A Tropical Squall Line Observed during the COPT 81 Experiment in West Africa. Part 1: Kinematic Structure Inferred from Dual-Doppler Radar Data

Abstract

This paper deals with the analysis of a tropical squall line, observed on 22 June 1981 during the COPT 81 (Convection Profonde Tropicale) experiment. The present Part I is restricted to the study of the kinematic structure of the system, which moved in a moderately unstable atmosphere, faster than the environmental air at all levels. At the observation time, the squall line was in a mature stage. The explicit description of the airflow within the system was inferred from dual- and single-Doppler radar data. The general characteristics of the squall line are found to be very similar to those of tropical squall lines observed during previous experiments such as VHIMEX or GATE: a large cloud system composed of an organized convective line ahead of an extensive trailing anvil cloud (stratiform rain), fast motion and long-lasting structure and a well-marked gust front signature at ground level. The overall airflow presents a three-dimensional structure. At the leading edge, convective-scale updrafts are fed by converging warm boundary layer air which is subsequently transferred rearward into the trailing anvil. Convective-scale downdrafts fed by midtropospheric air participate in the formation of a cold rear-to-front flow normal to the squall line. This flow strongly opposes the front-to-rear flow associated with warm air. A mesoscale updraft within the trailing anvil cloud and mesoscale downdraft below the anvil cloud are fed by midtropospheric convergence. This is consistent with the results of previous studies. However, detailed aspects of the three-dimensional wind field revel significant differences. The rear-to-front flow is observed in a deep layer 3 km thick. From mass transport estimation, it is found that convective and mesoscale downdrafts accounted for 40 and 60% of the deep rear-to-front flow, respectively. Through this mechanism, the mesoscale downdraft plays an important role in the generation of convective updrafts along the leading gust front. In fact, part of the mesoscale downdraft air spreads forward and overlies the outflow generated in the surface layer by the convective-scale downdrafts. This results from processes occurring in the trailing anvil. Indeed, relatively large mesoscale updraft and downdraft characterize this trailing part of the system which exhibits two regions of differentiated kinematic structure: the reflectivity trough (associated with weak precipitation) immediately behind the convective region, and the trailing stratiform region. The mesoscale downdraft in the reflectivity trough is observed in a deep layer from the surface to about 6 km altitude, while it occurs below the anvil-base level (?4 km) in the stratiform region.