Re: Gradient problem

From: Dennis Shea <shea_at_nyahnyahspammersnyahnyah>
Date: Mon, 18 May 2009 11:24:50 -0600

David Small wrote:
> Hello everyone,
>
> Iím trying to calculate the gradient of the geostrophic wind from
> ECMWF Reanalysis on a reduced gaussian grid. To do so, Iím using the
> centered finite difference routine in NCL to avoid a problem near the
> equator when using the spherical harmonic routines.
I do not understand this staement. The spherical harmonics
have no problems at the equator. The geostrophic winds are
problematical at the equator but not the spherical harmonics.

> I have no problem when I calculate the meridional derivative, it
> compares well to that which I calculated from the 2.5 degree ECMWF on
> a fixed grid. The problem is with the longitudinal derivative which is
> giving me junk. Does anyone have any clue what the problem might be?

I do not see anything wrong ... What is the "junk"?

>
> As always, thank you very much for all your help.
>
> Dave Small
>
>
> dlon = 1.125*0.0174533 ; convert to radians
> dlat = dlon*6378388.0
>
> do nl=0,72
>
> dX = 6378388.*cos(0.0174533*lat(nl))*dlon ; constant at this latitude
> v_grad_lon(:,:,nl,:) = center_finite_diff(v(:,:,nl,:), dX , True,0)
> u_grad_lon(:,:,nl,:) = center_finite_diff(u(:,:,nl,:), dX , True,0)
>
> end do
>
> --
> David Small
> Graduate Research Assistant
> Department of Atmospheric and Oceanic Sciences
> McGill University
> Montreal, Quebec, Canada
>
> ------------------------------------------------------------------------
>
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-- 
======================================================
Dennis J. Shea                  tel: 303-497-1361    |
P.O. Box 3000                   fax: 303-497-1333    |
Climate Analysis Section                             |
Climate & Global Dynamics Div.                       |
National Center for Atmospheric Research             |
Boulder, CO  80307                                   |
USA                        email: shea 'at' ucar.edu |
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Received on Mon May 18 2009 - 11:24:50 MDT

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