Northward bursts of the West African monsoon leading to rainfall over the Hoggar Massif, Algeria

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Abstract
The Hoggar Massif is a comparatively populated region in the Sahara, where water supply is a critical problem due to the lack of nearby sources and unaffordable water pumping. In the present paper, we analyse the influence of the West African monsoon (WAM) on precipitation over the Hoggar during summer. We investigate (1) two rainfall events during 23–27 July 2006, accounting for almost half of the precipitation of this year, and (2) the representativity of this period with regard to ERA‐40 ECMWF re‐analyses (1979 to 2001).By the end of July 2006, two consecutive northward bursts of the WAM flow reached the Hoggar and caused an increase in low‐level humidity. In the afternoon of these days, clouds formed at the top of the convective boundary layer and rapidly grew to more than ∼9 km above mean sea level. Due to the comparatively moist sub‐cloud layer (>45% mean relative humidity), considerable amounts of precipitation could reach the ground. The strong southerly WAM flow one day before and during the arrival of the moist air over the Hoggar is associated with both a south sector of an African easterly wave (AEW) and the cyclonic circulation around the south‐eastern flank of the Saharan heat low (SHL). In addition, the northward excursions of the monsoon were supported by convective cold‐pool outflows originating over Niger. The climatological analysis confirms the relation between precipitation over the Hoggar and AEW south sectors and also shows a conspicuous weakening of the SHL following rainfall. Copyright © 2009 Royal Meteorological Society

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