1. Direct LW radiative effect of Saharan dust aerosols Vincent Gimbert, H.E. Brindley, J.E. Harries Imperial College London GIST 24, 16 Dec 2005, Imperial College London

2. Outline of Presentation Mineral aerosols - Introduction Radiative effects Dust event March 2004 GERB Measurements Radiative Transfer Modelling Conclusion Future work

3. 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 Mineral dust aerosols

4. SEVIRI, 5 March 2004 12:00 Visible channel 0.6 Micron

5. AERONET time series – Year 2004 Dakar Capo Verde AOD (at 0.5 microns) ~ 0.5 throughout the year ~ 40% attenuation of sunlight at the surface due to Absorption+Scattering Daytime surface cooling

6. 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

7. 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 Image from Eumetsat

8. 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 OLR

9. 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

10. Precipitable Water (mm) from the 1st to the 18th of March 2004 at12:00 ECMWF operational model

11. 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

12. 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)

13. 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.

14. 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