How can I obtain ECMWF data?
- Real-time data
  You will find the Catalogue of Real-time data here.
  You can order data through one of our catalogue contact points or, if you are located outisde of the ECMWF Member States territory, directly through ECMWF Data Services.
- Archive data
  Depending on your location you can order archive data directly through our online ordering system (you may have to seek authorisation from your catalogue contact point).
  Specific data sets from our data server are available free of charge, subject to terms and conditions.
Our ordering/payment process is described here.

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What is MARS?

The Meteorological Archival and Retrieval System (MARS) is the main repository of meteorological data at ECMWF. It contains petabytes of operational and research data, as well as data from Special Projects.

What is the format of the data?

ECMWF data are normally archived in MARS in GRIB format. From web MARS and the data servers there is the option of extracting the data converted to netCDF format.

What is GRIB_API?
GRIB_API is the application program interface developed at ECMWF to provide an easy and reliable way of encoding and decoding WMO FM-92 GRIB edition 1 and edition 2 messages. See GRIB_API Documentation for more information.

Can GRIB_API run under Windows?
No, GRIB_API only runs under UNIX/Linux OS. However you can use Cygwin to run UNIX/Linux software. See Cygwin installation or check Cygwin FAQs.

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What is the horizontal resolution of the data?

The ECMWF model uses two different numerical representations of global fields:

A spectral method, based on a spherical harmonic expansion, truncated at a particular wave number, e.g. T1279 to identify truncation at wave number 1279. This representation is used in the model to compute some horizontal derivatives and for the implicit computations.
The reduced Gaussian grid. This is based on the so-called "Gaussian" grid, which is regular in longitude and almost regular in latitude (Hortal and Simmons, 1991). The standard Gaussian grid has the disadvantage that the east-west distance between the grid points decreases polewards. To avoid some numerical problems around the poles and to save computing time, a reduced Gaussian grid was introduced in 1991, which reduces the number of grid points along the shorter latitude lines near the poles, so as to keep the east-west separation between points on different latitudes almost constant. This grid is used in the model for the computation of the advection terms and the physics tendencies.

Some of the fields are archived in spectral space (temperature, geopotential and wind) to save storage space. Other fields are archived as grid-point fields (moisture, cloud variables and surface fields). The MARS system can convert from spectral to grid-point space.

The grid point resolution and spectral truncation in the model are related. The table below shows the correspondence between spectral, reduced Gaussian and latitude/longitude resolution for some ECMWF products.

Spectral Trunc.	Gaussian Reduced	Lat/Long	Application
T1279	N640	0.125	Deterministic forecast + data assimilation
T639	N320	0.25	EPS up to 10 days
T319	N160	0.5	EPS from 10 to 15 days
T319	N160	0.5	EPS from 16 to 32 days (in monthly forecast only)
T255	N128	0.75	Seasonal forecast system 4
-	-	0.25	Deterministic ocean wave
-	-	0.5	EPS ocean wave
T255	N128	0.75	ERA Interim
T159	N80	1.125	ERA 40

Other Gaussian grids used at ECMWF are described here.

What is the vertical resolution of the data?
ECMWF data are produced on model levels (hybrid pressure-sigma coordinates) and at the surface. ECMWF data are also interpolated to pressure levels and some datasets include data on isentropic levels and the PV=2PVU surface.
The atmosphere is divided into various numbers of levels depending on the application. ERA-40 and ERA-Interim use 60 levels with the model top at 0.1 hPa, The ensemble prediction system uses 62 levels with the top at 5 hPa and the high-resolution operational data assimilation and forecasting system uses 91 levels with the model top at 0.01 hPa. These so called "sigma-levels", which follow the earth's surface in the lower- and mid-troposphere, are used as vertical coordinates but are surfaces of constant pressure in the upper stratosphere and mesosphere. A smooth transition between these types of levels is ensured ( Untch et al, 1999). The vertical resolution (measured in geometric height) is highest in the planetary boundary layer and lowest in the stratosphere and lower mesosphere. Read more

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What is the difference between analyses, forecasts and accumulated forecasts?

ECMWF data can be split into 3 main categories: analyses, instantaneous forecasts and accumulated forecasts. Analyses are produced by combining short-range forecast data with observations to produce the best fit to both. The data are available a few times per day. Instantaneous forecast data are produced by the forecast model, starting from an analysis, and are available at various forecast steps (hours) from the analysis date/time. (Note, forecasts are not initiated from all analyses.) These data are relevant to a particular verifying date/time (analysis date/time plus step). Accumulated forecast parameters are accumulated from the beginning of the forecast. You can divide values by the lenght of the forecast step to calculate averages over the accumulation period. Some parameters are only analysed (eg. model bathymetry), some are only forecast (e.g. radiative fluxes) and some are both analysed and forecast (e.g. temperature, winds and pressure).

How are the data obtained? Do they come from observations or have they been derived form numerical models?

Reanalysis (as well as analysis) is a process by which model information and observations of many different sorts are combined in an optimal way to produce a consistent, global best estimate of the various atmospheric, wave and oceanographic parameters.
See The ERA-Interim reanalysis: configuration and performance of the data assimilation system (pages 553–597) and IFS documentation CY36r1 for more details.

