The West African Squall Line Observed on 23 June 1981 during COPT 81: Kinematics and Thermodynamics of the Convective Region

Abstract

The squall line observed during the night of 23-24 June 1981 was one of the most intense events during the COPT 81 (COnvection Profonde Tropicale) experiment, conducted in May and June 1981 at Korhogo in the northern Ivory Coast by French and Ivorian research institutes. The presquall environment possessed a very large convective instability with a stable layer in the lowest levels. The presence of dry air in the midlevels, which promoted the development of convective downdrafts, and the unusually low altitude of the strong easterly winds, which aided the development of a leading anvil above 6 km, are two noteworthy characteristics of the environment of this squall line. Its mesoscale structure, as deducted from radar data, is similar to that previously observed for tropical squall lines. Convective updrafts and downdraft occur in the leading heavy precipitation region (convective region) and a mesoscale updraft and downdraft is found within and below the trailing mid-to upper-level anvil clouds, respectively (stratiform region). The analysis of dual-Doppler radar data shows that the convective region was composed of short-lived cells characterized by intense updrafts and high reflectivity values, with convective downdrafts between and behind the cells. An improved method to retrieve thermodynamic fields from the radar data documents a Cold low-level frontward flow, inducing a dynamic pressure high in front of the line, which in turn forces the initial lifting of the inflowing air in the low levels. Positive temperature perturbations at midlevels and the hydrostatic pressure low beneath the convective region result from the large convective instability of the entering monsoon flow. Comparisons of vertical fluxes of horizontal momentum and thermodynamic properties in the convective region and vertical gradients of the environmental values show that, owing to the internal sources/sinks and two-dimensionality of the line, the component of momentum normal to the line and the “virtual-cloud” potential temperature are transported against their vertical gradients, while the parallel component of momentum is transported down its vertical gradient. Horizontal exchange rates of mass, momentum and sensible heat (also deduced from the radar data) illustrate the important role of the convective cells in modifying the kinematic and thermodynamic structure of the middle and upper troposphere.