If you have already followed the Introduction Tutorial on GRIB decoding with ecCodes GRIB tools: inspecting GRIB files there is no need to redo these preliminary steps.
Login to ecs-login and work in your $SCRATCH:
Make a copy of the practicals directory in your $SCRATCH to Atos:
This will create a directory in your $SCRATCH called grib_practicals containing the GRIB data files for this tutorial.
There are sub-directories for each practical:
For this tutorial, cd to the modify directory:
Remember to load the ecmwf-toolbox !
This tutorial covers the following topics:
The file tz_an_pl.grib1 contains parameters T and Z on six pressure levels.
How could you use grib_copy to create two files, one containing all of the pressure levels for parameter T, the other for parameter Z ? Check the content of the new files with grib_ls.
To copy only those messages for parameter T from the GRIB file tz_an_pl.grib1 use grib_copy with the -w option to specify a shortName=t:
Similarly to copy only those messages for parameter Z, use:
Or, more simply, and because the file contains only parameters T and Z, one can use:
Using grib_ls of the two files confirms that the contents are correct. For example:
You can take this further and split the file tz_an_pl.grib1 into separate files for each parameter/pressure level combination with:
Each file contains one message only, e.g.:
The field in the file tp.grib is supposed to contain total precipitation, created using MARS compute to sum the convective and large-scale (stratiform) precipitation.
What is the parameter set to in the file tp.grib ? How could you use grib_set to change it is set correctly to total precipitation (shortName=tp, paramId=228) ?
Use grib_ls to inspect the contents of the file tp.grib1, printing also the paramId with the –P option:
Alternatively, you could inspect the parameter namesapce with grib_ls:
The shortName is set to lsp and paramId to 142, i.e., large-scale precipitation.
The field was created using MARS compute to sum the convective and large-scale (stratiform) precipitation. When using MARS compute, the output field maintains the information from the GRIB header of the first input field so, in this case, we see that the field has paramId 142 even though the message is now supposed to contain the total precipitation.
To change the parameter to total precipitation, use grib_set as follows:
Check with grib_ls:
Note that changing the shortName has also changed the value of paramId. The same could be achieved by setting paramId=228:
How could you use the grib_to_netcdf tool to convert the GRIB messages in file1.grib to netCDF and store the result in a file named out1.nc using the default NC_SHORT netCDF data type ?
To convert the GRIB messages in file.grib1 to netCDF with data type NC_SHORT and store in a file named out1.nc use:
Load the netcdf4 module and use ncdump to check the values in the netCDF file created in the previous step as follows:
What are the data values set to in the out1.nc netCDF file ?
Using ncdump to print the values for the variable t2m shows:
The data values appear as integers because of the NC_SHORT data format. These values need to be unpacked using the scale_factor and add_offset NetCDF attributes.
How could you change the the grib_to_netcdf command to create a netCDF output with the values stored as floating point numbers ?
To store the values in the netCDF file as floating point numbers, use the grib_to_netcdf tool with the data format to NC_FLOAT or NC_DOUBLE with the -D option. For example:
Checking with ncdump, we see the values are now stored as floating point values:
In this case, the data values do not need further processing.
Try using grib_to_netcdf to convert the GRIB messages in file2.grib to netCDF. What happens....and why ? Can you fix it ?
Converting file2.grib to netCDF gives the following error:
The conversion fails because the GRIB data is represented on a reduced Gaussian grid (gridType=reduced_gg). Conversion to NetCDF is possible only for GRIB data with gridType=regular_ll or gridType=regular_gg.
To fix this it would be necessary to re-retrieve the data on a regular grid or interpolate to a regular grid.