Where can I find the ECMWF model level definitions?

The model level definitions can be found here.

Science related

What are the units of the fields?

The units of the fields are defined in the GRIB tables.

What is the land-sea mask (LSM)?

The model surface is logically divided into sea and land points by using a land-sea mask. A grid point is defined as a land point, if more than 50% of the actual surface of the grid-box is land, for example with a TL511 resolution, islands like Corsica, Crete and Cyprus are represented by around five land grid points each.

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

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

What is the direction convention for the U and V components of winds?

The meteorological convention for winds is that U component is positive for a west to east flow (west wind) and the V component is positive for south to sorth flow (south wind).

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

The meteorological convention for all vertical fluxes is that downwards is positive.

What is the direction convention for wave fields?

Mean wave directions and the surface wind direction archived as a wave model parameter are given in degrees and are defined using the meteorological convention that zero means "coming from north" and 90 "coming from east". However, wave spectra are archived in such a way that directional information given in the grib headers follows the oceanographic convention that zero indicates that the waves are propagating towards the north and 90 towards the east

What are the definitions of the radiation fields?

"Surface solar radiation downwards" 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 downwards" 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.

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

How are the soil temperature and soil wetness fields defined?

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

How can evaporation have both positive and negative values?

Evaporation is normally negative due to the convention for fluxes (see direction convention for fields). Positive evaporation represents condensation.

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

Over what horizontal area are grid point data values valid?

Grid point data values are not area averages, but are valid at the exact location of the grid points indicated by the grid. ECMWF provides grid point data on a reduced Gaussian grid, a regular or full Gaussian grid, a reduced latitude/longitude grid or a regular latitude/longitude grid.

Where can I find the definitions of the Gaussian grids?

View the definitions of the Gaussian grids.

How are data interpolated/transformed to a lower resolution grid?

The data servers and MARS can provide data at a lower resolution than that at which they were produced. Data that were originally on a grid are bi-linearly interpolated to the new grid. Spectral data are truncated to a lower spectral resolution before being transformed to the new grid. Read more.

What are 2D wave spectra?

The quantity that is actually computed at each grid point is the two-dimensional wave energy spectrum and the total surface stress for a given forcing by surface (10 metre) winds. The two-dimensional spectrum describes how the mean sea-surface elevation variance due to ocean waves is distributed as a function of frequency and propagation direction. The energy spectrum is discretised over a number of frequencies and directions. Read more.

Are the t=0 forecast fields the same as the analysis fields?

Yes, in most cases the forecast fields at time 0 are the same as the analysis fields. The exceptions are:

Soil temperature lev2
Soil temperature lev3
Soil temperature lev4
Sea-ice cover
Skin temperature
2m temperature
2m dew-point temperature

Soil temperature lev2, 3 and 4 and skin temperature are identical over land, while over sea there are differences, due to the re-computation of the sea-ice mixed temperature, using the new SST and ice fraction provided by the analysis.

Sea-ice cover differs because a threshold is imposed for minimum sea-ice cover (>0.2) which resets the values along the edge of the sea-ice.

The difference between the 2m temperature and the 2m dewpoint temperature is due to the nature of these two fields: they are diagnostic fields, computed at each forecast step (including step =0) as an interpolated value between the lowest level and the surface, whereas the analysed values are the result of a separate computation of the 2m analysis.

What is the 4DVAR analysis procedure?

The four-dimensional variational analysis (4DVAR), system introduced in November 1997, uses as optimization procedure whereby the initial condition is adjusted to obtain an optimal fit through all the observations in the assimilation interval and at the same time tries to stay as close as possible to the first guess.

Is the field surface pressure available on both model and pressure levels?

For ERA data the parameter "surface pressure" is only available on model levels, as lnsp [ln (sp) ]. It is not archived as a surface field, although MSL pressure is archived as a surface field.
For operational archive it is archived as "model level" and "surface field".

ERA Project

Where can I find general documentation on ERA reanalysis data?

The journal article describing the ERA-Interim reanalysis is available from the Quarterly Journal of the Royal Meteorological Society at: http://onlinelibrary.wiley.com/doi/10.1002/qj.828/abstract
The document describing the ERA-Interim archive can be found here.

The journal article describing the ERA-40 reanalysis is available from the Quarterly Journal of the Royal Meteorological Society and is referenced as:
Uppala, S.M., Kållberg, P.W., Simmons, A.J., Andrae, U., da Costa Bechtold, V., Fiorino, M., Gibson, J.K., Haseler, J., Hernandez, A., Kelly, G.A., Li, X., Onogi, K., Saarinen, S., Sokka, N., Allan, R.P., Andersson, E., Arpe, K., Balmaseda, M.A., Beljaars, A.C.M., van de Berg, L., Bidlot, J., Bormann, N., Caires, S., Chevallier, F., Dethof, A., Dragosavac, M., Fisher, M., Fuentes, M., Hagemann, S., Hólm, E., Hoskins, B.J., Isaksen, L., Janssen, P.A.E.M., Jenne, R., McNally, A.P., Mahfouf, J.-F., Morcrette, J.-J., Rayner, N.A., Saunders, R.W., Simon, P., Sterl, A., Trenberth, K.E., Untch, A., Vasiljevic, D., Viterbo, P., and Woollen, J. 2005: The ERA-40 re-analysis. Quart. J. R. Meteorol. Soc., 131, 2961-3012.doi:10.1256/qj.04.176
The document describing the ERA-40 archive can be found here.

