Content area
Full text
ABSTRACT
The West African squall line is a key quasi-linear storm system that brings much of the precipitation observed in the data-poor Sudanian climate zone. Squall lines propagate at a wide range of speeds and headings, but the lack of operational radar stations in the region makes quantifying the propagation of the squall lines difficult. A new method of estimating the propagation rate and heading for squall lines is proposed. Based on measurements of the time of onset of precipitation (OOP) at a network of rain gauge stations, an estimate of the propagation characteristics of the squall line can be inferred. By combining estimates of propagation rate with upper-air observations gathered at a nearby radiosonde station, the impact of various environmental factors on the propagation characteristics of West African squall lines is inferred. Results suggest that the propagation speed for West African squall lines is related to the conditions at midtropospheric levels, where dry air and an enhanced easterly flow favor faster propagation. Northerly anomalies at these levels are also associated with faster propagation. When applied to West African squall lines, the correlations between these environmental factors and the speed of propagation are significantly higher than those of methods developed for mesoscale convective systems in other parts of the world.
(ProQuest: ... denotes formulae omitted.)
1. Introduction
Westward-propagating squall lines produce much of the observed precipitation in sub-Saharan West Africa. Their contribution to the total rainfall varies significantly with respect to latitude, accounting for as much as 80%-90% of the annual rainfall in the Sahelian climate zone (12°-18°N) (Dhonneur 1981; Mathon et al. 2002a). In the wetter climate zones to the south, squall lines account for about half of the rainfall in the Sudanian climate zone (9°-12°N) (Eldridge 1957; Omotosho 1985) and as little as 16%-32% along the Guinea coast (Acheampong 1982; Omotosho 1985).
Hamilton and Archbold (1945) presented a compelling, early description of these systems. They stated that West African squall lines propagate to the westsouthwest at speeds of 25-30 mi h^sup -1^ (11.2-13.4 m s^sup -1^). In that study, the severe aspects of squall lines were emphasized, especially the heavy rainfall (often 60 mm h^sup -1^), reduced visibility (typically 100-200 m), frequent lightning, and strong leading gust front (with winds often in...