Coupled reanalysis of the satellite era (CERA-SAT)

CERA-SAT is a reanalysis dataset spanning 8 years between 2008 and 2016. It has been produced within the scope of the ERA-CLIM2 project as a proof-of-concept for a coupled reanalysis with the full observing system available in the modern satellite age. CERA-SAT has been created using the CERA assimilation system and comprises an ensemble of 10 individual members. The ensemble accounts for model- and observational errors and can be used to infer information on the uncertainty of the analysed fields. It is available on 37 pressure levels.

For any questions regarding the data access and the data itself, please go to the C3S user service desk and look at the Knowledge Base or contact the User Support via the Enquiry Portal.

Product description

CERA-SAT reconstructs the past weather 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-SAT provides a 10-member ensemble of reanalyses.

CERA-SAT was produced with IFS version Cy42r1. The air-sea interface is relaxed towards the high resolution (0.05 degree) OSTIA sea-surface temperature analysis to avoid model drift while enabling the simulation of coupled processes.

The CERA assimilation system is based on a variational method with a common 24-hour window shared by the atmospheric, ocean and sea ice 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 land surface and wave analyses are only weakly coupled to the rest of the Earth system domain through sharing of the coupled background trajectory.

The CERA-SAT Observation ECMWF Public Datasets web interface offers access to the atmospheric observation feedback. This includes the observations used in CERA-SAT, as well as departures before and after assimilation and usage flags. The observations assimilated in CERA-SAT include conventional and satellite measurements for the atmospheric, land, wave, ocean and sea ice components. 

To produce the CERA-SAT dataset in a reasonable period of time, the period 2008-2016 was divided into 4 different streams of 2-3 years. Each production stream was initialised from the uncoupled reanalyses ERA-Interim and ORAS5. The first 6 months of each production stream were used for spin-up to produce the final climate dataset for the period 2008-2016.

Spatial and temporal resolution

The CERA-SAT product describes the spatio-temporal evolution of the atmosphere (65km horizontal grid with 137 vertical levels, between the surface and 0.01 hPa), the land-surface (65km horizontal grid with 4 soil layers, the waves , the ocean (30km horizontal grid, 75 vertical levels) and the sea ice.

Atmospheric data are not only available on the native 137 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 2008 to 31 December 2016 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):

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 18 UTC cross calendar month boundaries, so these accumulations have been cut and spliced so the accumulations fall within the calendar month.

Missing data

For the periods 01/01/2010-15/07/2010 and 01/01/2012-18/08/2012, the forecast steps 21,24,27 and 30 are not available for all the vertical integral of surface parameters (stream=enda, type=fc, levtype=sfc).

For the periods 01/01/2010-24/07/2010 and 01/01/2012-27/08/2012, the forecast steps 3,6,9,12,15,18,21,24,27,30 are not available for all the wave parameters (stream=ewda, type=fc) .

Associated carbon reanalysis

CERA-SAT/Carbon is a land carbon cycle reanalysis forced by the atmospheric fluxes of CERA-SAT. It has been produced at LSCE with the ORCHIDEE biogeochemical model. It provides a reconstruction of the land carbon fluxes and stocks for different Plant Functional Types (PFTs). The dataset is available on the LSCE website.