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Imperial College London. Direct LW radiative effect of Saharan dust aerosols. Vincent Gimbert, H.E. Brindley, J.E. Harries. GIST 24, 16 Dec 2005, Imperial College London. Outline of Presentation. Mineral aerosols - Introduction Radiative effects Dust event March 2004 GERB Measurements
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Imperial College London Direct LW radiative effect of Saharan dust aerosols Vincent Gimbert, H.E. Brindley, J.E. Harries GIST 24, 16 Dec 2005, Imperial College London
Outline of Presentation • Mineral aerosols - Introduction • Radiative effects • Dust event March 2004 • GERB Measurements • Radiative Transfer Modelling • Conclusion • Future work
Mineral dust aerosols • Primary aerosols emitted from desert surfaces • Lifted in atmosphere by strong surface winds • Residence time 1 day ~ 1 week • Present in the Lower troposphere but can travel 1000s of km
SEVIRI, 5 March 2004 12:00 Visible channel 0.6 Micron
Dakar Capo Verde AERONET time series – Year 2004 AOD (at 0.5 microns) ~ 0.5 throughout the year ~ 40% attenuation of sunlight at the surface due to Absorption+Scattering Daytime surface cooling
Dust LW radiative effect • Dust are large aerosols (~ 1 micron) so also interact with IR terrestrial radiation. • LW effect is a greenhouse effect (as clouds) • Absorption of LW radiation and re-emission at level Temperature • Scattering of LW radiation (asymmetry parameter) • Both Absorption and Scattering decrease the TOA OLR Africa is the largest source of dust – GERB /SEVIRI excellente location for quantifying dust radiative effect
Image from Eumetsat SEVIRI, 3 March 2004 12:00 • RGB composite image from differences of Brightness temp. • R=IR12.0-IR10.8 G=IR10.8-IR8.7 R=IR10.8
3rd March 2004 12:00, Srong decrease in OLR GERB L2 unfiltered LW Radiances W/m2/sr from the 1st to the 18th of March 2004 (12:00)
3rd March 2004 12:00, Srong decrease in Surface Temperature Surface Temperature anomaly from the 1st to the 18th of March 2004 at12:00 wrt March 2004 12:00 average. ECMWF operational model
Precipitable Water (mm) from the 1st to the 18th of March 2004 at12:00 ECMWF operational model
ECMWF as input of RT model ECMWF model does seem to pick up the event (strong surf cooling) • SW extinction • Advection of cold and dry air from Europe Modelling of radiances using MODTRAN v4r3 from 3.5 μm to ∞ • Minor gases, heavy molecules from database (H. Brindley pers. comm.) • Surf Temp, Temp profile, Humidity profile from ECMWF op. model • 4 Surface types spectral emissivity (from Modtran Library and JHU) AERONET site in Agoufou, Mali (15N, 1W) • AOD at 0.55 μm • Very good check of PWV with ECMWF
Sensitivity to surface type – GERB radiance Modelling of integrated LW Radiance in Agoufou, Mali at12:00 through March 2004 (solid lines) 4 surface types (Max diff ~ 4 W/m2/sr) Cloudy days removed (RMIB flag)
Sensitivity aerosol representation in model Modelling of integrated LW Radiance in Agoufou, Mali at12:00 through March 2004 (solid lines) fixed surface type Dust aerosols (4 dust representations) LW TOA forcing 3-5 W/m2/sr per unit AOD, depending on dust model.
Conclusions and future work • Dust aerosols exert a significant radiative forcing, both in the SW and in the LW • March dust event clearly apparent on LW GERB data • RT model shows good qualitative agreement with GERB but need more constraints on surface type • Instantaneous LW radiative forcing of ~ 3-5 W/m2/sr per unit AOD but need more information on dust optical properties • Run RT model over the whole Sahara region on 3 March 2004 and compare to GERB • Model sensitivity study to temperature, humidity and dust height • Comparisons Model/SEVIRI NB radiances