Dusty gust fronts at synoptic scale, initiated and maintained by moist convection over the Sahara desert

So-called ‘dry' microburst outflows are well known phenomena in desert environments when rain from moist convection aloft evaporates into deep, dry-adiabatic boundary layers. Furthermore, moist convection outflows are well recognized as an efficient mechanism for dust mobilization over arid and semi arid regions. Extreme synoptic scale versions of this convective scale phenomenon have been documented in this study, in which the collective episodes of convective downdraft feed a common cold pool that expands as a gust front density current, raising large amount of dust in the boundary layer, and initiating new moist convection over the Sahara.

Satellite observations from the Spinning Enhanced Visible and Infra-Red Imager (SEVIRI) and the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) combined with selected West African surface station observations (thermodynamics and visibility) have been integrated to study the gust front and its associated dust activity in the period of August 3-6, 2006. The meteorological conditions accompanying this event have been described using the European Centre for Medium-range Weather Forecasts (ECMWF) analyses.

The gust front was initiated by a cluster of isolated cumulonimbus clouds over central Niger at 1400 UT on August 3 that lengthened to MCS size over Mali by the end of the day. At maximum expansion on August 5, the extending gust front exceeded 1500 km in length, with a transited area of lofted dust reaching a million square kilometers, mostly over southern Algeria and northern Mali. The aerosol optical depth (AOD) associated with the dust cloud was in the order of 1.5, as evaluated from the Moderate Resolution Imaging Spectroradiometer (MODIS) /AQUA Deep Blue Collection 005 over desertic surfaces. The dust load in the cold pool was estimated to 1.5 Tg. The northward gust front speed, estimated with SEVERI imagery, is rapid in initial stages (20 ms-1) but declines with time (10 ms-1) as the cold air absorbs heat from the hot desert surface and the gust front density contrast is diluted.

Young dusty gust fronts (i.e. during the first 24 hours of the event) were characterized by lidar reflectivity at 532 nm in excess of 3 x 10-3 km-1 sr-1, temperature drops exceeding 10°C, 1 km visibility and their associated dense dust cloud reached 2 km in altitude. Older dusty gust fronts (i.e. from August 5 on) were associated with weaker lidar reflectivities (below 3 x 10-3 km-1 sr-1), similar visibility conditions (~1km), temperature drops at the surface of about 5°C and their associated dust clouds reached higher altitudes (3-4 km).

The pronounced northward propagation of the dusty gust front and its associated moisture were favored by the presence of an active African Easterly Wave. Further work with CALIPSO and SEVIRI is aimed at determining the fate of the large area of dust raised by events of this kind.