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Reanalysis of surface observations in ERA-CLIM

Hans Hersbach and Paul Poli ECMWF. Reanalysis of surface observations in ERA-CLIM. Overview The ERA-CLIM pilot reanalyses Forcing model data Historical observation data sets Design of the reanalysis suite Final remarks.

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Reanalysis of surface observations in ERA-CLIM

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  1. Hans Hersbach and Paul Poli ECMWF Reanalysis of surface observationsin ERA-CLIM • Overview • The ERA-CLIM pilot reanalyses • Forcing model data • Historical observation data sets • Design of the reanalysis suite • Final remarks ACRE Workshop, KNMI, 21-23 September 2011

  2. ERA-CLIM:building on the 20th century observational network • Surface data has been available throughout, but initially sparsely distributed • The International Geophysical Year (IGY) marked the start of the extension of the • global radiosonde network and data exchange • The satellite era revolutionized upper-air and stratospheric coverage and general • coverage in the southern hemisphere ACRE Workshop, KNMI, 21-23 September 2011

  3. ERA-CLIM pilot reanalyses ERA-CLIM • The first pilot reanalysis using observations (ERA-20C) will focus on surface observations • All data used will be placed in a web-accessible observationfeedbackarchive(OFA) • A proper long-term evolution of forcing fields is important for all pilot reanalyses ACRE Workshop, KNMI, 21-23 September 2011

  4. Sea-surface temperature and sea-ice cover from HadISST2 • Produced by the Hadley Centre (follow-up of HadISST1, Rayner et al. 2003) • 1899-2010, 0.25-degree gridded daily fields of SST and SIC • 10 equally likely realizations • Fine-gridded climate + coarser-gridded EOF’s Monthly,Yearly, Decadalmoving average ACRE Workshop, KNMI, 21-23 September 2011

  5. CMIP5 forcing for radiation and surface parametrization • Radiation (are available in the latest ECMWF model cycle) • Solar forcing: total solar irradiance • Greenhouse gases: CO2, CH4, N20, CFC-11, CFC-12, … • Ozone:is prognostic variable, but prescribed inside radiation scheme • Tropospheric aerosols: sulphate, black carbon, organic, dust, sea salt • Stratospheric volcanic aerosols: sulphate, dust • Albedo Solid: SPARC, Dotted: GEMS 2006 • Surface parametrization • Vegetation type and cover, LAI,… Optical depth, from: Sato et. al, GISS ACRE Workshop, KNMI, 21-23 September 2011

  6. The ISPD data set International Surface Pressure Data bank(Version 2.2) The observation network has evolved quite a bit Courtesy :20th Century Reanalysis Project (NOAA/CIRES) • surface pressure and MSLP • Includes ICOADS pressure data • first data set being imported into OFA • Contains feedback info from 20CR ACRE Workshop, KNMI, 21-23 September 2011

  7. The ICOADS data set If time permits: International Comprehensive Ocean-Atmosphere Data Set ISD International Surface Data bank Land surface data HSDSD HistoricalSovjet Daily Snow Depth 1885-1991 Courtesy: ICOADS • ERA-CLIM will use MSLP, wind, T2m, Rh2m, • but possible not yet in ERA-20C • 2nd data set being imported into OFA ACRE Workshop, KNMI, 21-23 September 2011

  8. Data assimilation (at ECMWF) ACRE Workshop, KNMI, 21-23 September 2011

  9. Four-dimensional variational analysis of observations • The model equations are used to fill data gaps and to propagate information forward in time • Observations are used to constrain the model state • Additional parameters may be used to adjust for data biases(VarBC) • The quality of the analysis and background will evolve with the observation network (EDA, …) prior state constraints prior parameter constraints observational constraints ACRE Workshop, KNMI, 21-23 September 2011

  10. Towards a surface-data-only reanalysis • Has already been done in the US (pressure): 20CR(Compo et al., QJRMS 2011) • Ensemble Kalman filter • ERA-CLIM will use variational assimilation (4D-Var) in an ensemble (EDA) context • Production speed target is 100 days/day for 100 years of reanalysis • This implies a low resolution (horizontal: T159 ~ 125 km) • Try to make best use of observations; • Exploit the fact that re-analysis is not constrained by data timeliness • Test length of assimilation windows to optimize speed and quality • for surface pressure address biases in an automatic manner (VarBC). • address the evolution of the quality of the background (EDA). ACRE Workshop, KNMI, 21-23 September 2011

  11. What we can do with surface pressure-only observations: 4D-Var analysis of the 500hPa geopotential height surface 0 UTC, 15 February 2005 using all available observations using surface pressure observations only ACRE Workshop, KNMI, 21-23 September 2011

  12. Surface-pressure-only T159 (an T159/T95) vs. All obs T255 (an T159/T95)Forecast RMSE Temperature +3 days and +6 days NH 1000 hPa (K) PS +6d REF +6d ~ PS +3d REF +3d For NH (and SH), there is some forecast skill up for 1000 (and 500) hPa NH 100 hPa Problem with sudden strat. warming events (K) PS +3d ~ PS +6d REF +6d REF +3d ACRE Workshop, KNMI, 21-23 September 2011

  13. New bias correction scheme for surface station observations • Variational bias correction, by station identifier and/or location • Runs within the analysis; observations can be preparedahead of time without waiting forprevious analysis • Reproduces the behaviour of the old scheme for sfc. P., except • All stations get corrected (previously only those with large biases) • Greater stiffness Namibia Oper. (Pa) New (Pa) Zimbabwe Bias correction time-series ACRE Workshop, KNMI, 21-23 September 2011

  14. At ECMWF the initial phase of ERA-CLIM involves a consideral amount of preparation: • The design/construction of the observation feedback archive (OFA) • The ingestion of observation data bases from their native format to the OFA • The capability to assimilate data from the OFA into the ECMWF 4D-Var system • A proper long-term prescription of forcing fields, SST and sea-ice • The design of an analysis suite that meets the required integration speed and resolution • ERA20C will ingest surface data from the ISPD • but if time permits wind, T2m, Rh2m, snow from ICOADS, ISD, and HSDSD as well • The recovery of historical data sets is of great value to improve the reanalyses, • especially for the early 20th century Concluding remarks Pre-SAC Reanalysis briefing, 16 Sept 2011

  15. Towards 24-hour 4DVAR ACRE Workshop, KNMI, 21-23 September 2011

  16. Comparison to non-assimilated observations Radiosondemeridional wind (v) in NH ~1 ½ month bg an 24-hour 4DVAR 12-hour 4DVAR 1. Some redundancy of information between Ps and wind up to tropopause 2. 24-hour 4DVAR seems to better fit independent winds in troposphere, but degradation in stratosphere (model bias? incorrect B matrix?) ACRE Workshop, KNMI, 21-23 September 2011

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