[gradsusr] Moisture flux convergence/divergence

[Arlindo da Silva]

On Wed, Sep 21, 2011 at 12:00 AM, saeed bayat <saeedbayat7276 at gmail.com>wrote:

> Hi Arlindo,
> I am working with
> *Grid Analysis and Display System (GrADS) Version 2.0.a9.oga.1
> Copyright (c) 1988-2010 by Brian Doty and the
> Institute for Global Environment and Society (IGES)
> GrADS comes with ABSOLUTELY NO WARRANTY
> See file COPYRIGHT for more information
>
> Config: v2.0.a9.oga.1 little-endian readline printim grib2 netcdf hdf4-sds
> hdf5 opendap-grids,stn athena geotiff shapefile
> *
> I check the link you mentioned above, but I am confused that what am I type
>
> instead of expression exist there. I am working with output data of
> RegCM4.1
> and I do not know what exactly is the PSI, CHI, FISH_CHI and some another.
>
>
I am not sure I really understand what you are asking. If you enter:

ga-> q udf

you get a list of all the opengrads extension functions. Of relevance here
is the *fish* package which is documented here:

         http://opengrads.org/doc/udxt/fish/

Now, it is assumed that you know (from vector calculus or from your dynamic
meteorology classes) the definition of streamfunction (usually denoted psi)
and velocity potential (usually denoted chi).  Given the (u,v) components of
a horizontal vector (winds, mass flux, etc.) you can compute psi, chi with

ga-> psi = fish_psi(u,v)
ga-> chi = fish_chi(u,v)

  I hope this helps. If you are not familiar with the stream
function/velocity potential concepts, work on that first, then come back to
try the software.

     Arlindo


-- 
Arlindo da Silva
dasilva at alum.mit.edu
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