[MITgcm-support] Student with a question/issue with the KPP package

[Martin Losch]

Hi Taylor,

I would use the diagnostic MXLDEPTH, which independently of KPP computes some kind of MLD based on a density difference criterion. I am actually not sure if it does so also in offline simulations.

I thought that in offline simulations, KPP is basically passive and you need to provide (KPP) mixing coefficients along with your 3D flow field. I had a quick look at
MITgcm/verification/tutorial_cfc_offline/ 
(which does not use KPP) and there it seems that the GM-coefficents and some Convective adjustment parameter are given to the offline model. I assume that this is the same for KPP. There’s also code to compute KPP coefficients (diffKzS and ghat) from fields that are read.

Martin

> On 20 Jun 2018, at 20:13, Dhruv Balwada <db194 at nyu.edu> wrote:
> 
> Hi Taylor, 
> 
> KPP computes a mixed layer depth that depends on surface forcing, ocean buoyancy profile, and velocity profile. A Richardson number is calculated (equation 21 in Large 1994 Rev. of Geophysics), and the Hbl set to the smallest depth where Richardson number equals some critical value. As you are doing an offline calculation, I am assuming you have no forcing. Also, because you use daily averaged velocities, which are presumably smooth, you don't generate small Ri in the domain. You could try to look at what the boundary layer depth was in the online simulation that generated the velocity fields in the first place, if you just want an estimate. If you care about having a region of enhanced mixing near the surface in the biogeochem model, then you might need to think of a more clever work around. 
> 
> Best,
> Dhruv
> 
> On Wed, Jun 20, 2018 at 12:41 PM, Taylor Shropshire <tas14j at my.fsu.edu> wrote:
> Hello,
> 
> 
> Background:
> 
> I am running an offline biogeochemical model using MITgcm for the Gulf of Mexico. Flow fields are from previously computed daily averages and are prescribed daily in the model.
> 
> 
> Issue: 
> 
> I have KPP outputting its variables weekly: KPPviscAz, KPPdiffKzT, KPPdiffKzS, KPPGHAT, KPPHBL. 
> 
> 
> When I look at the output of KPPHBL (mixed layer depth) I find that it is always 5m at every record at every grid point. Why is this happening in my simulations?  
> 
> 
> Thanks for your time, 
> 
> -Taylor
> 
> 
> 
> 
> Taylor Shropshire
> PhD Candidate - Oceanography
> Center for Ocean - Atmospheric Prediction Studies
> Florida State University
> 
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