[MITgcm-support] DeepAtmosphere and rstar coordinates

[Jean-Michel Campin]

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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