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1 \subsection{GMREDI: Gent-McWilliams/Redi SGS Eddy Parameterization}
2 \label{sec:pkg:gmredi}
3 \begin{rawhtml}
4 <!-- CMIREDIR:gmredi: -->
5 \end{rawhtml}
6
7 There are two parts to the Redi/GM parameterization of geostrophic
8 eddies. The first, the Redi scheme \citep{re82}, aims to mix tracer properties
9 along isentropes (neutral surfaces) by means of a diffusion operator oriented
10 along the local isentropic surface.
11 The second part, GM \citep{gen-mcw:90,gen-eta:95}, adiabatically
12 re-arranges tracers through an advective flux where the advecting flow
13 is a function of slope of the isentropic surfaces.
14
15 The first GCM implementation of the Redi scheme was by \cite{Cox87} in the
16 GFDL ocean circulation model. The original approach failed to
17 distinguish between isopycnals and surfaces of locally referenced
18 potential density (now called neutral surfaces) which are proper
19 isentropes for the ocean. As will be discussed later, it also appears
20 that the Cox implementation is susceptible to a computational mode.
21 Due to this mode, the Cox scheme requires a background lateral
22 diffusion to be present to conserve the integrity of the model fields.
23
24 The GM parameterization was then added to the GFDL code in the form of
25 a non-divergent bolus velocity. The method defines two
26 stream-functions expressed in terms of the isoneutral slopes subject
27 to the boundary condition of zero value on upper and lower
28 boundaries. The horizontal bolus velocities are then the vertical
29 derivative of these functions. Here in lies a problem highlighted by
30 \cite{gretal:98}: the bolus velocities involve multiple
31 derivatives on the potential density field, which can consequently
32 give rise to noise. Griffies et al. point out that the GM bolus fluxes
33 can be identically written as a skew flux which involves fewer
34 differential operators. Further, combining the skew flux formulation
35 and Redi scheme, substantial cancellations take place to the point
36 that the horizontal fluxes are unmodified from the lateral diffusion
37 parameterization.
38
39 \subsubsection{Redi scheme: Isopycnal diffusion}
40
41 The Redi scheme diffuses tracers along isopycnals and introduces a
42 term in the tendency (rhs) of such a tracer (here $\tau$) of the form:
43 \begin{equation}
44 \bf{\nabla} \cdot \kappa_\rho \bf{K}_{Redi} \bf{\nabla} \tau
45 \end{equation}
46 where $\kappa_\rho$ is the along isopycnal diffusivity and
47 $\bf{K}_{Redi}$ is a rank 2 tensor that projects the gradient of
48 $\tau$ onto the isopycnal surface. The unapproximated projection tensor is:
49 \begin{equation}
50 \bf{K}_{Redi} = \frac{1}{1 + |S|^2} \left(
51 \begin{array}{ccc}
52 1 + S_y^2& -S_x S_y & S_x \\
53 -S_x S_y & 1 + S_x^2 & S_y \\
54 S_x & S_y & |S|^2 \\
55 \end{array}
56 \right)
57 \end{equation}
58 Here, $S_x = -\partial_x \sigma / \partial_z \sigma$ and $S_y =
59 -\partial_y \sigma / \partial_z \sigma$ are the components of the
60 isoneutral slope.
61
62 The first point to note is that a typical slope in the ocean interior
63 is small, say of the order $10^{-4}$. A maximum slope might be of
64 order $10^{-2}$ and only exceeds such in unstratified regions where
65 the slope is ill defined. It is therefore justifiable, and
66 customary, to make the small slope approximation, $|S| << 1$. The Redi
67 projection tensor then becomes:
68 \begin{equation}
69 \bf{K}_{Redi} = \left(
70 \begin{array}{ccc}
71 1 & 0 & S_x \\
72 0 & 1 & S_y \\
73 S_x & S_y & |S|^2 \\
74 \end{array}
75 \right)
76 \end{equation}
77
78
79 \subsubsection{GM parameterization}
80
81 The GM parameterization aims to represent the ``advective'' or
82 ``transport'' effect of geostrophic eddies by means of a ``bolus''
83 velocity, $\bf{u}^\star$. The divergence of this advective flux is added
84 to the tracer tendency equation (on the rhs):
85 \begin{equation}
86 - \bf{\nabla} \cdot \tau \bf{u}^\star
87 \end{equation}
88
89 The bolus velocity $\bf{u}^\star$ is defined as the rotational of a
90 streamfunction $\bf{F}^\star$=$(F_x^\star,F_y^\star,0)$:
91 \begin{equation}
92 \bf{u}^\star = \nabla \times \bf{F}^\star =
93 \left( \begin{array}{c}
94 - \partial_z F_y^\star \\
95 + \partial_z F_x^\star \\
96 \partial_x F_y^\star - \partial_y F_x^\star
97 \end{array} \right),
98 \end{equation}
99 and thus is automatically non-divergent. In the GM parameterization, the streamfunction is
100 specified in terms of the isoneutral slopes $S_x$ and $S_y$:
101 \begin{eqnarray}
102 F_x^\star & = & -\kappa_{GM} S_y \\
103 F_y^\star & = & \kappa_{GM} S_x
104 \end{eqnarray}
105 with boundary conditions $F_x^\star=F_y^\star=0$ on upper and lower boundaries.
