Migration to GRIB2 - changes in the MARS language

MARS requests for some parameters will also be affected by the migration to GRIB2.

Changes to param keyword

For GRIB1 parameters, the param keyword in the mars request can be set to param=indicatorOfParameter.table2Version. Requests with this structure will not work for GRIB2 as these two keys do not exist in this edition. The paramId must be used for GRIB2 parameters. This is the recommended way of specifying parameters in MARS requests.

An example is the retrieval of the U-component of the 10 metre wind, which is in GRIB1 in the indicatorOfParameter.table2Version representation param=165.128, but in GRIB2 param=165.

The following MARS retrieval is provided only as an example of the sort of change that users will need to make once the data are provided in GRIB2. If this request is run at the moment, it will return data encoded in GRIB1.

Retrieval of U-component of 10m wind in GRIB1

Retrieval of U-component of 10m wind in GRIB2

retrieve,
  class=od,
  stream=oper,
  expver=1,
  date=-2,
  time=00:00:00,
  type=fc,
  step=0,
  levtype=sfc,
  param=165.128,
  target="output_10u.grib1"

retrieve,
  class=od,
  stream=oper,
  expver=1,
  date=-2,
  time=00:00:00,
  type=fc,
  step=0,
  levtype=sfc,
  param=165,
  target="output_10u.grib2"

Changes for parameters archived with levtype=sol

Parameters on surface levels will be archived with levtype=sol (surface other levels).

For example, in GRIB1, a separate param is used to retrieve Volumetric soil water layer 1 and a different param o retrieve Volumetric soil water layer 2, etc with levtype=sfc . In GRIB2, only a single paramId is used for Volumetric soil water, with the layer being selected by the levelist keyword and using levtype=sol .

Instead of specifying a single parameter Id in the request, for some parameters additional keys will need to be added, for example for snow layer parameters the layer. These parameters will be archived with the new MARS keyword levtype=sol which has to be specified in the request together with the MARS keyword levelist .

Example of a retrieval of snow depth parameters with levtype=sol .

The levtype=sol keyword for multi-layer snow depth parameters in GRIB2 has been introduced with the Implementation of IFS Cycle 48r1.

Retrieval of snow depth in GRIB1

Retrieval of snow depth in GRIB2

retrieve,
  class=od,
  stream=oper,
  expver=1,
  date=-2,
  time=00:00:00,
  type=fc,
  step=0,
  levtype=sfc,
  param=141.128,
  target="output_sd.grib1"

retrieve,
  class=od,
  stream=oper,
  expver=1,
  date=-2,
  time=00:00:00,
  type=fc,
  step=0,
  levtype=sol,
  levelist=1/2/3/4/5,
  param=228141,
  target="output_sd.grib2"

Addition of new MARS keyword chem for retrieval of chemical species

Another example are chemical parameters. Besides using the MARS keyword param in the request to specify the parameter type, e.g., mass mixing ratio, an additional MARS keyword chem will be used to specify the chemical substance(s), e.g., nitrogen dioxide.

The following MARS retrieval is provided only as an example of the sort of change that users will need to make once the data are provided in GRIB2. The values taken by the chem keyword are not yet finalised.

Retrieval of NO₂ mass-mixing ratio in GRIB2 - current

Retrieval of NO₂ mass-mixing ratio in GRIB2 - new

retrieve,
  class=mc,
  stream=oper,
  expver=1,
  date=-2,
  time=00:00:00,
  type=fc,
  step=0,
  levtype=ml,
  levelist=1/to/137,
  param=210121,
  target="output_no2.grib1"

retrieve,
  class=mc,
  stream=oper,
  expver=1,
  date=-2,
  time=00:00:00,
  type=fc,
  step=0,
  levtype=ml,
  levelist=1/to/137,
  param=402000,      # Mass mixing ratio
  chem=no2,          # Nitrogen dioxide
  target="output_no2.grib2"

Change of MARS packing for GRIB2 parameters retrieved on a grid

GRIB2 parameters retrieved on a grid will be compressed with CCSDS. To decode these parameter, ecCodes must be linked to libaec. Alternatively, the data can be retrieved in another packing type such as grid_simple packing, which was in use for the model level GRIB2 parameters.