary):
370    \begin{itemize}
371    \item $u^{n+1}(y,z) = \bar{u}_{ob}(y) + u'(y,z)$, where $u_{n}'$ is the
372      deviation from the vertically averaged velocity one grid point
373      inward from the boundary.
374    \item If $u^{n+1}$ is directed into the model domain, the boudary
375      value for tracer $\chi$ is restored to the prescribed values:
376      \[\chi^{n+1} =   \chi^{n} + \frac{\Delta{t}}{\tau_\chi} (\chi_{ob} -
377      \chi^{n}),\] where $\tau_\chi$ is the relaxation time
378      scale \texttt{T/SrelaxStevens}.
379    \item If $u^{n+1}$ is directed out of the model domain, the tracer is
380      advected out of the domain with $u^{n+1}+c$, where $c$ is a phase
381      velocity estimated as
382      $\frac{1}{2}\frac{\partial\chi}{\partial{t}}/\frac{\partial\chi}{\partial{x}}$.
383      For test purposes, the phase velocity contribution or the entire
384      advection can
385      be turned off by setting the corresponding parameters
386      \texttt{useStevensPhaseVel} and \texttt{useStevensAdvection} to
387      \texttt{.FALSE.}.\end{itemize} See \citet{stevens:90} for details.
388    
389    \paragraph{OBCS\_BALANCE\_FLOW:} ~ \\
390    %
391    When turned on (\code{ALLOW\_OBCS\_BALANCE}
392    defined in \code{OBCS\_OPTIONS.h} and \code{useOBCSbalance=.true.} in
393    \code{data.obcs/OBCS\_PARM01}), this routine balances the net flow
394    across the open boundaries. By default the net flow across the
395    boundaries is computed and all normal velocities on boundaries are
396    adjusted to obtain zero net inflow.
397    
398    This behavior can be controlled with the runtime flags
399    \code{OBCS\_balanceFacN/S/E/W}. The values of these flags determine
400    how the net inflow is redistributed as small correction velocities
401    between the individual sections. A value ``\code{-1}'' balances an
402    individual boundary, values $>0$ determine the relative size of the
403    correction. For example, the values
404    \begin{tabbing}
405     \code{OBCS\_balanceFacE}\code{ = 1.,} \\
406     \code{OBCS\_balanceFacW}\code{ = -1.,} \\
407     \code{OBCS\_balanceFacN}\code{ = 2.,} \\
408     \code{OBCS\_balanceFacS}\code{ = 0.,}
409    \end{tabbing}
410    make the model
411    \begin{itemize}
412    \item correct Western \code{OBWu} by substracting a uniform velocity to
413    ensure zero net transport through the Western open boundary;
414    \item correct Eastern and Northern normal flow, with the Northern
415      velocity correction two times larger than the Eastern correction, but
416      \emph{not} the Southern normal flow, to ensure that the total inflow through
417      East, Northern, and Southern open boundary is balanced.
418    \end{itemize}
419    
420    The old method of balancing the net flow for all sections individually
421    can be recovered by setting all flags to -1. Then the normal
422    velocities across each of the four boundaries are modified separately,
423    so that the net volume transport across \emph{each} boundary is
424    zero. For example, for the western boundary at $i=i_{b}$, the modified
425    velocity is:
426    \[
427    u(y,z) - \int_{\mbox{western boundary}}u\,dy\,dz \approx OBNu(j,k) - \sum_{j,k}
428    OBNu(j,k) h_{w}(i_{b},j,k)\Delta{y_G(i_{b},j)}\Delta{z(k)}.
429    \]
430    This also ensures a net total inflow of zero through all boundaries,
431    but this combination of flags is \emph{not} useful if you want to
432    simulate, say, a sector of the Southern Ocean with a strong ACC
433    entering through the western and leaving through the eastern boundary,
434    because the value of ``\code{-1}'' for these flags will make sure that
435    the strong inflow is removed. Clearly, gobal balancing with
436    \code{OBCS\_balanceFacE/W/N/S} $\ge0$ is the preferred method.
