Extracting a subdomain from pp files using convsh/xconv

[cbirch]

Hi,

I have just uploaded pp files from three year-long, 12km-resolution global simulations to Jasmin from the Met Office. The files are massive and I would like to extract a subdomain of certain variables, save them as netcdf files and then scp to Leeds. There are multiple files so I can't use the xconv interface.

I have attached the (convsh) script which extracts selected variables at selected levels and converts them to netcdf. I got this from Nick Dixon, who I think got it from Grenville originally. Extracting a variable at a given level works fine on Jasmin but I can't get it to extract the subdomain properly.

Lines 72-76 in the file appear to do the sub-domain extraction. The numbers I have set in setgrid 1 and 2 are correct and the subdomain is saved out successfully. However there is something wrong with the values it extracts. Rather than simply extracting the grid boxes I want, it is doing some kind of interpolation (line 76). If I comment out this line the subdomain is not extracted. The comment on line 73 suggests that someone at some point has been playing with this (I didn't add that comment). I was wondering if you knew how to get this working?

Thanks,
Cathryn

[jeff]

Hi Cathryn

xconv can only subset over the level and time dimension. This script tries to use bilinear interpolation to do the same thing over the lat/lon dimensions. This might work if you choose the output grid to have exactly the same lat/lon values as contained in the input grid. Then the bilinear interpolation routine will choose a weight of 1 for the point you want and a weight of 0 for the other interpolation points. Of course rounding errors may make this not work as you might expect. Maybe you should use some other tool to do this.

Jeff.

[cbirch]

Hi Jeff,

Thanks for the info. The interpolation/roudning error is a bit of a risk. I'm looking into doing it with Python instead. This will take me longer so I just wanted to check there wasn't an easy way of doing it with convsh.

Thanks,
Cathryn

[grenville]

Cathryn

You could convert to netcdf, then use the NCO operators - I don't know how much data you'd need to convert, so this may be impractical?

Grenville

[cbirch]

Thanks for the suggestion. I hadn't thought of that. As you say the issue may be that I don't have the space on Jasmin to convert enough files to netcdf. Python would be better because I could read directly from the pp files (I think). I'll take a look at anyway NCO though.

Cathryn

[grenville]

Cathryn

David Hassell has written some very clever data handling tools (in python) - they take the pain out of reading huge files and lots of files (they can read pp, netcdf..) . I'm not sure if they can slice data exactly the way you want, but they are will wort a look. David is on vacation currently.

Grenville