CERA-20C is the ECMWF 10-member ensemble of coupled climate reanalyses of the 20th century, from 1901-2010. It is based on the CERA assimilation system, which assimilates only surface pressure and marine wind observations as well as ocean temperature and salinity profiles. It is an outcome of the ERA-CLIM2 project.

Product description

CERA-20C reconstructs the past weather and climate of the Earth system including the atmosphere, ocean, land, waves and sea ice. To account for errors in the observational record as well as model error, CERA-20C provides a 10-member ensemble of reanalyses.

CERA-20C was produced with IFS version Cy41r2 and the atmospheric forcings as the final version of the atmospheric model integration ERA-20CM and ERA-20C. The air-sea interface is relaxed towards the sea-surface temperature from the HADISST2 monthly product to avoid model drift while enabling the simulation of coupled processes. No data assimilation is performed in the land, wave and sea-ice components, but the use of the coupled model ensures a dynamically consistent Earth system estimate at each time.

The CERA assimilation system is based on a variational method with a common 24-hour window shared by the atmospheric and ocean components. The coupled model is introduced at the outer-loop level by coupling ECMWF’s Integrated Forecasting System (IFS) for the atmosphere, land and waves to the NEMO model for the ocean and to the LIM2 model for sea ice. This means that air–sea interactions are taken into account when observation misfits are computed and when the increments are applied to the initial condition. In this context, ocean observations can have a direct impact on the atmospheric analysis and, conversely, atmospheric observations can have an immediate impact on the analysed state of the ocean.

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

To produce the CERA-20C dataset in a reasonable period of time, the period 1900–2010 was divided into 14 different streams of 10 years. Each production stream was initialised from the uncoupled reanalyses ERA-20C and ORA-20C. The first two years of each production stream were used for spin-up (only one-year of spin-up for the first stream) to produce the final climate dataset for the period 1901-2010. The computation footprint of CERA-20C on ECMWF’s high-performance computing facility is significant, with seven months of production using 20,000 cores, which represents 5% of the total resources. 500,000 variational problems had to be solved processing up to 5,000,000 observations, at a pace of one every 30 seconds.

Spatial and temporal resolution

The CERA-20C product describes the spatio-temporal evolution of the atmosphere (125km horizontal grid with 91 vertical levels, between the surface and 0.01 hPa), the land-surface (125km horizontal grid with 4 soil layers, the waves ( for the period 1901-2010) , the ocean (110km horizontal grid with meridional refinement at the equator, 42 vertical levels) and the sea ice.

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.

The dataset is available from 1 January 1901 to 31 December 2010 with a temporal resolution of 3 hours for the daily products.

Forecast steps

All forecasts are integrated daily, from 18 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):

Significance of the forecast steps in CERA-20C
Step 3 6 9 12 15 18 21 24 27
Valid time, for instantaneous forecast parameters 21UTC 00UTC next day 03UTC next day 06UTC next day 09UTC next day 12UTC next day 15UTC next day 18UTC next day 21UTC next day
Accumulation period, for accumulated forecast parameters 18UTC to 21UTC 18UTC to 00UTC next day 18UTC to 03UTC next day 18UTC to 06UTC next day 18UTC to 09UTC next day 18UTC to 12UTC next day 18UTC to 15UTC next day 18UTC to 18UTC next day 18UTC to 21UTC next day

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, Snow melt, 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 18 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 18 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.