2016-11-18

GRIB2 file carpentry

We've learned from several users that the second WRF method in our post about How to work around operational changes to GFS/GDAS/FNL does not work.

Use the first method described.
First, you can work around the problem by removing the new vertical levels in the files after the GFS/GDAS change. Then all of your WPS input will be consistent. You can do this yourself with wgrib2, or our "Get a Subset" tool.
Here's the detailed recipe for those of you not familiar with wgrib2 tool for manipulating grib2 files.

We're going to use tip #4 and #4a from Wesley's Tricks for wgrib2 page to extract a range of records from a grib2 file.

Start by making a short inventory to identify the records that you want to keep and to remove.
wgrib2 gdas1.fnl0p25.2016051106.f00.grib2 > gdas1.fnl0p25.2016051106.f00.inv

wgrib2 gdas1.fnl0p25.2016051112.f00.grib2 > gdas1.fnl0p25.2016051112.f00.inv

Note that gdas1.fnl0p25.2016051106.f00.inv has 322 lines and gdas1.fnl0p25.2016051112.f00.inv has 352 lines. The extra 30 lines/records are in records 5-34.
We'll extract fields 1-4 and 35-352 separately and then recombine them with the UNIX command, cat.  It would be tedious to type in each and every file I want to convert. To save time, I put all the files I want to convert into one directory and ran this bash script.
#!/bin/bash

for file in gdas1*f00.grib2; do
  wgrib2 $file -for 1:4 -grib temp1
  wgrib2 $file -for 35:352 -grib temp2
  cat temp1 temp2 > $file.27
done

3 comments:

  1. for some reason I am not able to compile wgrib2 on bash. It only works on csh. Can you also provide the above code in csh as well?

    ReplyDelete
    Replies
    1. We don't provide or support wgrib2. It's NCEP-provided software. We recommend it's use. I don't know what your settings are on your local system, but it works in bash and csh for me. --Grace Peng

      Delete
  2. never mind I resolved my problem. Thanks for the code.

    ReplyDelete

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