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)

Slide 8

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

Slide 11

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?)