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20000905: Gempak, grid diagnostic function DDT

>From: Luis Farfan <address@hidden>
>Organization: UCAR/Unidata
>Keywords: 200009052020.e85KKBN28246

>
>I am interested in learning on the purpose, use and application of 
>the grid diagnostic function "DDT" which is supposed to compute the 
>time derivative.
>
>I am trying to estimate the time derivative of data contained in a
>gridded file and attempted to define the function DDT(VOR(WND)).
>There is an error label such as "Operand VORWND in DDT must be 
>read from grid file" when I use this function.
>
>Could you please provide help to understand what I am doing wrong?
>
>Thank you, Luis M Farfan.
>


Luis,

The parameter that is being time differenced in ddt() must exist for the 
2 separate times given in GDATTIM=time1:time2.

The problem you are having with the operand is that without the parameter
existing in the data file, the GDATTIM will be applied to the 
VOR(WND) parameter, and as a result, you won't have WND defined at TIME1:TIME2.
Instead, you have WND at TIME1 and WND at TIME2.

Typically the ETA does have AVOR defined already, but if you want to
look at VOR, you can use the program GDDIAG to  compute VOR(WND)
at the two times, say f000 and f024 for instance, and set the
output GRDNAM=RELVOR for instance. Then in GDCNTR, you will
be able to plot:
GDATTIM=f000:f024
GFUNC=ddt(relvor)

Steve Chiswell