106 In the end, the bolus transport in the GM parameterization is given by:
107 \begin{equation}
108 \bf{u}^\star = \left(
109 \begin{array}{c}
110 u^\star \\
111 v^\star \\
112 w^\star
113 \end{array}
114 \right) = \left(
115 \begin{array}{c}
116 - \partial_z (\kappa_{GM} S_x) \\
117 - \partial_z (\kappa_{GM} S_y) \\
118 \partial_x (\kappa_{GM} S_x) + \partial_y (\kappa_{GM} S_y)
119 \end{array}
120 \right)
121 \end{equation}
122
123 This is the form of the GM parameterization as applied by Donabasaglu,
124 1997, in MOM versions 1 and 2.
125
126 Note that in the MITgcm, the variables containing the GM bolus streamfunction are:
127 \begin{equation}
128 \left(
129 \begin{array}{c}
130 GM\_PsiX \\
131 GM\_PsiY
132 \end{array}
133 \right) = \left(
134 \begin{array}{c}
135 \kappa_{GM} S_x \\
136 \kappa_{GM} S_y
137 \end{array}
138 \right)= \left(
139 \begin{array}{c}
140 F_y^\star \\
141 -F_x^\star
142 \end{array}
143 \right).
144 \end{equation}
145
146 \subsubsection{Griffies Skew Flux}
147
148 \cite{gr:98} notes that the discretisation of bolus velocities involves
149 multiple layers of differencing and interpolation that potentially
150 lead to noisy fields and computational modes. He pointed out that the
151 bolus flux can be re-written in terms of a non-divergent flux and a
152 skew-flux:
153 \begin{eqnarray*}
154 \bf{u}^\star \tau
155 & = &
156 \left( \begin{array}{c}
157 - \partial_z ( \kappa_{GM} S_x ) \tau \\
158 - \partial_z ( \kappa_{GM} S_y ) \tau \\
159 (\partial_x \kappa_{GM} S_x + \partial_y \kappa_{GM} S_y)\tau
160 \end{array} \right)
161 \\
162 & = &
163 \left( \begin{array}{c}
164 - \partial_z ( \kappa_{GM} S_x \tau) \\
165 - \partial_z ( \kappa_{GM} S_y \tau) \\
166 \partial_x ( \kappa_{GM} S_x \tau) + \partial_y ( \kappa_{GM} S_y \tau)
167 \end{array} \right)
168 + \left( \begin{array}{c}
169 \kappa_{GM} S_x \partial_z \tau \\
170 \kappa_{GM} S_y \partial_z \tau \\
171 - \kappa_{GM} S_x \partial_x \tau - \kappa_{GM} S_y \partial_y \tau
172 \end{array} \right)
173 \end{eqnarray*}
174 The first vector is non-divergent and thus has no effect on the tracer
175 field and can be dropped. The remaining flux can be written:
176 \begin{equation}
177 \bf{u}^\star \tau = - \kappa_{GM} \bf{K}_{GM} \bf{\nabla} \tau
178 \end{equation}
179 where
180 \begin{equation}
181 \bf{K}_{GM} =
182 \left(
183 \begin{array}{ccc}
184 0 & 0 & -S_x \\
185 0 & 0 & -S_y \\
186 S_x & S_y & 0
187 \end{array}
188 \right)
189 \end{equation}
190 is an anti-symmetric tensor.
