Add Leith+E #250
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Add Leith+E #250
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This commit adds the 2D Leith+E closure, which uses a modified
2D Leith biharmonic viscosity paired with a harmonic backscatter.
('Modified' here is not used in the same sense as 'modified
2D Leith'; it just means that the biharmonic coefficient is
modified to account for enstrophy backscatter.) Variables
are often named 'leithy' to refer to Leith+E.
The parameterization is controlled by three main entries in
user_nl_mom:
1. USE_LEITHY = True
2. LEITH_CK = 1.0
3. LEITH_BI_CONST = 8.0
To use Leith+E you should have LAPLACIAN=True and BIHARMONIC=True.
(It doesn't hurt to be explicit and also set LEITH_AH=False, along
with any other viscous closures, but this is not required. If
USE_LEITHY=True it will not use any of the other schemes.)
LEITH_CK is the fraction of energy dissipated by the biharmonic
term that gets backscattered by the harmonic term (it's a target;
the backscatter rate is not exact.) Recommended values between 0
and 1.
LEITH_BI_CONST is Upsilon^6 where Upsilon is the ratio between
the dissipation scale for enstrophy and the grid scale. Values
should probably be greater than or equal to 1.
This commit also includes a merge with dev/ncar. Leith+E only
changes MOM_hor_visc.F90; all other changes are associated with
the merge.
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This commit adds the 2D Leith+E closure, which uses a modified 2D Leith biharmonic viscosity paired with a harmonic backscatter. ('Modified' here is not used in the same sense as 'modified 2D Leith'; it just means that the biharmonic coefficient is modified to account for enstrophy backscatter.) Variables are often named 'leithy' to refer to Leith+E.
The parameterization is controlled by three main entries in user_nl_mom:
To use Leith+E you should have LAPLACIAN=True and BIHARMONIC=True. (It doesn't hurt to be explicit and also set LEITH_AH=False, along with any other viscous closures, but this is not required. If USE_LEITHY=True it will not use any of the other schemes. It does use the background value of the biharmonic coefficient as a minimum, but ignores the
background harmonic value.) LEITH_CK is the fraction of energy dissipated by the biharmonic term that gets backscattered by the harmonic term (it's a target; the backscatter rate is not exact.) Recommended values between 0 and 1. LEITH_BI_CONST is Upsilon^6 where Upsilon is the ratio between the grid scale and the dissipation scale for enstrophy.
Values should be greater than or equal to 1; 8 is a good place to start.
The code is sensitive to the background value of Ah; specifically, if Ah is too large, the code is unstable. This is because the backscatter coefficient is proportional to Ah, and if Ah is large then you get large backscatter. If your code is unstable, consider reducing, e.g.,
AH_VEL_SCALE.