Slide 1
ACRE Workshop, KNMI, 21-23 September 2011
Reanalysis of surface observationsin ERA-CLIM
Hans Hersbach and Paul Poli
ECMWF
Slide 1
Overview
• The ERA-CLIM pilot reanalyses
• Forcing model data
• Historical observation data sets
• Design of the reanalysis suite
• Final remarks
Slide 2
ACRE Workshop, KNMI, 21-23 September 2011
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
Slide 2
Slide 3
ERA-CLIM pilot reanalyses
ER
A-C
LIM
ACRE Workshop, KNMI, 21-23 September 2011 Slide 3
• The first pilot reanalysis using observations (ERA-20C) will focus on surface observations• All data used will be placed in a web-accessible observation feedback archive (OFA)• A proper long-term evolution of forcing fields is important for all pilot reanalyses
Slide 4
ACRE Workshop, KNMI, 21-23 September 2011
Sea-surface temperature and sea-ice cover from HadISST2
Slide 4
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, Decadal moving average
Slide 5
ACRE Workshop, KNMI, 21-23 September 2011
CMIP5 forcing for radiation and surface parametrization
Slide 5
• 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
• Surface parametrization• Vegetation type and cover, LAI,…
Optical depth, from: Sato et. al, GISS
Solid: SPARC, Dotted: GEMS 2006
Slide 6
ACRE Workshop, KNMI, 21-23 September 2011
The ISPD data setInternational Surface Pressure Data bank (Version 2.2)
Courtesy :20th Century Reanalysis Project (NOAA/CIRES)
Slide 6
• surface pressure and MSLP• Includes ICOADS pressure data• first data set being imported into OFA• Contains feedback info from 20CR
The observation network has evolved quite a bit
Slide 7
ACRE Workshop, KNMI, 21-23 September 2011 Slide 7
The ICOADS data set
Courtesy: ICOADS
International Comprehensive Ocean-Atmosphere Data Set
• ERA-CLIM will use MSLP, wind, T2m, Rh2m,
but possible not yet in ERA-20C• 2nd data set being imported into OFA
If time permits:
ISDInternational Surface Data bankLand surface data
Historical Sovjet Daily Snow Depth1885-1991
HSDSD
Slide 8
ACRE Workshop, KNMI, 21-23 September 2011
Data assimilation (at ECMWF)
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Slide 9
Four-dimensional variational analysis of observations
h(x)β)(x,byRh(x)β)(x,by
β)(βBβ)(βx)(xBx)(xβ)J(x,
o1T
o
b1
βT
bb1
xT
b
• The model equations are used to fill data gaps and to propagate information forward in time
observational constraints
• Observations are used to constrain the model state
prior parameter constraints
• Additional parameters may be used to adjust for data biases (VarBC)
ACRE Workshop, KNMI, 21-23 September 2011
prior state constraints
• The quality of the analysis and background will evolve with the observation network (EDA, …)
Slide 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 Slide 10
Slide 11
ACRE Workshop, KNMI, 21-23 September 2011
using all available observations
4D-Var analysis of the 500hPa geopotential height surface 0 UTC, 15 February 2005
What we can do with surface pressure-only observations:
using surface pressure observations only
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Slide 12
Surface-pressure-only T159 (an T159/T95)
vs. All obs T255 (an T159/T95)
Forecast RMSE Temperature +3 days and +6 days
ACRE Workshop, KNMI, 21-23 September 2011 Slide 12
NH 100 hPa
REF +3d
PS +3d ~ PS +6dREF +6d
Problem with sudden strat. warming events
REF +3d
REF +6d ~ PS +3d
PS +6d
NH 1000 hPa
For NH (and SH), there is some forecast skill up for 1000 (and 500) hPa
(K)
(K)
Slide 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
ACRE Workshop, KNMI, 21-23 September 2011 Slide 13
(Pa)
(Pa)
Oper.New
Bias correction time-series
Namibia
Zimbabwe
Slide 14
Pre-SAC Reanalysis briefing, 16 Sept 2011
Concluding remarks
Slide 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
Slide 15
Towards 24-hour 4DVAR
ACRE Workshop, KNMI, 21-23 September 2011 Slide 15
Slide 16
Comparison to non-assimilated observations Radiosonde meridional wind (v) in NH
ACRE Workshop, KNMI, 21-23 September 2011 Slide 16
24-hour 4DVAR
12-hour 4DVAR
anbg
~1 ½ month
1. Some redundancy of information between Ps and wind up to tropopause2. 24-hour 4DVAR seems to better fit independent winds in troposphere, but degradation in stratosphere (model bias? incorrect B matrix?)