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1 cnh 1.16 % $Header: /u/u0/gcmpack/mitgcmdoc/part3/case_studies/barotropic_gyre/baro.tex,v 1.15 2008/01/15 17:05:12 jmc Exp $
2 cnh 1.2 % $Name: $
3 adcroft 1.1
4     \bodytext{bgcolor="#FFFFFFFF"}
5    
6     %\begin{center}
7     %{\Large \bf Using MITgcm to Simulate a Barotropic Ocean Gyre In Cartesian
8     %Coordinates}
9     %
10     %\vspace*{4mm}
11     %
12     %\vspace*{3mm}
13     %{\large May 2001}
14     %\end{center}
15    
16 cnh 1.11 \section[Barotropic Gyre MITgcm Example]{Barotropic Ocean Gyre In Cartesian Coordinates}
17 cnh 1.16 \label{www:tutorials}
18 cnh 1.8 \label{sect:eg-baro}
19 edhill 1.12 \begin{rawhtml}
20     <!-- CMIREDIR:eg-baro: -->
21     \end{rawhtml}
22 jmc 1.15 \begin{center}
23     (in directory: {\it verification/tutorial\_barotropic\_gyre/})
24     \end{center}
25 adcroft 1.1
26     This example experiment demonstrates using the MITgcm to simulate
27 molod 1.14 a Barotropic, wind-forced, ocean gyre circulation. The files for this
28     experiment can be found in the verification directory tutorial\_barotropic\_gyre.
29     The experiment is a numerical rendition of the gyre circulation problem similar
30 adcroft 1.1 to the problems described analytically by Stommel in 1966
31     \cite{Stommel66} and numerically in Holland et. al \cite{Holland75}.
32    
33     In this experiment the model
34     is configured to represent a rectangular enclosed box of fluid,
35     $1200 \times 1200 $~km in lateral extent. The fluid is $5$~km deep and is forced
36     by a constant in time zonal wind stress, $\tau_x$, that varies sinusoidally
37 cnh 1.4 in the ``north-south'' direction. Topologically the grid is Cartesian and
38 adcroft 1.1 the coriolis parameter $f$ is defined according to a mid-latitude beta-plane
39     equation
40    
41     \begin{equation}
42 cnh 1.8 \label{EQ:eg-baro-fcori}
43 adcroft 1.1 f(y) = f_{0}+\beta y
44     \end{equation}
45    
46     \noindent where $y$ is the distance along the ``north-south'' axis of the
47     simulated domain. For this experiment $f_{0}$ is set to $10^{-4}s^{-1}$ in
48 cnh 1.8 (\ref{EQ:eg-baro-fcori}) and $\beta = 10^{-11}s^{-1}m^{-1}$.
49 adcroft 1.1 \\
50     \\
51     The sinusoidal wind-stress variations are defined according to
52    
53     \begin{equation}
54 cnh 1.8 \label{EQ:eg-baro-taux}
55 adcroft 1.1 \tau_x(y) = \tau_{0}\sin(\pi \frac{y}{L_y})
56     \end{equation}
57    
58     \noindent where $L_{y}$ is the lateral domain extent ($1200$~km) and
59     $\tau_0$ is set to $0.1N m^{-2}$.
60     \\
61     \\
62 cnh 1.8 Figure \ref{FIG:eg-baro-simulation_config}
63 cnh 1.4 summarizes the configuration simulated.
64 adcroft 1.1
65 edhill 1.10 %% === eh3 ===
66 adcroft 1.1 \begin{figure}
67 edhill 1.10 %% \begin{center}
68     %% \resizebox{7.5in}{5.5in}{
69     %% \includegraphics*[0.2in,0.7in][10.5in,10.5in]
70     %% {part3/case_studies/barotropic_gyre/simulation_config.eps} }
71     %% \end{center}
72     \centerline{
73     \scalefig{.95}
74     \epsfbox{part3/case_studies/barotropic_gyre/simulation_config.eps}
75     }
76 adcroft 1.1 \caption{Schematic of simulation domain and wind-stress forcing function
77     for barotropic gyre numerical experiment. The domain is enclosed bu solid
78     walls at $x=$~0,1200km and at $y=$~0,1200km.}
79 cnh 1.8 \label{FIG:eg-baro-simulation_config}
80 adcroft 1.1 \end{figure}
81    
82 cnh 1.3 \subsection{Equations Solved}
83 adcroft 1.9 \label{www:tutorials}
84 cnh 1.3 The model is configured in hydrostatic form. The implicit free surface form of the
85 adcroft 1.6 pressure equation described in Marshall et. al \cite{marshall:97a} is
86 cnh 1.3 employed.
