ERA-Interim

The local copy of this data set is for PSD internal use only.

Update, April 25, 2013: 13 surface variables were added to the existing 10 pressure level variables.

Brief Description:

• ERA-Interim is a global reanalysis of recorded climate observations over the past 3.5 decades. It is presented as a gridded data set at approximately 0.7 degrees spatial resolution, and 37 atmospheric levels. ERA-Interim is produced by the European Centre for Medium-Range Weather Forecasts (ECMWF).

Temporal Coverage:

• 4 times daily, January 1979 to December 2012 (currently).

Spatial Coverage:

• Global grid, 0.7 degrees approximate resolution.
• 89.5S - 89.5N, 0E - 359.3E.
• Grid dimensions are 256 latitudes x 512 longitudes.
• Grid type: N128 Gaussian grid. Equal spacing on longitudes.

Levels:

• All 37 original pressure levels, 1 to 1000 hPa, quasi-logarithmic.
• Levels: 1, 2, 3, 5, 7, 10, 20, 30, 50, 70, 100, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 550, 600, 650, 700, 750, 775, 800, 825, 850, 875, 900, 925, 950, 975, 1000 hPa.

Update Schedule:

• By request only. Please send requests to Dave Allured.
• Latency of the source data for PSD access is currently about 3 weeks behind the ECMWF posted date, and 3 months behind real time.
• For current availability of the source data, and source update policy, please visit the ECMWF ERA-Interim status page.

Data Access:

• Summary of currently supported variables:
  • 10 out of 14 available pressure level variables
  • 13 out of 48 available surface analysis variables
  •   0 out of 84 available surface forecast variables
• Access is through the PSD local file system only, at the following Unix paths.
• This file system is automatically mounted on Linux systems.
• For Macs, this file system is mounted on request only. For access, send a request to PSD IT support, to have /Projects/erai mounted on your computer.

Usage Restrictions:

• Restricted data set, for PSD internal use only. Do not redistribute the PSD files outside of PSD.
• The PSD local copy is restricted by the terms of the following agreements:
  • ECMWF license agreement
  • NCAR access restrictions
• For public access, grib files by time step are available from NCAR's CISL Research Data Archive.

Detailed Description:

• The full ERA-Interim archives contain several data categories. Currently, this PSD local data set includes only selected variables from the surface analysis and upper air (pressure level) categories.
• Currently, no surface level fields are included.
• ERA-Interim files were modifed from the original source by extracting data for one variable at a time, and merging into large NetCDF files for single variables, all time steps.
• Individual data values were preserved exactly.
• Original lat/lon grids and pressure levels were preserved.
• Original ECMWF variable names were changed to shorter internal names. Most are standard short names matching PSD current usage for reanalysis data sets.

Caveats:

• This is an experimental file format. Some users have found that accessing entire time series in a single file is more efficient and easier to code than spanning multiple files.
• Feedback on the format, metadata, and naming conventions is welcome. Please send comments to Dave Allured.
• Relative humidity: The physical limits are zero to 100 percent. However, fields in this data set are routinely far outside this range. Values range as low as -47 percent and as high as +257 percent. The most extreme cases are listed in the data summary below.
• Specific humidity: The physical lower limit is zero. However, small negative values are commonly found in this data set. Most values are not lower than about -0.00015 kg/kg. The most extreme cases are listed in the data summary below.

Data Summaries:

• See the file metadata (ncdump -h) for the current actual range of values (global minimum and maximum values) for each field. Actual ranges are also included in the following summaries, although they might not always be completely up to date.
• The most extreme values are located for quality control purposes, and listed in the following summary reports. These are for pressure level variables only:
  • Divergence
  • Geopotential
  • Omega
  • Potential vorticity
  • Relative humidity
  • Specific humidity
  • Temperature
  • U wind
  • V wind
  • Vorticity (relative)

Related File Naming and Structure Information:

File Names:

• pressure level: /Projects/erai/pressure/<var_name>.<year-range>.nc

Dataset Format and Size:

• NetCDF-4 Classic; single large file for each variable.
• Each file contains data for one variable, all years and all 37 pressure levels.
• File size varies. The range is about 470 to 620 Gbytes for 34 years of record.
• NetCDF-4 lossless compression (zlib/deflate + shuffle) at level 5 is used. Actual compressed file sizes are about 54% to 64% of the uncompressed file sizes.

Missing Data:

• No missing values (preliminary observation). This data set is the product of a global climate reanalysis model that provides estimates at all locations.
• See headers of NetCDF files for missing value codes.

Citation:

• D. P. Dee et al (2011), The ERA-Interim reanalysis: configuration and performance of the data assimilation system, Q.J.R. Meteorol. Soc., Vol. 137: 553-597, DOI: 10.1002/qj.828

References:

• See http://reanalysis.org/atmosphere/erainterim-references

Original Source:

• European Centre for Medium-Range Weather Forecasts (ECMWF)

Contact:

• Dave Allured, NOAA/PSD/CIRES