Dust longwave forcing from gerb and seviri
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Imperial College London. Dust Longwave Forcing from GERB and SEVIRI. Vincent Gimbert, H.E. Brindley, J.E. Harries. Thanks to Rainer Hollmann, Nicolas Clerbaux, Alessandro Ipe and Richard Allan, Andy Smith. GIST 26, 03 May 2007, RAL, Abingdon. Motivation.

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Dust longwave forcing from gerb and seviri

Imperial College


Dust Longwave Forcing from GERB and SEVIRI

Vincent Gimbert, H.E. Brindley, J.E. Harries

Thanks to Rainer Hollmann, Nicolas Clerbaux, Alessandro Ipe and Richard Allan, Andy Smith

GIST 26, 03 May 2007, RAL, Abingdon


  • Use GERB and SEVIRI to measure TOA LW radiative forcing of Dust aerosols over the Sahara

  • Model Clear-sky TOA LW Radiances Using ECMWF Analyses to separate out effect of Dust from that of variable meteorology

  • Test the ECMWF model against observations and provide uncertainties in estimate of Dust forcing

Outline of presentation
Outline of Presentation

  • Method, measurements, model

  • March 2004: LW forcing of dust storm from GERB

  • Model comparison with Clearsky observations from GERB and SEVIRI IR Channels

  • Dust spectral forcing

  • Results from March 2006

  • Conclusion and future work

GERB LW radiance 200403 12 UTC

  • Strong OLR perturbation associated with dust event

  • Daytime SAFNWC Dust detection

RT modelling / GERB-SEVIRI data

Spectral Radiances modelled using MODTRAN 4 from 3.5 μm to ∞

  • Temperature, humidity, ozone from ECMWF analyses

  • Unity surface emissivity

  • 1 * 1 Degree resolution - 60/91 vertical levels

  • Spectral integration over appropriate instrument/channel response

Comparison with GERB and SEVIRI measurement


  • SEVIRI L1.5 Radiances (7 IR channels, 062,073,087,097,108,120,134)

  • 6-hourly 00:00, 06:00, 12:00, 18:00 (No GERB data at 00:00)

  • March 2004 and 2006

12:00 GERB - Model difference

  • GERB/model anomaly coincides with dust front on 3rd March 2004

  • Anomalies over clear sky as well

GERB and Model 12UTC through March

Cloud/dust free pixels

averages over dust

front region

(~10*10 deg square)

Over dust front (20040303) : LWRF ~ 20(+/- 3) W/m2

Time dependence of clearsky errors

  • Model warm bias at 06 and 18

  • Larger errors at 12 (larger Std Dev than 06 and 18)

  • Use SEVIRI to test model...

WV 6.2 μm, High/mid atm WV 7.3μm, Mid/Low

SEVIRI/model Water Vapour channels

Window channels

  • Persistent model warm bias at night (σ~1-2K)

  • Daytime cold bias (~3K), σ~3.3K

  • Discrepancies over LAND

  • Same results from 10.8μm channel

The case of 8.7μm Channel





Method useful for removing consistent bias

No a priori knowledge of surface spectral emissivity needed

8.7μm: Emissivity + Tsurf

12.0μm: Surface Temperature

Latitudinal dependence of Clearsky errors

  • No dependance in WV channels

Dust LW Spectral forcing

No correction: unrealistic forcing, No uncertainties

6.2um: non- physical

7.3um: Negative at 12UTC…

8.7um: Too high (emissivity bias)

March 2006 Dust event

20060307 12UTC

EUMETSAT Dust RGB colour scheme

March 2006 Dust event

20060308 12UTC

EUMETSAT Dust RGB colour scheme

Dust LW Spectral forcing

  • Maximum forcing on 20060308

  • Similar spectral dependence as March 2004

  • Consistent with models

Conclusion and future work

  • Mean 200403 Dust storm OLR forcing: ~20W/m2

  • Diurnal cycle of Clear-sky GERB-model errors

  • Good agreement in SEVIRI WV channels (small biases)

  • Land Emissivity and Temperature Regional Biases in Window Channels

  • Very similar results for March 2004 and 2006:

  • Dust Spectral Forcing

Future work:

  • Validation at Niamey ARM site:

  • Dust detection reliability

  • Diurnal cycle of Tsurf

  • Consistence of GERB-SEVIRI vs Model biases

Daytime Conditional Bias

Model does not reproduce the range of GERB OLR at 12

Similar finding as Trigo and Viterbo (2003) with ECMWF model compared to MS7

Window channel

We look at possible sources of errors at the surface