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Changing topography in CAM4 (CESM1.2.2)

Hi all,

I'm using CESM1.2.2 with CAM4 and I want to look at impact of Greenland topography change.
To reduce the elevation of the Greenland, I am assuming I need to make the PHIS variable in the bnd_topo file zero (for the extreme case) in the Greenland. But I am not sure what to do with SGH (standard deviation of 10min elevations) and SGH30 (standard deviation of elevations from 30s to 10m) variables.

Is this the right way to change the topography? And what to do with SGH and SGH30 variables.
Thanks for your time,
Deepashree
 

juliob

New Member
Hello Deepashree,
Setting PHIS to zero over Greenland is the right way to "flatten out" Greenland.

What you should do with SGH and SGH30 will depend on the goal of your simulation. These variables drive parameterizations of subgrid drag. If you make them zero, then in addition to eliminating resolved topographic effects you also eliminate any subgrid topographic drag - so Greenland is truly a flat featureless plain. The only surface roughness will be that assigned by the land model.

If you leave non-zero SGH30 or SGH you are imagining a surface that is flat in the grid mean, but has subgrid obstacles (hills and valleys).

The model should run either way.

Julio Bacmeister
 
Hello Deepashree,
Setting PHIS to zero over Greenland is the right way to "flatten out" Greenland.

What you should do with SGH and SGH30 will depend on the goal of your simulation. These variables drive parameterizations of subgrid drag. If you make them zero, then in addition to eliminating resolved topographic effects you also eliminate any subgrid topographic drag - so Greenland is truly a flat featureless plain. The only surface roughness will be that assigned by the land model.

If you leave non-zero SGH30 or SGH you are imagining a surface that is flat in the grid mean, but has subgrid obstacles (hills and valleys).

The model should run either way.

Julio Bacmeister
Hi Julio,
Thank you so much for your explanation!
It is really helpful.
 
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