[MITgcm-support] KPP with inverse stratification

[Andrew Grace]

Hi Jody, thanks for the reply.


If you look on line 390 in kpp_calc.F (http://lxr.mitgcm.org/lxr2/source/MITgcm/pkg/kpp/kpp_calc.F), there is a comment that says


bo = -g *( alpha * surfaceForcingT + beta * surfaceforcingS) / rho


which is what lead me to assume that the equation of state was hardcoded to be linear.


However, that assumption is wrong because I see a little while later that the expansion coefficient, alpha, is calculated via a function that talks with the equation of state. This means that alpha could be positive or negative which takes care of the case when you have an inverse stratification.


I guess I just didn't look far enough.


Thanks

________________________________
From: MITgcm-support <mitgcm-support-bounces at mitgcm.org> on behalf of Jody Klymak <jklymak at uvic.ca>
Sent: January 3, 2020 3:31:57 PM
To: mitgcm-support at mitgcm.org
Subject: Re: [MITgcm-support] KPP with inverse stratification

Hi Andrew,

I didn’t spend a lot of time looking, but KPP seems to call `FIND_RHO_2D` to calcualate (for instance) `dbloc`, which should return whatever density you specified in your equation of state.

Where did you see that it is hardcoded?

Cheers,   Jody

On 3 Jan 2020, at 11:32, Andrew Grace <andrew.grace at uwaterloo.ca<mailto:andrew.grace at uwaterloo.ca>> wrote:

Hi all,

I'm trying to use the KPP package in a freshwater setup but the water temperatures are all below the density maximum, meaning that cooler water over warm water is stable. I noticed that in kpp_calc.F, the surface buoyancy is calculated using a hardcoded linear equation of state, and therefore, when then code determines the stability of the water column, it is deemed unstable when it should not be. This is how I understand it.

I just want to confirm that the KPP package requires modification before it can be used in situations where an inverse stratification is stable. I also apologize if this question has been asked before.

Thanks in advance.

Andrew P. Grace
Graduate Student
Department of Applied Mathematics
University of Waterloo
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Jody Klymak
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