87 cnh 1.4 A horizontal Laplacian operator $\nabla_{h}^2$ provides viscous
88 adcroft 1.1 dissipation. The wind-stress momentum input is added to the momentum equation
89     for the ``zonal flow'', $u$. Other terms in the model
90 cnh 1.4 are explicitly switched off for this experiment configuration (see section
91 cnh 1.3 \ref{SEC:code_config} ), yielding an active set of equations solved in this
92     configuration as follows
93 adcroft 1.1
94     \begin{eqnarray}
95 cnh 1.8 \label{EQ:eg-baro-model_equations}
96 cnh 1.3 \frac{Du}{Dt} - fv +
97     g\frac{\partial \eta}{\partial x} -
98     A_{h}\nabla_{h}^2u
99 adcroft 1.1 & = &
100     \frac{\tau_{x}}{\rho_{0}\Delta z}
101     \\
102     \frac{Dv}{Dt} + fu + g\frac{\partial \eta}{\partial y} -
103 cnh 1.3 A_{h}\nabla_{h}^2v
104 adcroft 1.1 & = &
105     0
106     \\
107     \frac{\partial \eta}{\partial t} + \nabla_{h}\cdot \vec{u}
108     &=&
109     0
110     \end{eqnarray}
111    
112     \noindent where $u$ and $v$ and the $x$ and $y$ components of the
113 cnh 1.3 flow vector $\vec{u}$.
114 adcroft 1.1 \\
115 cnh 1.3
116    
117     \subsection{Discrete Numerical Configuration}
118 adcroft 1.9 \label{www:tutorials}
119 cnh 1.3
120     The domain is discretised with
121     a uniform grid spacing in the horizontal set to
122     $\Delta x=\Delta y=20$~km, so
123     that there are sixty grid cells in the $x$ and $y$ directions. Vertically the
124     model is configured with a single layer with depth, $\Delta z$, of $5000$~m.
125 adcroft 1.1
126     \subsubsection{Numerical Stability Criteria}
127 adcroft 1.9 \label{www:tutorials}
128 adcroft 1.1
129 cnh 1.4 The Laplacian dissipation coefficient, $A_{h}$, is set to $400 m s^{-1}$.
130 adcroft 1.5 This value is chosen to yield a Munk layer width \cite{adcroft:95},
131 adcroft 1.1
132     \begin{eqnarray}
133 cnh 1.8 \label{EQ:eg-baro-munk_layer}
134 adcroft 1.1 M_{w} = \pi ( \frac { A_{h} }{ \beta } )^{\frac{1}{3}}
135     \end{eqnarray}
136    
137     \noindent of $\approx 100$km. This is greater than the model
138     resolution $\Delta x$, ensuring that the frictional boundary
139     layer is well resolved.
140     \\
141    
142     \noindent The model is stepped forward with a
143     time step $\delta t=1200$secs. With this time step the stability
144 adcroft 1.5 parameter to the horizontal Laplacian friction \cite{adcroft:95}
145 adcroft 1.1
146    
147    
148     \begin{eqnarray}
149 cnh 1.8 \label{EQ:eg-baro-laplacian_stability}
150 adcroft 1.1 S_{l} = 4 \frac{A_{h} \delta t}{{\Delta x}^2}
151     \end{eqnarray}
152    
153     \noindent evaluates to 0.012, which is well below the 0.3 upper limit
154     for stability.
155     \\
156    
157     \noindent The numerical stability for inertial oscillations
158 adcroft 1.5 \cite{adcroft:95}
159 adcroft 1.1
160     \begin{eqnarray}
161 cnh 1.8 \label{EQ:eg-baro-inertial_stability}
162 adcroft 1.1 S_{i} = f^{2} {\delta t}^2
163     \end{eqnarray}
164    
165     \noindent evaluates to $0.0144$, which is well below the $0.5$ upper
166     limit for stability.
167     \\
168    
169 adcroft 1.5 \noindent The advective CFL \cite{adcroft:95} for an extreme maximum
170 adcroft 1.1 horizontal flow speed of $ | \vec{u} | = 2 ms^{-1}$
171    
172     \begin{eqnarray}
173 cnh 1.8 \label{EQ:eg-baro-cfl_stability}
174 adcroft 1.1 S_{a} = \frac{| \vec{u} | \delta t}{ \Delta x}
175     \end{eqnarray}
176    
177     \noindent evaluates to 0.12. This is approaching the stability limit
178     of 0.5 and limits $\delta t$ to $1200s$.
