...
Most of the data at ECMWF is global. A sub-area can be created using the area keyword by defining its latitude and longitude boundaries: North/West/South/East.
...
GRIB coded fields have a specified number of bits per packed value which can be changed with keyword accuracy. This might be useful when trying to retrieve fields from MARS identical to those you get from dissemination.
...
The truncation can be controlled using the resol keywordkeywords truncation and intgrid. Default truncations are detailed in the following table.
...
Fields on spherical harmonics or Gaussian grid can be rotated with the keyword rotation. The rotation is performed prior to any other conversion. Therefore:
...
- Whenever possible, use a local file system to store the target file (e.g. $SCRATCH on ecgate for external users) as this will avoid unnecessary network traffic.
- Estimate the data volume to be retrieved before issuing a request. It is easy to retrieve Gigabytes of data. Check that computer resources and limits are adequate for the amount of data to retrieve/interpolate in order to avoid unnecessary processing (MARS will fail if a quota is exceeded or in the case of any Unix problem regarding resources such as memory or CPU time).
- Estimate the number of fields to be retrieved before issuing a request. Try to retrieve a sensible number, up to tens of thousands of fields.
- Reduce the number of tapes involved. The number of tapes a MARS request is going to access will have an impact on its scheduling on the server. As a rule of thumb, two or more separate requests accessing files on different tapes are scheduled more efficiently than a single request accessing two or more tapes. A large number of tapes implies more waiting time. Use multiple requests in one MARS call whereby all the data is written to one output file.
- When retrieving large datasets (e.g. Re-Analysis), try to retrieve as many data from the same tape file as possible. Then, avoid caching the data on the server by specifying use = infrequent.
- Avoid constantly accessing the same tape. If you issue a large number of requests, all accessing data on the same tape, this can keep that tape in the drive for many hours and potentially cause some damage. Once you access a tape read as many fields as possible and if needed split the output by level, parameter etc. using the multi-target feature. This would reduce the amount of requests and make your extractions much faster.
...