191
192 This formulation of the GM parameterization involves fewer derivatives
193 than the original and also involves only terms that already appear in
194 the Redi mixing scheme. Indeed, a somewhat fortunate cancellation
195 becomes apparent when we use the GM parameterization in conjunction
196 with the Redi isoneutral mixing scheme:
197 \begin{equation}
198 \kappa_\rho \bf{K}_{Redi} \bf{\nabla} \tau
199 - u^\star \tau =
200 ( \kappa_\rho \bf{K}_{Redi} + \kappa_{GM} \bf{K}_{GM} ) \bf{\nabla} \tau
201 \end{equation}
202 In the instance that $\kappa_{GM} = \kappa_{\rho}$ then
203 \begin{equation}
204 \kappa_\rho \bf{K}_{Redi} + \kappa_{GM} \bf{K}_{GM} =
205 \kappa_\rho
206 \left( \begin{array}{ccc}
207 1 & 0 & 0 \\
208 0 & 1 & 0 \\
209 2 S_x & 2 S_y & |S|^2
210 \end{array}
211 \right)
212 \end{equation}
213 which differs from the variable Laplacian diffusion tensor by only
214 two non-zero elements in the $z$-row.
215
216 \fbox{ \begin{minipage}{4.75in}
217 {\em S/R GMREDI\_CALC\_TENSOR} ({\em pkg/gmredi/gmredi\_calc\_tensor.F})
218
219 $\sigma_x$: {\bf SlopeX} (argument on entry)
220
221 $\sigma_y$: {\bf SlopeY} (argument on entry)
222
223 $\sigma_z$: {\bf SlopeY} (argument)
224
225 $S_x$: {\bf SlopeX} (argument on exit)
226
227 $S_y$: {\bf SlopeY} (argument on exit)
228
229 \end{minipage} }
230
231
232
233 \subsubsection{Variable $\kappa_{GM}$}
234
235 \cite{visbeck:97} suggest making the eddy coefficient,
236 $\kappa_{GM}$, a function of the Eady growth rate,
237 $|f|/\sqrt{Ri}$. The formula involves a non-dimensional constant,
238 $\alpha$, and a length-scale $L$:
239 \begin{displaymath}
240 \kappa_{GM} = \alpha L^2 \overline{ \frac{|f|}{\sqrt{Ri}} }^z
241 \end{displaymath}
242 where the Eady growth rate has been depth averaged (indicated by the
243 over-line). A local Richardson number is defined $Ri = N^2 / (\partial
244 u/\partial z)^2$ which, when combined with thermal wind gives:
245 \begin{displaymath}
246 \frac{1}{Ri} = \frac{(\frac{\partial u}{\partial z})^2}{N^2} =
247 \frac{ ( \frac{g}{f \rho_o} | {\bf \nabla} \sigma | )^2 }{N^2} =
248 \frac{ M^4 }{ |f|^2 N^2 }
249 \end{displaymath}
250 where $M^2$ is defined $M^2 = \frac{g}{\rho_o} |{\bf \nabla} \sigma|$.
251 Substituting into the formula for $\kappa_{GM}$ gives:
252 \begin{displaymath}
253 \kappa_{GM} = \alpha L^2 \overline{ \frac{M^2}{N} }^z =
254 \alpha L^2 \overline{ \frac{M^2}{N^2} N }^z =
255 \alpha L^2 \overline{ |S| N }^z
256 \end{displaymath}
257
258
259 \subsubsection{Tapering and stability}
260
261 Experience with the GFDL model showed that the GM scheme has to be
262 matched to the convective parameterization. This was originally
263 expressed in connection with the introduction of the KPP boundary
264 layer scheme \citep{lar-eta:94} but in fact, as subsequent experience
265 with the MIT model has found, is necessary for any convective
266 parameterization.
