Begin Main Content

What is it?

This page documents otherwise undocumented lats4d options for use at the NASA/GSFC Global Modeling and Assimilation office (GMAO).

Setting your GrADS environment

On those Linux workstations mounting the /share file system, add this to your .cshrc or .login:

source /share/dasilva/opengrads/setup.csh

If your workstation does not mount the /share file system, please contact support@gmao.gsfc.nasa.gov and make a request. You can customize this setup by incorporating the contents of setup.csh above in your .cshrc or .login.

Regridding

There are a couple of shorthands for using lats4d with Mike Fiorino's regrid2 user defined function. If you set your environment as above then regrid2 will work for you.

To convert the sample GrADS file model.ctl to DAO's standard grids at 4x5, 2x2.5 or 1x1 degree resolution you can enter:

lats4d -i model -o model4x5 -geos4x5
lats4d -i model -o model2x25 -geos2x2.5
lats4d -i model -o model1x1 -geos1x1

These options will use a simple bilinear interpolation scheme to convert the file. This is usually fine when the input and output file have similar resolutions.

When the input has a finer resolution compared to the output file it is recommended that you use the box averaging method. In this case you enter:

lats4d -i model -o model4x5 -geos4x5a
lats4d -i model -o model2x25 -geos2x2.5a
lats4d -i model -o model1x1 -geos1x1a

When the input has a coarser resolution compared to the output file you may consider using a higher order interpolation scheme such as the 4-point Bessel interpolation that regrid2 implements. In this case you enter:

lats4d -i model -o model4x5 -geos4x5s
lats4d -i model -o model2x25 -geos2x2.5s
lats4d -i model -o model1x1 -geos1x1s

although the original bilinear interpolation could work just as fine. Keep in mind that the 4-point bessel interpolation scheme can produce negative values even when the input field is all positive (say, specific humidity). As with all these interpolation algorithms, use it with caution, at your own risk, and make sure to carefully examine the output produced.

Last but not least, whenever possible consider using the option -format grads_grib to produce files which are about half the size of the default HDF files. However, if your file has more than 1 pressure level above 1 hPa, GRIB will not work for you due to a limitation of the format.

The shorthands above internally sets the "-de" and "-func" options, automatically generating an appropriate dimension environment file. For additional information on regrid2 options consult its documentation.