A note on: Estimation of rainfall due to squall lines over West Africa using meteosat imagery

Abstract

The results of the first step of a project to develop a method to estimate precipitation over the Soudano-Sahelian belt of West Africa are reported. The study has been performed over the period from 10 June to 9 July 1986 using hourly METEOSAT infrared images. 122 individual cloud clusters associated with squall lines or tropical storms have been tracked. For each event, the time variations of a convection index giving the volume of cloud cooler than −40°C has been determined every hour. The convection index exhibits a strong diurnal cycle. From daily rainfall amounts obtained at about 300 stations, and assuming a time apportion of rainfall within a cloud cluster, the time variations of the hourly total rainfall produced by the cluster can be determined and represents the precipitation index. Because of insufficient rainfall, the precipitation index has been determined for only 17 events. For 2/3 of the 17 cases, there is a significant correlation between the two indices. For each of the 17 events, precipitation has been regressed on the associated convective index and relative time variations. In that case, the results indicate that a convective index representing the life history of the cloud cluster can be calibrated with corresponding raingage measurements provided raingage data are available. Then, estimation of rainfall due to that event over data void regions can be obtained. However, this study shows that no universal relationship exists between precipitation: no rainfall can be estimated if there is not enough raingage measurements to construct a precipitation index for a tracked cloud cluster. This represents a limitation to the method.