179    
180     \subsection{Code Configuration}
181 adcroft 1.9 \label{www:tutorials}
182 cnh 1.8 \label{SEC:eg-baro-code_config}
183 adcroft 1.1
184     The model configuration for this experiment resides under the
185 jmc 1.15 directory {\it verification/tutorial\_barotropic\_gyre/}.
186     The experiment files
187 adcroft 1.1 \begin{itemize}
188     \item {\it input/data}
189     \item {\it input/data.pkg}
190     \item {\it input/eedata},
191     \item {\it input/windx.sin\_y},
192     \item {\it input/topog.box},
193     \item {\it code/CPP\_EEOPTIONS.h}
194     \item {\it code/CPP\_OPTIONS.h},
195     \item {\it code/SIZE.h}.
196     \end{itemize}
197 cnh 1.4 contain the code customizations and parameter settings for this
198     experiments. Below we describe the customizations
199 adcroft 1.1 to these files associated with this experiment.
200    
201     \subsubsection{File {\it input/data}}
202 adcroft 1.9 \label{www:tutorials}
203 adcroft 1.1
204     This file, reproduced completely below, specifies the main parameters
205     for the experiment. The parameters that are significant for this configuration
206     are
207    
208     \begin{itemize}
209    
210     \item Line 7, \begin{verbatim} viscAh=4.E2, \end{verbatim} this line sets
211 cnh 1.4 the Laplacian friction coefficient to $400 m^2s^{-1}$
212 adcroft 1.1 \item Line 10, \begin{verbatim} beta=1.E-11, \end{verbatim} this line sets
213     $\beta$ (the gradient of the coriolis parameter, $f$) to $10^{-11} s^{-1}m^{-1}$
214    
215     \item Lines 15 and 16
216     \begin{verbatim}
217     rigidLid=.FALSE.,
218     implicitFreeSurface=.TRUE.,
219     \end{verbatim}
220     these lines suppress the rigid lid formulation of the surface
221     pressure inverter and activate the implicit free surface form
222     of the pressure inverter.
223    
224     \item Line 27,
225     \begin{verbatim}
226     startTime=0,
227     \end{verbatim}
228     this line indicates that the experiment should start from $t=0$
229 cnh 1.4 and implicitly suppresses searching for checkpoint files associated
230 adcroft 1.1 with restarting an numerical integration from a previously saved state.
231    
232     \item Line 29,
233     \begin{verbatim}
234     endTime=12000,
235     \end{verbatim}
236     this line indicates that the experiment should start finish at $t=12000s$.
237     A restart file will be written at this time that will enable the
238     simulation to be continued from this point.
239    
240     \item Line 30,
241     \begin{verbatim}
242     deltaTmom=1200,
243     \end{verbatim}
244     This line sets the momentum equation timestep to $1200s$.
245    
246     \item Line 39,
247     \begin{verbatim}
248     usingCartesianGrid=.TRUE.,
249     \end{verbatim}
250     This line requests that the simulation be performed in a
251 cnh 1.4 Cartesian coordinate system.
252 adcroft 1.1
253     \item Line 41,
254     \begin{verbatim}
255     delX=60*20E3,
256     \end{verbatim}
257     This line sets the horizontal grid spacing between each x-coordinate line
258     in the discrete grid. The syntax indicates that the discrete grid
259     should be comprise of $60$ grid lines each separated by $20 \times 10^{3}m$
260     ($20$~km).
261    
262     \item Line 42,
263     \begin{verbatim}
264     delY=60*20E3,
265     \end{verbatim}
266     This line sets the horizontal grid spacing between each y-coordinate line
267     in the discrete grid to $20 \times 10^{3}m$ ($20$~km).
268    
269     \item Line 43,
270     \begin{verbatim}
271     delZ=5000,
272     \end{verbatim}
273     This line sets the vertical grid spacing between each z-coordinate line
274     in the discrete grid to $5000m$ ($5$~km).
