On the Generation of African Squall Lines

Abstract

Squall lines (SLs) form an important component of the meteorology of northern Africa, and in particular, contribute substantially to rainfall totals. Their generation requires the existence of a potentially unstable low-level supply of moisture overlain by dry desert air and vertical wind shear beneath the midlevel African easterly jet. The instability may be released (and an SL initiated) by factors such as surface heating, topography, African waves, or surface evaporation. The relative importance of each of these factors and the means by which they impact on SL generation is reviewed. This is followed by a detailed analysis of one month of satellite imagery and surface data for August 1985 over a portion of central northern Africa. The novelty of our study lies in the temporal resolution of the satellite imagery, which with 21 images per day allows the identification of a large number of short-lived SLs (4-h duration or less). On the southern fringes of the Sahara these are likely to contribute significantly to rainfall totals, and so cannot be neglected. The analysis is also entirely objective, an important feature if future studies are to produce a homogeneous SL climatology. Our results show, for the period and area of study, a preference for SLs to generate during the midafternoon, with generation probability also enhanced by above-average low-level westerly flow and by surface features such as the Air Mountains, the Jos Plateau, and the northernmost section of the river Niger. African waves and the strength of the African easterly jet were not found to affect SL generation for the period and region studied. Where these results do not support previous studies, we speculate that this may be due to differences in location or time of year, but only a more extensive analysis will resolve these issues.