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HOME > Monitoring_and_Data > Oceanic and Atmospheric Data > Reanalysis: Atmospheric Data > wgrib2 - example converting MERRA HDF to grib2
 

wgrib2: Converting MERRA HDF file to GRIB-2

Introduction

MERRA is a NASA reanalysis project that has created reanalyses in HDF format. For some people, grib is a more convenient format. Here is our first attempt to convert a MERRA HDF file into grib-2.

Surveying the situation we see that Lats or the simpler to use Lats4d can convert HDF into grib-1. Grib-1 can be converted into grib-2 by the cnvgrib utility except that lats creates a custom grib table. Therefore a program would have to be written that converts the grib-1 file with the custom grib table into a grib table using the standard NCEP table. Doable but the program wouldn't be very generic. In addition the process would take 3 steps (convert to grib1, translate grib1, convert to grib2). Can we do better?

Looking at the programs that can read HDF, we see a familiar face, GrADS. GrADS can read the HDF and write it out as a binary file. wgrib2 can then read the binary file, a grib-2 template and metadata and then create a grib-2 file.

1: The template

Grib-2 files include much metadata that is constant such as grid, center that created the data, the process that created the data, etc. Making a template takes some knowledge and we'll assume it has already been done. For the MERRA template, I used copygb/cnvgrib to create a grib2 file with the proper grid definition and then used wgrib2 to fill in some of the metadata (center). Probably should have filled in more of the metadata but I didn't.

2: Writing the binary and meta data

The script g2write takes three arguements, the GrADS variable/expression, the output file and some metadata (variable:level:forecast/analysis). The script then creates the binary and metadata for one or more grids depending whether t is varying. These two files along with the template can be used to make a grib2 file.

3: All together

  • Start gradshdf.
  • open the HDF file, sdfopen hdf-file
  • run the script, merra2grb2.gs

    4: What does merra2grb2 do?

  • set the grid .. grads often uses an extra column .. 0..360 degrees
  • set the time
  • writes out ps with the metadata PRES:surface:anl
  • writes out slp with the metadata PRES:mean sea level:anl
  • loops over subset of available pressure levels
  • writes out h with the metadata HGT:xx mb:anl
  • writes out t with the metadata TMP:xx mb:anl
  • writes out u with the metadata UGRD:xx mb:anl
  • writes out v with the metadata VGRD:xx mb:anl
  • writes out qv with the metadata SPFH:xx mb:anl
  • writes out o3 with the metadata O3MR:xx mb:anl
  • after finishing looping over the vertical levels, the binary file are closed
  • the binary data is stored in test
  • the metadata is stored in test.inv, one line per field
  • finally wgrib2 is run which takes the binary data (test), inventory (test.inv) and template (hires-merra.template.grb) and creates a grib2 file (test.grb2)

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