267
268 \fbox{ \begin{minipage}{4.75in}
269 {\em S/R GMREDI\_SLOPE\_LIMIT} ({\em
270 pkg/gmredi/gmredi\_slope\_limit.F})
271
272 $\sigma_x, s_x$: {\bf SlopeX} (argument)
273
274 $\sigma_y, s_y$: {\bf SlopeY} (argument)
275
276 $\sigma_z$: {\bf dSigmadRReal} (argument)
277
278 $z_\sigma^{*}$: {\bf dRdSigmaLtd} (argument)
279
280 \end{minipage} }
281
282 \begin{figure}
283 \begin{center}
284 \resizebox{5.0in}{3.0in}{\includegraphics{s_phys_pkgs/figs/tapers.eps}}
285 \end{center}
286 \caption{Taper functions used in GKW91 and DM95.}
287 \label{fig:tapers}
288 \end{figure}
289
290 \begin{figure}
291 \begin{center}
292 \resizebox{5.0in}{3.0in}{\includegraphics{s_phys_pkgs/figs/effective_slopes.eps}}
293 \end{center}
294 \caption{Effective slope as a function of ``true'' slope using Cox
295 slope clipping, GKW91 limiting and DM95 limiting.}
296 \label{fig:effective_slopes}
297 \end{figure}
298
299
300 \subsubsection*{Slope clipping}
301
302 Deep convection sites and the mixed layer are indicated by
303 homogenized, unstable or nearly unstable stratification. The slopes in
304 such regions can be either infinite, very large with a sign reversal
305 or simply very large. From a numerical point of view, large slopes
306 lead to large variations in the tensor elements (implying large bolus
307 flow) and can be numerically unstable. This was first recognized by
308 \cite{Cox87} who implemented ``slope clipping'' in the isopycnal mixing
309 tensor. Here, the slope magnitude is simply restricted by an upper
310 limit:
311 \begin{eqnarray}
312 |\nabla \sigma| & = & \sqrt{ \sigma_x^2 + \sigma_y^2 } \\
313 S_{lim} & = & - \frac{|\nabla \sigma|}{ S_{max} }
314 \;\;\;\;\;\;\;\; \mbox{where $S_{max}$ is a parameter} \\
315 \sigma_z^\star & = & \min( \sigma_z , S_{lim} ) \\
316 {[s_x,s_y]} & = & - \frac{ [\sigma_x,\sigma_y] }{\sigma_z^\star}
317 \end{eqnarray}
318 Notice that this algorithm assumes stable stratification through the
319 ``min'' function. In the case where the fluid is well stratified ($\sigma_z < S_{lim}$) then the slopes evaluate to:
320 \begin{equation}
321 {[s_x,s_y]} = - \frac{ [\sigma_x,\sigma_y] }{\sigma_z}
322 \end{equation}
323 while in the limited regions ($\sigma_z > S_{lim}$) the slopes become:
324 \begin{equation}
325 {[s_x,s_y]} = \frac{ [\sigma_x,\sigma_y] }{|\nabla \sigma|/S_{max}}
326 \end{equation}
327 so that the slope magnitude is limited $\sqrt{s_x^2 + s_y^2} =
328 S_{max}$.
329
330 The slope clipping scheme is activated in the model by setting {\bf
331 GM\_tap\-er\_scheme = 'clipping'} in {\em data.gmredi}.
332
333 Even using slope clipping, it is normally the case that the vertical
334 diffusion term (with coefficient $\kappa_\rho{\bf K}_{33} =
335 \kappa_\rho S_{max}^2$) is large and must be time-stepped using an
336 implicit procedure (see section on discretisation and code later).
337 Fig. \ref{fig-mixedlayer} shows the mixed layer depth resulting from
338 a) using the GM scheme with clipping and b) no GM scheme (horizontal
339 diffusion). The classic result of dramatically reduced mixed layers is
340 evident. Indeed, the deep convection sites to just one or two points
341 each and are much shallower than we might prefer. This, it turns out,
342 is due to the over zealous re-stratification due to the bolus transport
343 parameterization. Limiting the slopes also breaks the adiabatic nature
344 of the GM/Redi parameterization, re-introducing diabatic fluxes in
345 regions where the limiting is in effect.
346
347 \subsubsection*{Tapering: Gerdes, Koberle and Willebrand, Clim. Dyn. 1991}
348
349 The tapering scheme used in \cite{gkw:91}
350 addressed two issues with the clipping method: the introduction of
351 large vertical fluxes in addition to convective adjustment fluxes is
352 avoided by tapering the GM/Redi slopes back to zero in
353 low-stratification regions; the adjustment of slopes is replaced by a
354 tapering of the entire GM/Redi tensor. This means the direction of
355 fluxes is unaffected as the amplitude is scaled.
356
357 The scheme inserts a tapering function, $f_1(S)$, in front of the
358 GM/Redi tensor:
359 \begin{equation}
360 f_1(S) = \min \left[ 1, \left( \frac{S_{max}}{|S|}\right)^2 \right]
361 \end{equation}
362 where $S_{max}$ is the maximum slope you want allowed. Where the
363 slopes, $|S|<S_{max}$ then $f_1(S) = 1$ and the tensor is un-tapered
364 but where $|S| \ge S_{max}$ then $f_1(S)$ scales down the tensor so
365 that the effective vertical diffusivity term $\kappa f_1(S) |S|^2 =
366 \kappa S_{max}^2$.
