[MITgcm-support] KPP implementation

[Renske Gelderloos]

Dear fellow MITgcm-ers,

I have a couple of questions about the implementation of the KPP scheme in the MITgcm code. I am interested in using the option of estimating reference values instead of using the surface values, because my model setup has a high vertical resolution. There are a couple of things that have me puzzled in that part of the code:

1) In general, the boundary layer depth is defined as the shallowest depth where the bulk Richardson number is equal to its critical value. Since we need the boundary layer depth to calculate the surface layer depth where MOST holds, I understand that this provides a chicken-and-egg-type difficulty in practice. It looks like the boundary layer depth (hMix) in this case is set equal to the shallowest depth where dB/dz exceeds a hard-coded value of 5.2e-5 s^-2.
1a) While this parameter value can be changed in the data.kpp file, I can’t find a reference to how this particular value was chosen. Was this based on literature, expert judgement, something else?
1b) Once the boundary layer depth using this threshold value is determined, this is what is used; in other words, it is not recalculated after the boundary layer depth has been estimated based on the Richardson number criterion. Shouldn't it be? Or is the difference not large enough to warrant the additional computational expense?
1c) Since the search for hMix starts at the bottom, wouldn’t this way of estimating the boundary layer depth potentially pose a problem in the presence of a bottom boundary layer?


2) The comments in kpp_forcing_surf.F reiterate a few times that the current implementation does not handle cases where the surface layer depth exceeds the thickness of the first layer (i.e., high resolution). However, the statement that rules out that option (z0(i,j) = MIN(zRef(i,j),drF(1)) is commented out, and so the IF statement in the calculation of uRef and vRef handles both high resolution and coarse resolution cases. Are the comments out of date, or am I missing something/interpreting this incorrectly?

Thanks for any input on this!
Renske




Dr. Renske Gelderloos (she/her)
Associate Research Scientist | Earth and Planetary Sciences
Krieger School of Arts & Sciences
Johns Hopkins University
Olin Hall 328, Baltimore, MD 21218
O: 410-516-5631
[Johns Hopkins University]<https://www.jhu.edu/>
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