Atmospheric Aerosol Dynamics

Atmospheric aerosols play an important though uncertain role in the Earth's climate system. Our analysis of satellite derived dust estimates has identified the Bodele Depression in Chad as the planet's largest source of dust. Very little is known about the atmospheric circulation that maintain this source. Our research has made the following contributions to understanding dust production:

Figure 1: Example of dust plume from Bodele Depression (3 March 2003) as observed from MODIS

Figure 1: Example of dust plume from Bodele Depression (3 March 2003) as observed from MODIS.


1) We have found that maxima in dust loadings, surface wind stress and topographic depressions are co-located in many of the world's key dust sources, e.g. Bodele, Chad, Taklamakan and the Simspon Desert in Australia. See Figures 8 and 13 in our global dust paper, Washington et al. 2003.

2) We have identified a new feature of the atmospheric circulation, the Bodele Low Level Jet (LLJ) Washington and Todd, 2005, GRL, which explains the atmospheric control on the planet's largest source of mineral aerosols. This work resulted from a rigorous analysis of numerical climate model products. We have quantified the structure and charactertistics of the Bodele Low Level Jet which has a maximum speed near 18°N, 19°E at 925 hPa (see figures 2-4). The jet is strongest in the northern winter, receding with the advance of summer in phase with dustiness in the Bodele. Variability of dust over the Bodele occurs contemporaneously with the ridging of the Libyan High and pulsing of the pressure gradient which drives the northeasterlies in which the LLJ is embedded. Similarly, interannual variability of aerosol is associated with modulation of the jet strength, thus proving that the atmosphere is alone able to exert a crucial role in the control of dust loadings.

3) Following from the first field experiment to the Bodele, called BoDEx, carried out in February and March 2005 which was featured in Nature (view article), we have confirmed the existence of the Bodele Low Level Jet, constrained the threshold windspeed for dust initiation, and quantified the diurnal variability of the low level circulation, a factor crucial to satellite estimates of dust which are made from daytime only passes. See our JGR paper, Washington et al. 2006. for details.

4) In collaboration with UCL, we have run a number of idealized experiments with the regional model MM5, which proves that the topography of North Africa is responsible for increasing the Low Level Jet strength by 40%.

5) Based on our understanding of the Bodele Low Level Jet and the regional modelling work, we have examined whether the colocation of high friction velocity and surface sediments is purely coincidental in the Bodele. In this paper submitted to GRL. we argue that they are linked.

6) We are on the steering committee for the aerosol component of AMMA (http://amma.mediasfrance.org/), the largest land based climate experiment ever undertaken, one of the aims of which is to gain aircraft measurements of aerosols.

Our work is not confined to the Bodele, but includes much of North Africa (e.g. Sebastian Engelstaedter) and China (e.g. Hang Gao). In the case of West Africa, we have shown that dry convection is a main driver of deflation in the boreal summer months over the key sources in Mali, Mauritania and Algeria.

Figure 2. Mean (1979-1992) ERA-40 zonal winds (m.s-1) in Bodele region during January to April, longitude-height section along 18°N (location of Bodele Depression) between 10°W and 30°E
Figure 2. Mean (1979-1992) ERA-40 zonal winds (m.s-1) in Bodele region during January to April, longitude-height section along 18°N (location of Bodele Depression) between 10°W and 30°E.

Figure 3. Mean (1979-1992) ERA-40 zonal winds (m.s-1) in Bodele region during January to April at 925hPa.
Figure 3. Mean (1979-1992) ERA-40 zonal winds (m.s-1) in Bodele region during January to April at 925hPa.

Figure 4. Latitude-height section of mean ERA-40 zonal wind (m.s-1) composite difference anomalies along 18°E  for a sample of 8 high, minus 8 low, dust months during January-April 1979-92.
Figure 4. Latitude-height section of mean ERA-40 zonal wind (m.s-1) composite difference anomalies along 18°E for a sample of 8 high, minus 8 low, dust months during January-April 1979-92.


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