ECMWF Reanalysis of the 20th Century using surface observations only (ERA-20C)

ERA-20C is ECMWF's first atmospheric reanalysis of the 20th century, from 1900-2010. It assimilates observations of surface pressure and surface marine winds only. It is an outcome of the ERA-CLIM project.

Product description

The configuration of the ERA-20C reanalysis is described in  Poli et al. (2016).

ERA-20C was produced with IFS version Cy38r1 and used the same surface and atmospheric forcings as the final version of the atmospheric model integration ERA-20CM. A coupled Atmosphere/Land-surface/Ocean-waves model is used to reanalyse the weather, by assimilating surface observations. The ERA-20C products describe the spatio-temporal evolution of the atmosphere (on 91 levels in the vertical, between the surface and 0.01 hPa), the land-surface (on 4 soil layers), and the ocean waves (on 25 frequencies and 12 directions).

The horizontal resolution is approximately 125 km (spectral truncation T159). Note, atmospheric data are not only available on the native 91 model levels, but also on 37 pressure levels (as in ERA-Interim), 16 potential temperature levels, and the 2 PVU potential vorticity level.

Daily, invariant, and monthly mean data are available from the ERA-20C ECMWF Public Datasets web interface.

The ERA-20C Observation ECMWF Public Datasets web interface also offers access to the observation feedback (see the report by Hersbach et al. (2015) for a description of the contents). This includes the observations used in ERA-20C, as well as departures before and after assimilation and usage flags. The observations assimilated in ERA-20C include surface and mean sea level pressures from ISPDv3.2.6 and ICOADSv2.5.1, and surface marine winds from ICOADSv2.5.1.

The assimilation methodology is 24-hour 4D-Var analysis, with variational bias correction of surface pressure observations. Analysis increments are at T95 horizontal resolution (approx. 210 km). The analyses provide the initial conditions for subsequent forecasts that serve as backgrounds to the next analyses. The spatio-temporal evolution of background errors was provided by a 10-member ensemble produced a priori  (Poli et al. (2013)).

ERA-20C was produced in 2014 in about 6 weeks on ECMWF's IBM Power7 high-performance computer. This production employed 22 parallel streams: Jan 1899-Dec 1904, Jan 1904-Dec 1909, etc... The first year of each stream was only used for spin-up and discarded; the final product is made of the last years of each stream.

Temporal resolution

The temporal resolution of the daily products is usually 3-hourly.

Exceptions are in the analysis products, where the following surface parameters are only available 6-hourly: Sea-ice cover, Snow density, Sea surface temperature, Volumetric soil water layer 1, Volumetric soil water layer 2, Volumetric soil water layer 3, Volumetric soil water layer 4, Soil temperature level 1, Snow depth, 2 metre temperature, 2 metre dewpoint temperature, Soil temperature level 2, Soil temperature level 3, Temperature of snow layer.

Forecast steps

All forecasts are integrated daily, from 06 UTC, for +step hours. The significance of the forecast step depends on whether the forecast parameter is instantaneous or accumulated (from the beginning of the forecast):

Unless otherwise noted, all forecast parameters are instantaneous. The list below indicates the accumulated forecast parameters. These are only found on model levels and at the surface:

• model levels:
  Tendency of short wave radiation, Tendency of long wave radiation, Tendency of clear sky short wave radiation, Tendency of clear sky long wave radiation, Updraught mass flux, Downdraught mass flux, Updraught detrainment rate, Downdraught detrainment rate, Total precipitation flux, Turbulent diffusion coefficient for heat, Tendency of temperature due to physics, Tendency of specific humidity due to physics, Tendency of u component due to physics, Tendency of v component due to physics
• surface:
  Surface runoff, Sub-surface runoff, Clear sky surface photosynthetically active radiation, Total sky direct solar radiation at surface, Clear-sky direct solar radiation at surface, Snow evaporation, Snowmelt, Large-scale precipitation fraction, Downward UV radiation at the surface, Photosynthetically active radiation at the surface, Accumulated Carbon Dioxide Net Ecosystem Exchange, Accumulated Carbon Dioxide Gross Primary Production, Accumulated Carbon Dioxide Ecosystem Respiration, Large-scale precipitation, Convective precipitation, Snowfall, Boundary layer dissipation, Surface sensible heat flux, Surface latent heat flux, Surface solar radiation downwards, Surface thermal radiation downwards, Surface net solar radiation, Surface net thermal radiation, Top net solar radiation, Top net thermal radiation, Eastward turbulent surface stress, Northward turbulent surface stress, Evaporation, Sunshine duration, Eastward gravity wave surface stress, Northward gravity wave surface stress, Gravity wave dissipation, Runoff, Top net solar radiation, clear sky, Top net thermal radiation, clear sky, Surface net solar radiation, clear sky, Surface net thermal radiation, clear sky, TOA incident solar radiation, Vertically integrated moisture divergence, Total precipitation, Convective snowfall, Large-scale snowfall

Monthly means

For both analyses and forecasts, two sets of monthly means are available:

•  synoptic monthly means:
  In the case of analyses, these are averages throughout the calendar month for each available synoptic hour, whereas in the case of forecasts (all issued daily from 06 UTC), they are averages throughout the calendar month for each available forecast step up to 24 hours.
• monthly means of daily means:
  In the case of analyses, these are averages throughout the calendar month across all the available synoptic hours,  whereas in the case of forecasts (all issued daily from 06 UTC), they are averages throughout the calendar month across all the available forecast steps up to 24 hours, for instantaneous forecasts, or just for step=24 hours, for accumulated forecasts.

Although it is not always clear in the GRIB headers, due to limitations on the meta data that can be included in GRIB, the forecast monthly means only include forecast steps that either have a valid time in the calendar month (for instantaneous forecasts) or have an accumulation in the calendar month. For example, 21 and 24 hour forecast accumulations from 06 UTC cross calendar month boundaries, so these accumulations have been cut and spliced so the accumulations fall within the calendar month.