Interaction between microphysics and dynamics in the West African squall lines.

Abstract

The rainfall variability due to squall lines directly affects West African populations. The African Monsoon Multidisciplinary Analyses (AMMA) campaign was designed in order to get a better knowledge of this phenomenon and was therefore based on a large field experiment in 2006. In this context the radar Ronsard was deployed. allowing us to retrieve wind fields and polarimetric classifications. The main interest of this study is to combine the use of observations, polarimetric classification from Ronsard and modeling using a microphysical retrieval model. This model was originally developed for the European fronts, was adapted to tropical conditions, and squall line's intensity. The microphysical model allowed us to perform retrievals that are in agreement with Ronsard observations. For a realistic restitution of convective and stratiform part of a squall line, we showes that at least two species of precipitating ice (graupel and snow) are necessary. We highlighted the competition between water vapor deposition on the precipatating ice and condensation of water vapor into liquid cloud droplets in forming precipitating ice, and the role of these in the formation of rain. The microphysical model has allowed us to identify important processes in the formation of precipitation, especially the condensation of water vapor into liquid cloud, melting, riming and the accretion of liquid cloud by rain. The use of a diagnostic microphysics microphysics and of a radiative transfer model enabled to link the ice content to the brightness temperatures, but also to establish a relationships between microphysical processes, brightness temperatures and integrated water content.