The document describing the ERA-15 archive can be found here.

What is the resolution of ERA data?

ERA-Interim has a resolution of T255 (triangular truncation at 255), N128 (128 latitude circles, pole to equator), L60 (model levels), 37 pressure levels and 15/16 isentropic levels.
ERA-40 has a resolution of T159 (triangular truncation of 159), N80 (80 latitude circles, pole to equator), L60 (model levels), 23 pressure levels and 15 isentropic levels. ERA-15 has a resolution of T106 with 31 vertical hybrid levels.

Where can I find documentation on the scientific aspects of ERA data?

The model used to produce ERA-Interim is IFS CY31r1
The model used to produce ERA-40 is IFS CY23r4

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 have only analysis data.

If step 0 is chosen, then only analysed fields, which are produced for 0000, 0600, 1200 and 1800 UTC, are available.

If step 3, 6, 9 or 12 is selected then only forecast fields which are produced from forecasts beginning at 0000 and 1200 UTC, are available.

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

Accumulated forecast fields are prone to spin-up/spin-down problems, therefore a choice of forecast step has to be made. The spin-down for ERA-Interim precipitation in the tropics is not as severe as it was in ERA-40, but is still present. A report investigating this problem is available here.

What observations were assimilated in ERA-Interim?

The observations assimilated in ERA-Interim are described in Section 4 of an article on the ERA-Interim reanalysis in the Quarterly Journal of the Royal Meteorological Society, available at: http://onlinelibrary.wiley.com/doi/10.1002/qj.828/abstract

How can I get the monthly means of daily forecast accumulations (e.g. for precipitation) from the ERA-Interim Data Server?
For the ERA-Interim Data Server it is planned to include the monthly means of daily forecast accumulations (for step 12) under the section entitled "Monthly means of daily means". However, this has not yet been implemented. In the meantime, you can calculate the monthly means of daily forecast accumulations (for step 12) by adding together the synoptic monthly means at time 00, step 12 and time 12, step 12. c

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What is the accumulation period in ERA-40?

On the ERA-40 Data Server, accumulated fields (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. The ERA-40 accumulated fields in the monthly means of daily means (stream=moda in MARS) have an accumulation period that does not fully coincide with the calender month (see the archive document for more details). The monthly diurnal forecast accumulations (stream=mdfa in MARS) do not suffer from this problem, but these data are not available from the ERA-40 data server.

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.

Which satellites are used in assimilation for ERA-40?

Polar orbiting satellites in use are: NOAA:VTPR,TOVS,ATOVS, DMSP:SSM/I, ERS/SCATTEROMETER.

Geostationary satellites in use are: DMSP, GOES, Meteosat.

What are the soil level depths and thicknesses for the TESSEL scheme used by ERA-40 and ERA-Interim?

The soil level depths and thicknesses for the TESSEL scheme used by
ERA-40 and ERA-Interim are as follows:

Level	Top (m)	Bottom (m)	Thickness (m)
1	0.0	0.07	0.07
2	0.07	0.28	0.21
3	0.28	1.0	0.72
4	1.0	2.89	1.89

Does the ERA-40 dataset contain near-surface humidity data?

Yes, in the form of dew point temperature. The archive contains near-surface (2m from the surface) temperature and dew point temperature. For ERA-Interim you can use the following procedure to calculate specific and relative humidity. For ERA-40, however, you cannot do this from the data server because surface pressure is not available. (ERA-40 surface pressure is available to Member State users who have access to the full archive in MARS.)

To obtain humidity, take a look at the latest documentation of the IFS.
Look at equations 7.4 and 7.5 (pages 91-92) in part IV , physical processes. Use the 2m dew point temperature and surface pressure to obtain specific humidity. The relative humidity is defined as:
RH=100*es(Td)/es(T)

Technical

Why don't I see the parameters I have requested in my GRIB file but 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 GRIB_API parameters. Please refer to the manual pages of your decoding software to configure it properly. We encourage users to download and install GRIB API, the ECMWF GRIB decoding software.

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 have been packed using 16 bits to represent real values.

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 receive this error message, select a lat/lon grid by clicking on the grid icon.

Do you provide the software to convert data from model levels into pressure levels?

No, Data Services do not have any software to do vertical interpolation.

Is there any software available for horizontal interpolation?

The software EMOSLIB can be used for horizontal interpolation.

Are there any deficiencies in your data archives?

Yes. Read a brief description of the deficiencies in our archives

Where can I find the model identification numbers?

View the model identification numbers.

Where can I find information on the evolution of your forecasting system?

View the evolution of the ECMWF system.

What is GRIBEX?
The GRIBEX software has been replaced by GRIB_API, please use GRIB_API.

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

06.01.2012