XDFOPEN For Nonstandard NetCDF File

[Jennifer M. Adams]

Dave,
The simplest option is to use the Z dimension as a proxy for M, since
your variable doesn't appear to vary in Z. Using the Z dimension
makes it easier to slice and dice (and average) through your ensemble
dimension, but you should document in the descriptor file (with
comments and perhaps in the title too) that Z is ensemble member
instead of a vertical axis such as depth or millibars. You can still
use xdfopen if you do it this way, just give the name of the ensemble
dimension with your ZDEF entry.

The second option is to list each ensemble member as a separate
variable, with declarations like this:
t2m=>t2m0  0  t,0,y,x  2m Temperature Ensemble 0
t2m=>t2m1  0  t,1,y,x  2m Temperature Ensemble 1
t2m=>t2m2  0  t,2,y,x  2m Temperature Ensemble 2
...
With this option you have to use the 'open' command with a full
'dtype netcdf' descriptor. This reminds me of a question from Vincent
Moron (a colleague of yours at IRI) who asked about reading a netcdf
with 5 dims. I believe the 2nd option would work in his case ...

A variable that looked like this (with ncdump)
    float u(T,M,Z,Y,X)
could be described this way (assuming Z dimension has 5 elements):
    u=>u0  5  t,0,z,y,x  U-Wind Ensemble 0
    u=>u1  5  t,1,z,y,x  U-Wind Ensemble 1
    ...

This option didn't occur to me at the time when he asked his
question, but I do think that will work, although I haven't tested it
(yet)!

Jennifer



On Mar 16, 2006, at 1:19 PM, David G DeWitt wrote:

> Hi Folks,
> I am trying to open a netcdf file that has an extra dimension, i.e.
> ensemble
> member. Reading the documentation it appears that GrADS can handle
> this
> with a comma delimited list for  the units parameter, i.e.
>
>  VAR=>hist0   0   0,y,x   First historgram interval for VAR
>
> is the example given in the documentation. For a variable with the
> following ncdump what would this syntax look like:
>
> float t2m(T, M, Y, X)
>
> where:
> T is time
> M is ensemble member (the non-standard dimension)
> Y is latitude
> X is longitude
>
> Thanks for any advice you can give me on this. I have tried what I
> thought
> this would be and I clearly do not understand the example.
>
> Dave
>

Jennifer M. Adams
IGES/COLA
4041 Powder Mill Road, Suite 302
Beltsville, MD 20705
jma at cola.iges.org