An Analysis of the Environments of Intense Convective Systems in West Africa in 2003

Abstract

Abstract The local- and regional-scale environments associated with intense convective systems in West Africa during 2003 were diagnosed from soundings, operational analysis, and space-based datasets. Convective system cases were identified from the Tropical Rainfall Measuring Mission (TRMM) microwave imagery and classified by the system minimum 85-GHz brightness temperature and the estimated elapsed time of propagation from terrain greater than 500 m. The speed of the midlevel jet, the magnitude of the low-level shear, and the surface equivalent potential temperature θe were greater for the intense cases compared to the nonintense cases, although the differences between the means tended to be small: less than 3 K for surface θe and less than 2 × 10−3 s−1 for low-level wind shear. Hypothesis testing of a series of commonly used intensity prediction metrics resulted in significant results only for low-level metrics such as convective available potential energy and not for any of the mid- or upper-level metrics such as the 700-hPa θe. None of the environmental variables or intensity metrics by themselves or in combination appeared to be reliable direct predictors of intensity. In the regional-scale analysis, the majority of intense convective systems occurred in the surface baroclinic zone where surface θe exceeded 344 K and the 700-hPa zonal wind speeds were less than −6 m s−1. Fewer intense cases compared to nonintense cases were associated with African easterly wave troughs. Fewer than 25% of these cases occurred in environments with detectable Saharan dust loads, and the results for intense and nonintense cases were similar. Although the discrimination between the intense and nonintense environments was narrow, the results were robust and consistent with the seasonal movement of the West African monsoon, regional differences in topography, and African easterly wave energetics.