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Direct LW radiative effect of Saharan dust aerosols
Vincent Gimbert, H.E. Brindley, J.E. Harries
Imperial CollegeLondon
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
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
SEVIRI, 5 March 2004 12:00
Visible channel 0.6 Micron
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+ScatteringDaytime 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
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
GERB L2 unfiltered LWRadiances W/m2/srfrom the 1st to the 18th ofMarch 2004 (12:00)
3rd March 2004 12:00,Srong decrease in OLR
3rd March 2004 12:00,Srong decrease in SurfaceTemperature
Surface Temperature anomalyfrom the 1st to the 18th of March2004 at12:00 wrt March 2004 12:00 average.ECMWF operational model
Precipitable Water (mm)from the 1st to the 18th of March2004 at12:00ECMWF 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 dustmodel.
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