[MITgcm-support] DeepAtmosphere and rstar coordinates

[Angela Zalucha]

Jean-Michel:

I am not tied to the deep-atmosphere mode, I really just care about 
getting gravity to be a function of k in the model.  Is it really as 
simple as replacing the model variable "gravity" with a k dependence 
wherever it appears in the model?  This does not interfere with any 
assumptions made in the governing equations (one example I could think of 
of is gravity being taken out of an integral over z under the assumption 
that gravity is a constant)?  I also wonder if modifications would need to 
be made to the geopotential Phi.

   Angela




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Angela Zalucha, Ph.D.
Research Scientist
Search for Extraterrestrial Intelligence (SETI) Institute


Office located at Southwest Research Institute
1050 Walnut Street, Suite 300
Boulder, CO 80302
USA
(720) 208-7211


http://www.boulder.swri.edu/~angela/
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On Mon, 14 Jan 2013, Jean-Michel Campin wrote:

> Hi Angela,
>
> Few things here:
> 1) deep-atmosphere would work the same way in p* coordinate as it does in
> pure pressure coordinate. The reason is that the way the code takes
> into account deep-atmosphere effect is horizontally uniform, with radius
> and grid size that only varies with k (i.e., pressure or p* level).
> This is only an approximation (the true radius should varies from place to
> place, function of the local geopotential height), and the assumption that
> p* is close to p in the deep-atmosphere effect is very likely to have less
> impact than the assumption above, which is a horizontally uniform
> radius for a given pressure level k.
>
> 2) Note that the deep-atmosphere code in general, and specially in pressure
> coordinate, has not been extensively tested, plus I don't remember if
> it has been consistently taken into account everywhere in the dynamics.
> So I would recommend to be rather careful in switching this on.
>
> 3) gravity variation with height: although it seems natural to associate this
> to the deep-atmosphere formulation, it is not currently part of it.
> But if we agree on the main assumption above (in point 1), meaning that
> a pressure or p* level is at a fixed distance from the planet center
> (fixed radius), it's not difficult to consider gravity as a function of k
> (and this does not require significant code modifications, especially in
> pressure coords).
>
> Cheers,
> Jean-Michel
>
> On Thu, Jan 10, 2013 at 04:52:48PM -0700, Angela Zalucha wrote:
>> Hi,
>>
>> Am I correct in saying that setting deepAtmosphere to true while using rstar coordinates is not supported by the model?
>>
>> I have been running the MITgcm applied to Pluto (and now Triton) for some time, and one of the poorer assumptions I must make is that the atmosphere is shallow.  Pluto's solid body radius is about 1150 km, yet the "lower" atmosphere extends to 600-800 km.  Thus, the assumption that the gravitational acceleration is constant with height is poor at high levels.
>>
>> My configuration is essentially Held-Suarez 94 with rstar coordinates (plus a Pluto-specific heating/cooling scheme that I call from external_forcing.F).  Is there any possibility of getting g to vary with height in rstar coordinates?
>>
>>     Angela
>>
>
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