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

Update, August 7, 2014:
  • Selected theta level variables were added back in March. See listing below. Thanks to Maria Gehne, NOAA/PSL/CIRES, for adding these.
  • Surface variables were updated. This brings all local files up to date through the end of 2013, or later.
Update, February 3, 2014: All pressure level files were reconstructed from scratch, to remove low levels of data corruption. At the same time, the time range was extended through October 2013, and file names were changed to fixed names that include "erai.pl".

The data problem was an old issue internal to PSL. Only pressure level files were affected, not surface files. For more information about this update, please contact Dave Allured.

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 2013 (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.


  • All 37 original pressure levels, 1 to 1000 hPa, quasi-logarithmic.
  • Pressure 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.
  • All 15 original isentropic (theta) levels, 265 to 850 Kelvin.
  • Theta levels: 265, 275, 285, 300, 315, 330, 350, 370, 395, 430, 475, 530, 600, 700, 850 Kelvin.

Update Schedule:

  • Annually, or by request. Please send requests to Dave Allured.
  • Latency of the source data for PSL 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:
    • 14 out of 14 available pressure level variables.
    • 13 out of 48 available surface analysis variables.
    •   0 out of 84 available surface forecast variables.
    • 4 theta level variables.
  • Access is through the PSL 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 PSL IT support, to have /Projects/erai mounted on your computer.
Pressure Level Variables ERA
Statistic Level Units File
Cloud cover CC 4-times Daily Pressure (0 - 1) /Projects/erai/pressure/cloud_cover.erai.pl.nc
Cloud ice water content CIWC 4-times Daily Pressure kg kg**-1 /Projects/erai/pressure/cloud_ice_water.erai.pl.nc
Cloud liquid water content CLWC 4-times Daily Pressure kg kg**-1 /Projects/erai/pressure/cloud_ice_water.erai.pl.nc
Divergence D 4-times Daily Pressure s**-1 /Projects/erai/pressure/div.erai.pl.nc
Geopotential Z 4-times Daily Pressure m**2 s**-2 /Projects/erai/pressure/geopot.erai.pl.nc
Omega (vertical velocity) W 4-times Daily Pressure Pa s**-1 /Projects/erai/pressure/omega.erai.pl.nc
Ozone mass mixing ratio O3 4-times Daily Pressure kg kg**-1 /Projects/erai/pressure/ozone.erai.pl.nc
Potential vorticity PV 4-times Daily Pressure K m**2 kg**-1 s**-1 /Projects/erai/pressure/pv.erai.pl.nc
Relative humidity R 4-times Daily Pressure % /Projects/erai/pressure/rhum.erai.pl.nc
Specific humidity Q 4-times Daily Pressure kg kg**-1 /Projects/erai/pressure/shum.erai.pl.nc
Temperature T 4-times Daily Pressure K /Projects/erai/pressure/temp.erai.pl.nc
U wind U 4-times Daily Pressure m s**-1 /Projects/erai/pressure/uwnd.erai.pl.nc
V wind V 4-times Daily Pressure m s**-1 /Projects/erai/pressure/vwnd.erai.pl.nc
Vorticity (relative) VO 4-times Daily Pressure s**-1 /Projects/erai/pressure/vort.erai.pl.nc
Surface and Monolevel
Statistic Level Units File
2 metre dewpoint temperature 2D 4-times Daily Surface K /Projects/erai/surface/dewpoint_temp.erai.sfc.nc
2 metre temperature 2T 4-times Daily Surface K /Projects/erai/surface/temp.erai.sfc.nc
10 metre U wind component 10U 4-times Daily Surface m s**-1 /Projects/erai/surface/uwnd.erai.sfc.nc
10 metre V wind component 10V 4-times Daily Surface m s**-1 /Projects/erai/surface/vwnd.erai.sfc.nc
Geopotential Z 4-times Daily Surface m**2 s**-2 /Projects/erai/surface/geopot.erai.sfc.nc
High cloud cover HCC 4-times Daily Surface (0 - 1) /Projects/erai/surface/high_cloudcover.erai.sfc.nc
Low cloud cover LCC 4-times Daily Surface (0 - 1) /Projects/erai/surface/low_cloud_cover.erai.sfc.nc
Mean sea level pressure MSL 4-times Daily Surface Pa /Projects/erai/surface/sea_level_pres.erai.sfc.nc
Sea surface temperature SSTK 4-times Daily Surface K /Projects/erai/surface/sst.erai.sfc.nc
Skin temperature SKT 4-times Daily Surface K /Projects/erai/surface/skin_temp.erai.sfc.nc
Surface pressure SP 4-times Daily Surface Pa /Projects/erai/surface/surface_pres.erai.sfc.nc
Total cloud cover TCC 4-times Daily Surface (0 - 1) /Projects/erai/surface/tot_cloud_cover.erai.sfc.nc
Total column water TCW 4-times Daily Surface kg m**-2 /Projects/erai/surface/tot_columnwater.erai.sfc.nc
Theta Level Variables ERA
Statistic Level Units File
Potential vorticity PV 4-times Daily Theta K m**2 kg**-1 s**-1 /Projects/erai/theta/pv.erai.pt.nc
Pressure PRES 4-times Daily Theta Pa /Projects/erai/theta/pres.erai.pt.nc
U wind U 4-times Daily Theta m s**-1 /Projects/erai/theta/uwnd.erai.pt.nc
V wind V 4-times Daily Theta m s**-1 /Projects/erai/theta/vwnd.erai.pt.nc

Usage Restrictions:

Detailed Description:

  • The full ERA-Interim archives contain several data categories. Currently, this PSL local data set includes only selected variables from the surface analysis, upper air (pressure level), and theta level categories.
  • 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 levels were preserved.
  • Original ECMWF variable names were changed to shorter internal names. Most are standard short names matching PSL current usage for reanalysis data sets.


  • 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:

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.


  • 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


Original Source:


This page was last updated on August 7, 2014.