[MITgcm-support] changing from a fixed temperature to fixed heat flux as a boundary condition

[Julian Mak]

Dear all,

I am quite new to using MITgcm and have been trying to implement 
something in a channel; this question is about implementing boundary 
conditions and it seems like it should be a fairly trivial thing to do 
but I am still a bit confused, so any help would be appreciated.

Set up: re-entrant channel in x, walls on y, z being vertical, to mimic 
a part of the ACC. Surface and northern boundary is relaxed to some 
specified temperature profile (no salinity here) using the "rbcs" 
package with appropriate masking and short-ish restoring time.

Problem: Change over to a Neumann condition on both the surface and 
northern boundary, to specify the heat flux instead of the temperature 
instead. Rationale is that a Dirichlet condition specifying the 
temperature pins the ends of the isotherms and we would like to relax 
this, so the eddies have more freedom to move the isotherms around. The 
idea would be to diagnose the heat flux from the Dirichlet condition 
runs (I checked today that "TFLUX", "Trelax" and "surForcT" all give me 
the same thing***) and put it back in as the boundary conditions for the 
Neumann runs.

So I can see changing over to the surface one is fairly trivial, by 
specifying surfQfile. The northern one though is stumping me: I am 
thinking I probably need to hack "rbcs" and do something to add the 
tendencies in accordingly? I just thought there would be an easier way 
to do this, or I may have missed something obvious? I know there are 
other implementations to relax the pinning on the isotherms on the 
northern boundary but a flux condition seems like it should be easy to 
put in.

Thanks for your time,
Julian

*** I am not entirely clear on what the differences and relations 
relations between "TFLUX", "Trelax", "surForcT" and "oceQnet" are, even 
after reading a previous archived e-mail on this, so if someone could 
clarify I would be thankful. I think I understand why the first three 
are the same in this case (although I am guessing the relation). These 
quantities are presumably different if restoring is not used on its own 
or at all, and when salinity is involved (so "oceQnet" would be 
something like "TFLUX" + "SFLUX" + ... ?)

-- 
Julian Mak,
School of Mathematics,
The University of Edinburgh,
James Clerk Maxwell Building,
The King's Buildings,
Edinburgh, EH9 3FD
e-mail: j.mak at ed.ac.uk
tel: +44 131 650 5040
www: https://sites.google.com/site/julianclmak/home


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