© Crown copyright Met Office

Assimilating infra-red sounder data over land

John Eyre for Ed PavelinMet Office, UK

Acknowledgements: Brett Candy

DAOS-WG, 4th meeting, Exeter, 27-28 June 2011

© Crown copyright Met Office

Assimilating infra-red sounder data over land

Outline

• The problem• Met Office implementation: 1D-var + 4D-Var• Retrieval assimilation studies:

• on simulated data• on real data

• Conclusions

© Crown copyright Met Office

Pre

ssur

e (h

Pa)

IASI temperature Jacobians

Surface-sensitivechannels

Large proportion oftropospheric channels!

The problem

© Crown copyright Met Office

Land surface emissivity (8.5m)from NASA AIRS product, June-August 2008

© Crown copyright Met Office

Use of IR radiances over land

• Current operations:

• Assume infrared emissivity = 0.98 over land

• Not good enough – don’t use channels sensitive below ~ 400hPa

• Options to increase data use over land• Use fixed emissivity “atlas”• Use land surface model / surface type atlas• Retrieve surface emissivity from observations

© Crown copyright Met Office

Met Office assimilation of AIRS and IASI radiances

• 1D-Var analysis for each radiance vector:• … to estimate atmospheric profile + surface variables• … to provide:

• QC on radiances passed to 4D-Var• estimate of variables not in 4D-Var control variable:

• skin temperature• [temperature profile above model top]• cloud top height, cloud fraction• ***NEW*** – surface emissivity variables

• Radiances and 1D-Var-retrieved variables passed to 4D-Var

© Crown copyright Met Office

1D-Var fit to observations

Window channel, assuming = 0.98

Over sea:use ISEM emissivity model(quite accurate)

Over land:assume emissivity=0.98Large mis-fit

© Crown copyright Met Office

0.98

© Crown copyright Met Office

Simulation experiment: retrievals from simulated AIRS radiances (1)

Simulated radiances:

• simulated using RTTOV radiative transfer model• 13495 atmospheric profiles from ECMWF ERA40 dataset• surface emissivity from UWisc/CIMSS IR emissivity atlas

(2006 data used)• derived from MODIS observations, fitted to laboratory spectra• independent of training dataset

• simulated observation errors

© Crown copyright Met Office

Simulation experiment: retrievals from simulated AIRS radiances (2)

1D-Var retrievals:

• 12 emissivity PCs in retrieval vector

• first guess emissivity = 0.98

• emissivity retrieved simultaneously with T, q and cloud

• idealised case: • using all AIRS channels• clear sky only

© Crown copyright Met Office

Atlas: 9.32 m (mineral signal)

True Emissivity

© Crown copyright Met Office

Retrieval: 9.32 m (mineral signal)

Retrieved Emissivity

© Crown copyright Met Office

920 hPa temperature bias:without emissivity retrieval

© Crown copyright Met Office

920 hPa temperature bias:with emissivity retrieval

© Crown copyright Met Office

Promising results from simulations…

Test with real data

…but first: what about clouds?

© Crown copyright Met Office

Need good and

Tskin to detect cloud

Need good and

Tskin to detect cloud

Need knowledge of cloud to

analyse sfc!

Need knowledge of cloud to

analyse sfc!

© Crown copyright Met Office

Need good and

Tskin to detect cloud

Need good and

Tskin to detect cloud

Need knowledge of cloud to

analyse sfc!

Need knowledge of cloud to

analyse sfc!

Use a piori emissivity

atlas?

Use a piori emissivity

atlas?

Use a priori

emissivity atlas

Use a priori

emissivity atlas

© Crown copyright Met Office

First-guess emissivity:a global IR emissivity atlas

• CIMSS/Univ. Wisconsin IR emissivity atlas• based on MODIS products• emissivities fitted to high-spectral-resolution

laboratory measurements• monthly variability, high spatial resolution• becoming widely used

• Other atlases available from other sources• Products from AIRS, IASI, …

© Crown copyright Met Office

Global NWP trials

• Bottom line: impact on Met Office “global NWP index”• Winter: +0.0 vs obs, -0.1 vs anal• Summer: +0.1 vs obs, +0.3 vs anal

• Sufficiently +ve to include in next operational change (July 2011)

• However:

• Problems with model Tskin biases during daytime

• At present data included nighttime only, and only IASI• First step - improvements expected

© Crown copyright Met Office

Conclusions

• Currently, low-peaking IR sounding channels are rejected over land - uncertainties in emissivity and Tskin

• Improvements implemented via 1D-Var:• analysis of spectrally-varying surface emissivity• “first-guess” global emissivity atlas

• Poor fit to observations over daytime deserts• possible systematic biases in model surface temperature

© Crown copyright Met Office

Questions ?