Re: Masking Tibetan Plateau

Hi A.R.,

Others may have better ways to do this, but I'll share what I've done
and hope that others will chime in to correct/improve the method I'm
outlining.

What I did was use the surface geopotential and the 850 mb heights
from NCEP/NCAR Reanalysis to create a mask. The tricky part about
masking out topography for fields on pressure levels, of course, is
this fact that geopotential height of the pressure levels change with
time. First I found 5th percentile* of 850 mb geopotential height for
each gridbox over the entire reanalysis period. Any gridbox where the
surface geopotential equalled or exceeded this value was set to 0; all
other gridboxes were set to 1. Then apply this masking array to the
850 mb wind data using the NCL mask() function.

I'm not going to suggest any specific NCL code (I used CDO for this
particular task) but I think the above could be done in just a few
lines.

~Rob

* The "5th percentile" choice is really quite arbitrary, but I chose
it because using the minimum value for each gridbox ended up masking
out vast swaths of the globe to the point of making the fields useless
for my purpose.

On Thu, Apr 8, 2010 at 9:04 PM, A.R Ragi <ar.ragi@gmail.com> wrote:
> Dear NCL users..,
>
> I would like to know is there any way to mask the Tibetan Plateau while
> plotting 850hpa  winds in NCL??
>
> --
> *********************************************************************
> Thanks,
> A.R.Ragi
> M.Tech Atmospheric science
> Department of Atmospheric science
> Cochin University of Science and Technology
> Email : ar.ragi@gmail.com
>
> ***********************************************************************
> "I want to know how God created this world.I am not interested in this or
> that phenomenon. I want to know, his thought, the rest are details"   .
>                          ---Albert Einstein
>
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Received on Fri Apr 9 11:26:17 2010