Imperial CollegeLondon

Saharan Dust Longwave RadiativeForcing using GERB and SEVIRI

Vincent Gimbert1, H.E. Brindley1, Nicolas Clerbaux2, J.E. Harries1

1. Blackett Laboratory, Imperial College, London2. Royal Meteorological Institute of Belgium, Brussels

IUGG Conference, 10 July 2007, Perugia, Italy

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Aims and Motivation

Use geostationary broadband and spectral data to measure TOA Direct LW radiative forcing of Dust over the Sahara

Model clear-sky TOA LW radiances and fluxes 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

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Outline of Presentation

Data and Methods

Case Study: March 2004 Dust Storm

Broadband forcing from GERBSpectral forcing from SEVIRI

Clear-sky Model Evaluation

Summary and Discussion

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Geostationary Earth Radiation Budget - GERB

GERB: European consortium led by the UK (Harries et al., 2005)Imperial College: PI (J.E. Harries), Instrument calibration and Science

4 instruments onboard Meteosat Second GenerationCalibrated LW and SW Broadband radiancesResolution: ~ 50km at Nadir, 15 Minutes

Edition 1 data available from Feb 2004(12-14 years of data)

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Case Study: March 2004 Dust Storm

Spinning Enhanced Visible and InfraRedImager (SEVIRI) RGB Composite IR12.0-IR10.8, IR10.8-IR8.7, IR10.83 March 2004 - 1200 UTC

Credit: EUMETSAT

Dust storm associated with strong decreasein OLR. How much did the dust contribute?

Ground-based and model data show strongchanges in temperature and atmosphericcomposition. Cooling by cold air advection.(Knippertz and Fink, 2006)

Need to model Clear-sky OLR to estimatedust forcing.

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Estimating the dust Direct Radiative Effect

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RT Modelling, Data and Scene Identification

Spectral Radiances modeled using MODTRAN 4• Temperature, humidity, ozone from ECMWF operational model analyses• Unity surface emissivity• 1 × 1 Degree resolution - 60 vertical levels• Study region: [0º-40ºN] and [25ºW-60ºE]

Comparison with GERB and SEVIRI measurement• GERB Level 2 Average Rectified Geolocated (L2 ARG) product• SEVIRI L1.5 Radiances (7 IR channels, 062,073,087,097,108,120,134)• 6-hourly 0000, 0600, 1200, 1800 UTC (No GERB data at 0000)• March 2004

Scene Identification from SEVIRI (NWCSAF)• Cloud and Dust detection (Visible and infrared channels)

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Broadband DRE from GERB

Mean DRE = 22.2 ± 4.6 W.m-2

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Spectral DRE from SEVIRI

Similar approach with the 7 SEVIRI infrared channelsSpectral integration of high resolution radiance spectra

Strong emissivity correction in 8.7µm channelSignificant DRE in CO2 and O3 channelsDust signature in 7.3 µm water vapour channel

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Monthly Mean Daytime Clear-sky Model-GERB

Large regional biases:

• Positive ~ [20-30N] and Arabian Peninsula: Land surface emissivity• Negative ~ [10-15N]: Daytime underestimation of Land Surface

Temperature

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Model Evaluation

Similar regional patterns in March 2006

Narrowband comparisons useful to understand model errors

• Very good agreement in WV channels• Most Model-GERB errors arise from the land surface• Surface emissivity shows strong spectral dependance• ECMWF model underestimates amplitude of diurnal cycle of Ts

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Summary and Discussion

Combination of model and observations to infer DRE without information on dust properties and vertical distributionAverage DRE of 22.2 ± 4.6 W.m-2 on 3 March 2004 at 1200 UTCSpectral DRE form SEVIRIClear-sky errors

• Dust detection issues:

- Detection of light dust loadings- Cloud/dust misidentification

• Geostationary satellite for diurnal cycle of DRE• Ground-based model validation + Information on dust

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Contact and references

Thanks to:• Rainer Hollmann, DWD, Germany• Alessandro Ipe, RMIB, Belgium• GERB International Science Team

GERB Project: http://www3.imperial.ac.uk/spat/research/missions/atmos_missions/gerb

References:• Harries, J. E., et al. (2005), The Geostationary Earth Radiation Budget project,

Bull. Am. Meteorol. Soc., 86, 945–960.• Knippertz, P., and A. H. Fink (2006), Synoptic and dynamic aspects of an extreme

springtime Saharan dust outbreak, Q. J. R. Meteorol. Soc., 132, 1153–1177.

Please contact me: vincent.gimbert03@imperial.ac.uk

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6-hourly Clear-sky Model-GERB Errors

Time Surface

06 UTC 12 UTC 18 UTC

LANDMean (1σ), W.m-2 8.7 (4.4) 3.4 (11.0) 6.8 (4.8)

WATERMean (1σ), W.m-2 5.4 (1.8) 4.8 (1.9) 4.6 (1.9)

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

Time Channel

00 UTC 06 UTC 12 UTC 18 UTC

IR_062Mean (1σ), K

-1.67 (0.8)

-1.59 (0.6)

-1.51 (0.6)

-1.53 (0.6)

IR_073Mean (1σ), K

0.95 (0.8) 0.95 (0.7) 2.05 (1.0) 1.34 (0.8)

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Additional Slides

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Additional Slides

12.0µm

Night Day

8.7µm

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Additional Slides

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Additional Slides

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Additional Slides

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Additional Slides