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19990129: GEMPAK Question

>From: "John H. E. Clark" <address@hidden>
>Organization: .
>Keywords: 199902041927.MAA21272

>Steve,
>
>Thank-you for replying to my question about using GDDIAG. I have a few
>questions in response to your reply:
>
>1) I am not sure what a variable is called after running GDDIAG. In your
>example is it actually named
>LEVL1      LEVL2    VCORD    PARM
>  500       700     PRES     LAPSE
>
>This seems like an awkward name for a variable. Where is the variable
>created in GDDIAG stored?
>

As I mentioned, This was only a demonstration of how to control the
naming of the grid. If you don't like the name LAPSE for the variable,
call it something else such as DT. I tried to show how to use the GRDNAM 
variable
to define the grid name, as well as the resulting level/layer values
that would be defined in glevel. I discussed that if you didn't
want to have the resulting grid defined as a layer quantity between 500 and 700 
mb,
then you could define it to be the value at a specific level- such as 600 mb.

The output of the grid will be written to the file named in GDOUTF.


>2) When I call the variable created in GDDIAG for use in, say, GDPLOT, do I
>call it
> LEVL1     LEVL2   VCORD     PARM
>  500       700     PRES     LAPSE

In this case, the values you need are:
gvcord = PRES
gfunc = LAPSE
glevel = 500:700

The above output is how programs like gdinfo list the information for
grids. You typically have 6 columns of output. These columns show the
values for TIME1, TIME2, LEVL1, LEVL2, VCORD, and PARM. In the
example I showed you, I omitted the TIME columns since they were irrelevant 
for the calculation at hand. The six columns listed relate to the variables
you need to refer to a grid diagnostic as:
gvcord = VCORD
glevel = LEVL1:LEVL2
gfunc = PARM
gdattime = TIME1:TIME2

Since many parameters are defined at a single LEVEL and TIME, eg
500mb height, you refer to those with glevel = LEVL1 and gdattim = TIME1.
For grids defined over a layer such as the difference of temperature
over a layer, or where 2 times are involved such as 24hr height change,
two values are used (sepatated by the : character).

I showed that you could define your grid output at a single level, such
as 600mb rather than as a lavel 500:700 mb if that is how you needed to
refer to a variable. The tutorial shows how to use inline modifiers
to override the GLEVEL, GVCORD and GDATTIM variables when your
calculations involve parameters defined on different coordinate systems,
levels or times.

>
>3) I tried running GDDIAG today and I always get the diagnostic 'data file
>not open'

Make sure you have your input and output files in your duirectory
or are where they are accessible from the path name. Also, make sure you have
space available in the output file to write the data- eg use gdinfo to
see how many grids are in the data file, and the maximum number of
grids which will be allowed in the grid file. 
>
>John Clark
>
>


Steve Chiswell