ECMWF Data Frequently Asked Questions

• What are the units of fields?
• Many fields have seconds in the units e.g. precipitation and radiation fields. How can instantaneous values be calculated?
• What is the direction convention for U and V winds?
• What is the direction convention for fields such as radiation and precipitation?
• What are the definitions of the radiation fields?
• Geopotential is defined with units of m**2/s**2 both on pressure levels and on the surface ( orography). How can the height in metres be calculated?
• How are the soil temperature and soil wetness fields defined?
• How can evaporation have both positive and negative values?
• How are low, medium and high cloud cover defined?
• Why don't I see the parameters I have requested in my GRIB file and see other parameters instead?
• What are 2D wave spectra?

You may also wish to refer to the

Data Servers FAQ

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1. What are the units of fields?

All ECMWF data use the ECMWF local table 2 versions and most of the commonly used atmospheric fields are from ECMWF Table 128 while the wave fields are from Table 140.

2. Many fields have seconds in the units e.g. precipitation and radiation fields. How can instantaneous values be calculated?

Some forecast fields (fluxes) are accumulated in time since the start of the forecast, so to get averaged values you need to divide by the number of seconds in the length of the forecast (the forecast step). In this way the average is obtained between the start of the forecast and the forecast step.   To obtain the average between two forecast steps (e.g. STEP1 and STEP2) it is necessary to retrieve the field for the two steps ( FLD ( STEP1) and FLD( STEP2)), to take the difference and divide by the time difference in seconds i.e.: ( FLD( STEP2)- FLD( STEP1))/(( STEP2-STEP1)*3600).

3. What is the direction convention for U and V winds?

The ECMWF convention for winds that U winds are positive for a West to East flow (west wind) and V winds are positive for South to North flow (south wind).

4. What is the direction convention for fields such as radiation and precipitation?

The ECMWF convention for all flux fields is that downward is positive.

5. What is the direction convention for wave direction fields?

The directions follow compass conventions (0 is North) and are measured in degrees, but the wave directions follow meteorological convention in that a 0 direction means waves traveling from North to South.

6. What are the definitions of the radiation fields?

• Surface solar radiation downward is the incident solar (shortwave) radiation and surface solar radiation is the net solar radiation. Surface net solar radiation, clear sky is a hypothetical field assuming no clouds.
• Surface thermal radiation downward is the incident thermal ( longwave) radiation and surface thermal radiation is the net thermal radiation. Surface net thermal radiation, clear sky is a hypothetical field assuming no clouds.
• Top solar radiation is the net solar (shortwave) radiation at the top of the atmosphere. Top net solar radiation, clear sky is a hypothetical field assuming no clouds.
• Top thermal radiation is the net thermal ( longwave) radiation at the top of the atmosphere. Top net thermal radiation, clear sky is a hypothetical field assuming no clouds.

7. Geopotential is defined with units of m**2/s**2 both on pressure levels and on the surface ( orography). How can the height in metres be calculated?

The geopotential is the geopotential height multiplied by the WMO defined gravity constant of 9.80665 m/s**2 which is constant for all latitudes and all heights.

8. How are the soil temperature and soil wetness fields defined?

The sub-surface field definitions have changed with time, but all three ECMWF Re-analysis archives (ERA-15, ERA-40 and ERA-Interim) use the Tessel formulation.

9. How can evaporation have both positive and negative values?

Evaporation is normally negative due to the convention for fluxes, but over ice or very cold sea water there can be condensation which is treated as part of evaporation.

10. How are low, medium and high cloud cover defined?

Let sigma = Pressure / Surface pressure.

• Low cloud cover is for 1.0 > sigma > 0.8
• Medium cloud cover is for 0.8 >= sigma > 0.45
• High cloud cover is for 0.45 >= sigma

11. Why don't I see the parameters I have requested in my GRIB file and see other parameters instead?

If you are not using GRIB decoders developed by ECMWF, make sure that your software is configured to use the correct GRIB table to decode the data. ECMWF data use the ECMWF local table 2 versions. Please refer to the manual pages of your decoding software to properly configure it. We encourage users to download and install the GRIB API, ECMWF GRIB decoding software.

12. What are 2D wave spectra?

2D wave spectra are described here.