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