Setting Up Operational COSMO Runs using GRIB2

Last updated: August 2018

Introduction

For operational and experimental runs of the COSMO-Model system, both programs, INT2LM and the COSMO-Model, need to know about the vertical grid and the reference atmosphere used. But only INT2LM has the appropriate namelist variables to specify both, the vertical grid and the reference atmosphere.

When using GRIB1, we mis-used some of the GRIB1 meta data to save the information from these INT2LM namelist variables to all GRIB1 messages. When COSMO read such messages, it could extract the necessary information from the GRIB1 meta data. Then it could compute the vertical grid and also the reference atmosphere in the same way than INT2LM did.

This is NO MORE possible with GRIB2, because now we adhere to the GRIB standard and do no more mis-use of the meta data. But still COSMO needs to have the same vertical grid as INT2LM does, and COSMO also needs information about the reference atmosphere it should use.

Using GRIB2 we implemented the following modus of operation for these two issues:

• Vertical Grid:
  • Information to construct the vertical grid is given to INT2LM via namelist variables ivctype, vcflat and vcoord_d. Depending on ivctype some more namelist variables might be necessary.
  • INT2LM computes the field with the heights of the half levels, the HHL fields.
  • The HHL fields are written to the laf file, which is read from the COSMO-Model.
  • In case of a data assimilation cycle, the COSMO-Model reads the HHL fields from the laf file and writes them again to the new analysed laf file.
    Note that in this case the HHL fields have to be brought into a laf file once!
• Reference Atmosphere:
  • Information to construct the reference atmosphere is given to INT2LM via namelist variables irefatm, p0sl, t0sl, dt0lp, delta_t and h_scal.
  • INT2LM computes the reference atmosphere P0 and a corresponding pressure deviation pp to do horizontal and vertical interpolations. At the end, the full pressure P is written to GRIB2 files (while in GRIB1 only the pressure deviation PP was written.
  • And because COSMO can read the full pressure, it does not need to know about the reference atmosphere which was used in INT2LM. It can compute a reference atmosphere of its own. For this purpose the same namelist variables from INT2LM have also been implemented in the COSMO-Model in the group /LMGRID/ and can be specified by the user.

Getting the Definition Files for grib_api

INT2LM and the COSMO-Model are working with the grib_api library from ECMWF. For a convenient implementation we make use of the shortName concept offered by this library. In this concept products can be identified by the short names already used in COSMO (e.g. HSURF, or TOT_PREC) and by using grib_set (...,'shortName',...) we need not set all meta data associated with the short names.

In order to work properly, we need several definition files, which set up the correspondence between the short names and the associated meta data. Also for other purposes, definition files are necessary. And some of these definition files are written escpecially for DWD and its WMO number "78" (or "edzw").

But other centers can use these definition files as well. We implemented a method to link all necessary definition files for DWD with a 78 in the file name to corresponding files with the number of a desired center in the name. In this way all centers can use the DWD definition files.

Preparing a HHL File

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