367
368 The GKW91 tapering scheme is activated in the model by setting {\bf
369 GM\_tap\-er\_scheme = 'gkw91'} in {\em data.gmredi}.
370
371 \subsubsection*{Tapering: Danabasoglu and McWilliams, J. Clim. 1995}
372
373 The tapering scheme used by \cite{dm:95} followed a similar procedure but used a different
374 tapering function, $f_1(S)$:
375 \begin{equation}
376 f_1(S) = \frac{1}{2} \left( 1+\tanh \left[ \frac{S_c - |S|}{S_d} \right] \right)
377 \end{equation}
378 where $S_c = 0.004$ is a cut-off slope and $S_d=0.001$ is a scale over
379 which the slopes are smoothly tapered. Functionally, the operates in
380 the same way as the GKW91 scheme but has a substantially lower
381 cut-off, turning off the GM/Redi SGS parameterization for weaker
382 slopes.
383
384 The DM95 tapering scheme is activated in the model by setting {\bf
385 GM\_tap\-er\_scheme = 'dm95'} in {\em data.gmredi}.
386
387 \subsubsection*{Tapering: Large, Danabasoglu and Doney, JPO 1997}
388
389 The tapering used in \cite{lar-eta:97} is based on the
390 DM95 tapering scheme, but also tapers the scheme with an additional
391 function of height, $f_2(z)$, so that the GM/Redi SGS fluxes are
392 reduced near the surface:
393 \begin{equation}
394 f_2(z) = \frac{1}{2} \left( 1 + \sin(\pi \frac{z}{D} - \frac{\pi}{2})\right)
395 \end{equation}
396 where $D = L_\rho |S|$ is a depth-scale and $L_\rho=c/f$ with
397 $c=2$~m~s$^{-1}$. This tapering with height was introduced to fix
398 some spurious interaction with the mixed-layer KPP parameterization.
399
400 The LDD97 tapering scheme is activated in the model by setting {\bf
401 GM\_tap\-er\_scheme = 'ldd97'} in {\em data.gmredi}.
402
403
404
405
406 \begin{figure}
407 \begin{center}
408 %\includegraphics{mixedlayer-cox.eps}
409 %\includegraphics{mixedlayer-diff.eps}
410 Figure missing.
411 \end{center}
412 \caption{Mixed layer depth using GM parameterization with a) Cox slope
413 clipping and for comparison b) using horizontal constant diffusion.}
414 \label{fig-mixedlayer}
415 \end{figure}
416
417 \subsubsection{Package Reference}
418 \label{sec:pkg:gmredi:diagnostics}
419
420 {\footnotesize
421 \begin{verbatim}
422 ------------------------------------------------------------------------
423 <-Name->|Levs|<-parsing code->|<-- Units -->|<- Tile (max=80c)
424 ------------------------------------------------------------------------
425 GM_VisbK| 1 |SM P M1 |m^2/s |Mixing coefficient from Visbeck etal parameterization
426 GM_Kux | 15 |UU P 177MR |m^2/s |K_11 element (U.point, X.dir) of GM-Redi tensor
427 GM_Kvy | 15 |VV P 176MR |m^2/s |K_22 element (V.point, Y.dir) of GM-Redi tensor
428 GM_Kuz | 15 |UU 179MR |m^2/s |K_13 element (U.point, Z.dir) of GM-Redi tensor
429 GM_Kvz | 15 |VV 178MR |m^2/s |K_23 element (V.point, Z.dir) of GM-Redi tensor
430 GM_Kwx | 15 |UM 181LR |m^2/s |K_31 element (W.point, X.dir) of GM-Redi tensor
431 GM_Kwy | 15 |VM 180LR |m^2/s |K_32 element (W.point, Y.dir) of GM-Redi tensor
432 GM_Kwz | 15 |WM P LR |m^2/s |K_33 element (W.point, Z.dir) of GM-Redi tensor
433 GM_PsiX | 15 |UU 184LR |m^2/s |GM Bolus transport stream-function : X component
434 GM_PsiY | 15 |VV 183LR |m^2/s |GM Bolus transport stream-function : Y component
435 GM_KuzTz| 15 |UU 186MR |degC.m^3/s |Redi Off-diagonal Tempetature flux: X component
436 GM_KvzTz| 15 |VV 185MR |degC.m^3/s |Redi Off-diagonal Tempetature flux: Y component
437 \end{verbatim}
438 }
439
440 \subsubsection{Experiments and tutorials that use gmredi}
441 \label{sec:pkg:gmredi:experiments}
442
443 \begin{itemize}
444 \item{Global Ocean tutorial, in tutorial\_global\_oce\_latlon verification directory,
445 described in section \ref{sec:eg-global} }
446 \item{ Front Relax experiment, in front\_relax verification directory.}
447 \item{ Ideal 2D Ocean experiment, in ideal\_2D\_oce verification directory.}
448 \end{itemize}
449
450 % DO NOT EDIT HERE
451
452
453

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