ECMWF Data Servers Frequently Asked Questions

1. What are the 'steps' for the surface daily fields in ERA Interim?
2. What is the best forecast period to use for accumulating fields in ERA Interim?
3. What is the accumulation period in ERA 40?
4. What observations were assimilated in ERA 40?
5. Why do my netCDF data only contain integers?
6. Why does the retrieval abort with a message like "Sorry!: Subarea not supported for reduced gaussian grids"?

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1. What are the 'steps' for the surface daily fields in ERA Interim?

The ERA Interim Data Server surface archive has a mixture of analysis fields, forecast fields and fields available from both the analysis and forecast. The other daily archives only have analysis data.

If step = 0 is chosen, then this is analysis fields at the times 0000, 0600, 1200 and 1800 UTC. If this step is chosen then forecast only fields will not be available.

If step 3, 6, 9 or 12 is selected then the time becomes the forecast base time and only options 0000 and 1200 UTC will be available. In this case analysis only fields will not be available.

2. What is the best forecast period to use for accumulating fields in ERA Interim?

Accumulating forecast fields are prone to spin-up/spin-down problems and therefore, a choice of forecast step has to be made. This optimal choice of steps for precipitation data is currently being investigated. The spin-down for ERA-Interim precipitation in the tropics is not as severe as it was in ERA-40, but is still present.

3. What is the accumulation period in ERA 40?

Analysis fields of fluxes (e.g. precipitation) are accumulations of the relevant parameter during the first 6 hours of forecasts initialized by ERA-40 analyses. Unlike ERA Interim, the ERA 40 data server does not explicitly state the accumulation period.

4. What observations were assimilated in ERA 40?

These maps contain the observations, in a 1° x 1° grid, assimilated in ERA-40, organised by observation type and by month.

5. Why do my netCDF data only contain integers?

The netCDF data format supports packing in order to reduce data size as explained here in the section on packed data values. The data on this server has been packed using 16 bits to represent real values.

6. Why does the retrieval abort with a message like "Sorry!: Subarea not supported for reduced gaussian grids"?

Some data servers allow the retrieval of data on reduced gaussian grids. Our interpolation software is not yet able to handle the extraction of sub-areas on reduced gaussian grids, as explained here in the "area" section. If you get this error, select a lat/lon grid by clicking on the grid icon.

If the FAQ does not solve your problem, please contact ECMWF mars support