A Tropical Squall Line Observed during the COPT 81 Experiment in West Africa. Part II: Water Budget

Abstract

The relative contributions of the different processes involved in the water budget of the 22 June 1981 tropical squall line are investigated. The kinematic and thermodynamic fields derived from Doppler radar data are used to calculate the sources and sinks of condensate in the convective and trailing stratiform regions of the system. Both regions play an important role in providing the surface rain. Fifty-five to 65% of the total rain precipitated at the surface is convective, while the remaining 35?45% is supplied by the trailing stratiform cloud. This partition corresponds to a precipitation efficiency of 47?57% for the convective region, and 45-57% for the stratiform region. Though these efficiencies are of the same order of magnitude, the sink of water substance in each region (before reaching the surface) is attributable to different processes. In the convective region, low of water substance is mainly due to the transfer of condensate into the trailing anvil cloud. This transfer represents 32% of the condensate formed in the convective updrafts and constitutes an important source (47%) of condensate for the stratiform cloud. In the stratiform region, the evaporation of precipitation which occurs beneath the trailing anvil cloud is predominant: 33% of the total condensate supplied to the cloud is lost in the mesoscale evaporative downdraft. Cooling effects due to this important evaporation, together with those associated with melting, help to maintain the observed mesoscale downdraft. Finally, this study indicates that the transfer of condensate from the convective to the stratiform region and mesoscale evaporation of precipitation may have a direct influence on the apparent feedback between convective-scale and mesoscale airflows.