Characterisation of Dust Emission from Alluvial Sources in the Sahara

Stefanie Feuerstein 1, Jamie R. Banks2, Kerstin Schepanski1
1Leibniz Institute for Tropospheric Research, Leipzig, Germany, 2Space & Atmospheric Physics Group, Imperial College London, London, UK

The Sahara is the world's largest source of atmospheric dust. 55% of the global dust emission can be linked to North Africa [1]. Thus, understanding the North African dust sources is of great importance to estimate their impact on the atmospheric dust content. However, the interannual variability in the frequency of dust emission and its connection to local weather conditions is still poorly understood. Alluvial dust sources are considered to have a great impact on dust export, especially in rugged terrain, where heavy rainfall can lead to strong surface runoff and flash floods [2]. Thereby, enormous amounts of sediments are carried downhill and are accumulated at the foothills of mountains. These alluvial fans are considered to be predestined for wind erosion. Despite their importance, alluvial dust sources have not been characterized very well in the past and are often underestimaed in mesoscale and global climate models [2].

Here, we present results from a study characterizing alluvial sediments acting as dust sources. They are analysed in three steps: (1) the surface runoff is estimated by determining the amount of precipitation using TRMM precipitation estimates and by identifying the catchment area of the ephemeral river system. (2) Dust emission subsequent to strong precipitation events is detected using satellite based AOD data and MSG SEVIRI's Dust Product with its very high resolution. (3) Changes in the surface's sediments structure due to accumulation and erosion by water and wind are determined using time series of Landsat optical and Sentinel-1 SAR remote sensing data. Classification algorithms of the remote sensing data show an increase of alluvial sediments downstream of ephemeral channels and on foothills of mountains due to strong precipitation events and a decline of sediment coverage after longer periods of drought. The disappearance of alluvial sediments can at least partly be related to wind erosion, especially since satellite based dust products show dust plumes emitted from the considered area within the analysed time period.

The analysis represents a proof of concept of alluvial dust sources in Saharan mountains like described in [2]. It quantifies the connection between the amount of precipitation and the supply of potentially erodible sediments; the processes are put into a spatial and temporal scale. This will help to improve the representation of alluvial dust sources in climate and dust emission models.

[1] Ginoux, P., Prospero, J.M., Gill, T.E., Hsu, N.C., and Zhao, M. (2012). Global-Scale Attribution of Anthorpogenic and Natural Dust Sources and their Emission Rates based on MODIS Deep Blue Aerosol Products. Rev. Geophys., 50, RG3005. DOI: 10.1029/2012rg000388

[2] Schepanski, K., Wright, T.J., Knippertz, P. (2012). Evidence for Flash Floods over Deserts from loss of coherence in InSAR Imagery. J. Geophys. Res., 117, D20101. DOI: 10.1016/j.rse.2012.03.019