Single-Doppler Observations of a West African Squall Line on 27-28 May 1981 during COPT 81: Kinematics, Thermodynamics and Water Budget

Abstract

A large and intense West African squall line has been observed during the night of 27/28 May 1981 at Korhogo (in the north of Ivory Coast) during the COPT 81 experiment. The environmental conditions in which it occurred and its overall structure are slightly different from the previously analyzed COPT 81 cases on 22 and 23–24 June 1981. Unfortunately, in this situation, as only single-Doppler radar data available, it is not possible to obtain detailed information on the small scale structure of the leading convective region. Nevertheless talking advantage of the apparent stationarity of the mesoscale features, a new processing method (referred to as ABCD) has been developed for a combined processing of 12 successive conical scans, which allows deduction of three-dimensional wind and reflectivity fields in a domain of 260 × 60 × 15 km3, with resolutions of 20 km in the horizontal and 500 m in the vertical. The deduced circulation displays classical characteristics of trailing straitform parts of mesoscale convective systems, with a mesoscale updraft above 4 km, a mesoscale downdraft below, and a relative rear-to-front flow of moderate intensity in the midlevels; all these features are slightly more intense than on 22 and 23–24 June 1981. One particularity here is that, because of the propagation of the squall line in a direction slightly different from that of the environmental winds, the velocity component parallel to the line is relatively important. Pressure and temperature fields retrieved from an improved analysis of the momentum equation and of the thermodynamic equation show a warming due to condensation in the mesoscale updraft, cooling due to evaporation below, hydrostatic low pressure in the midlevels and high pressure near the surface. The vertical component of vorticity is weak and correlated with the horizontal divergence, while there is an anticorrelation between the shearing deformation and the horizontal divergence. The cross-line component of vorticity is weak but not negligible, as compared with the along-line component. The baroclinic generation, due to the horizontal buoyancy gradient, is the dominant term in the budget of the along-line component of vorticity, however, the influence of the three-dimensional effects through the shearing-tilting term are ~ important in the leading part of the considered domain. The water budget, obtained through a microphysical mtdcvw in one vertical plane, provides values of the different components relatively similar to those obtained in previous studies, with, however, a ~r condensation rate due to the more important upward motions.