437    
438    \paragraph{OBCS\_APPLY\_*:} ~ \\
439  ~  ~
440    
441  \paragraph{ORLANSKI} Orlanski radiation conditions \\  \paragraph{OBCS\_SPONGE:} ~ \\
442  %  %
443  ~  The sponge layer code (turned on with \code{ALLOW\_OBCS\_SPONGE} and
444    \code{useOBCSsponge}) adds a relaxation term to the right-hand-side of
445    the momentum and tracer equations. The variables are relaxed towards
446    the boundary values with a relaxation time scale that increases
447    linearly with distance from the boundary
448    \[
449    G_{\chi}^{\mbox{(sponge)}} =
450    - \frac{\chi - [( L - \delta{L} ) \chi_{BC} + \delta{L}\chi]/L}
451    {[(L-\delta{L})\tau_{b}+\delta{L}\tau_{i}]/L}
452    = - \frac{\chi - [( 1 - l ) \chi_{BC} + l\chi]}
453    {[(1-l)\tau_{b}+l\tau_{i}]}
454    \]
455    where $\chi$ is the model variable (U/V/T/S) in the interior,
456    $\chi_{BC}$ the boundary value, $L$ the thickness of the sponge layer
457    (runtime parameter \code{spongeThickness} in number of grid points),
458    $\delta{L}\in[0,L]$ ($\frac{\delta{L}}{L}=l\in[0,1]$) the distance from the boundary (also in grid points), and
459    $\tau_{b}$ (runtime parameters \code{Urelaxobcsbound} and
460    \code{Vrelaxobcsbound}) and $\tau_{i}$ (runtime parameters
461    \code{Urelaxobcsinner} and \code{Vrelaxobcsinner}) the relaxation time
462    scales on the boundary and at the interior termination of the sponge
463    layer. The parameters \code{Urelaxobcsbound/inner} set the relaxation
464    time scales for the Eastern and Western boundaries,
465    \code{Vrelaxobcsbound/inner} for the Northern and Southern boundaries.
466    
467  \paragraph{OB's with nonlinear free surface} ~ \\  \paragraph{OB's with nonlinear free surface} ~ \\
468  %  %
# Line 246  Diagnostics output is available via the Line 494  Diagnostics output is available via the
494  Available output fields are summarized in  Available output fields are summarized in
495  Table \ref{tab:pkg:obcs:diagnostics}.  Table \ref{tab:pkg:obcs:diagnostics}.
496    
497  \begin{table}[h!]  \begin{table}[!ht]
498  \centering  \centering
499  \label{tab:pkg:obcs:diagnostics}  \label{tab:pkg:obcs:diagnostics}
500  {\footnotesize  {\footnotesize
# Line 263  Table \ref{tab:pkg:obcs:diagnostics}. Line 511  Table \ref{tab:pkg:obcs:diagnostics}.
511  %----------------------------------------------------------------------  %----------------------------------------------------------------------
512    
513  \subsubsection{Reference experiments}  \subsubsection{Reference experiments}
514    In the directory \code{verifcation}, the following experiments use
515    \code{obcs}:
516    \begin{itemize}
517    \item \code{exp4}: box with 4 open boundaries, simulating flow over a
518      Gaussian bump based on \citet{adcroft:97}, also tests
519      Stevens-boundary conditions;
520    \item \code{dome}: based on the project ``Dynamics of Overflow Mixing
521      and Entrainment''
522      (\url{http://www.rsmas.miami.edu/personal/tamay/DOME/dome.html}), uses
523      Orlanski-BCs;
524    \item \code{internal\_wave}: uses a heavily modified \code{S/R~OBCS\_CALC}
525    \item \code{seaice\_obcs}: simple example who to use the sea-ice
526      related code, based on \code{lab\_sea};
527    \item \code{tutorial\_plume\_on\_slope}: uses Orlanski-BCs, see also
528      section~\ref{sec:eg-gravityplume}.
529    \end{itemize}
530    
531    
532    
# Line 270  Table \ref{tab:pkg:obcs:diagnostics}. Line 534  Table \ref{tab:pkg:obcs:diagnostics}.
534    
535  \subsubsection{References}  \subsubsection{References}
536    
537    \subsubsection{Experiments and tutorials that use obcs}
538    \label{sec:pkg:obcs:experiments}
539    
540    \begin{itemize}
541    \item \code{tutorial\_plume\_on\_slope} (section~\ref{sec:eg-gravityplume})
542    \end{itemize}
543    
544    
545    %%% Local Variables:
546    %%% mode: latex
547    %%% TeX-master: "../../manual"
548    %%% End:

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