275    
276     \item Line 46,
277     \begin{verbatim}
278     bathyFile='topog.box'
279     \end{verbatim}
280     This line specifies the name of the file from which the domain
281     bathymetry is read. This file is a two-dimensional ($x,y$) map of
282     depths. This file is assumed to contain 64-bit binary numbers
283     giving the depth of the model at each grid cell, ordered with the x
284     coordinate varying fastest. The points are ordered from low coordinate
285     to high coordinate for both axes. The units and orientation of the
286     depths in this file are the same as used in the MITgcm code. In this
287     experiment, a depth of $0m$ indicates a solid wall and a depth
288     of $-5000m$ indicates open ocean. The matlab program
289     {\it input/gendata.m} shows an example of how to generate a
290     bathymetry file.
291    
292    
293     \item Line 49,
294     \begin{verbatim}
295     zonalWindFile='windx.sin_y'
296     \end{verbatim}
297     This line specifies the name of the file from which the x-direction
298     surface wind stress is read. This file is also a two-dimensional
299     ($x,y$) map and is enumerated and formatted in the same manner as the
300     bathymetry file. The matlab program {\it input/gendata.m} includes example
301     code to generate a valid {\bf zonalWindFile} file.
302    
303     \end{itemize}
304    
305     \noindent other lines in the file {\it input/data} are standard values
306     that are described in the MITgcm Getting Started and MITgcm Parameters
307     notes.
308    
309     \begin{small}
310     \input{part3/case_studies/barotropic_gyre/input/data}
311     \end{small}
312    
313     \subsubsection{File {\it input/data.pkg}}
314 adcroft 1.9 \label{www:tutorials}
315 adcroft 1.1
316     This file uses standard default values and does not contain
317 cnh 1.4 customizations for this experiment.
318 adcroft 1.1
319     \subsubsection{File {\it input/eedata}}
320 adcroft 1.9 \label{www:tutorials}
321 adcroft 1.1
322     This file uses standard default values and does not contain
323 cnh 1.4 customizations for this experiment.
324 adcroft 1.1
325     \subsubsection{File {\it input/windx.sin\_y}}
326 adcroft 1.9 \label{www:tutorials}
327 adcroft 1.1
328     The {\it input/windx.sin\_y} file specifies a two-dimensional ($x,y$)
329     map of wind stress ,$\tau_{x}$, values. The units used are $Nm^{-2}$.
330     Although $\tau_{x}$ is only a function of $y$n in this experiment
331     this file must still define a complete two-dimensional map in order
332     to be compatible with the standard code for loading forcing fields
333     in MITgcm. The included matlab program {\it input/gendata.m} gives a complete
334     code for creating the {\it input/windx.sin\_y} file.
335    
336     \subsubsection{File {\it input/topog.box}}
337 adcroft 1.9 \label{www:tutorials}
338 adcroft 1.1
339    
340     The {\it input/topog.box} file specifies a two-dimensional ($x,y$)
341     map of depth values. For this experiment values are either
342     $0m$ or {\bf -delZ}m, corresponding respectively to a wall or to deep
343     ocean. The file contains a raw binary stream of data that is enumerated
344     in the same way as standard MITgcm two-dimensional, horizontal arrays.
345     The included matlab program {\it input/gendata.m} gives a complete
346     code for creating the {\it input/topog.box} file.
347    
348     \subsubsection{File {\it code/SIZE.h}}
349 adcroft 1.9 \label{www:tutorials}
350 adcroft 1.1
351     Two lines are customized in this file for the current experiment
352    
353     \begin{itemize}
354    
355     \item Line 39,
356     \begin{verbatim} sNx=60, \end{verbatim} this line sets
357     the lateral domain extent in grid points for the
358     axis aligned with the x-coordinate.
359    
360     \item Line 40,
361     \begin{verbatim} sNy=60, \end{verbatim} this line sets
362     the lateral domain extent in grid points for the
363     axis aligned with the y-coordinate.
364    
365     \end{itemize}
366    
367     \begin{small}
368     \input{part3/case_studies/barotropic_gyre/code/SIZE.h}
369     \end{small}
370    
371     \subsubsection{File {\it code/CPP\_OPTIONS.h}}
372 adcroft 1.9 \label{www:tutorials}
373 adcroft 1.1
374     This file uses standard default values and does not contain
375 cnh 1.4 customizations for this experiment.
376 adcroft 1.1
377    
378     \subsubsection{File {\it code/CPP\_EEOPTIONS.h}}
379 adcroft 1.9 \label{www:tutorials}
380 adcroft 1.1
381     This file uses standard default values and does not contain
382 cnh 1.4 customizations for this experiment.
383 adcroft 1.1

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