Raindrop Size Distribution and Radar Parameters at Cape Verde

Abstract

Precipitation measurement using passive or active microwaves from space- or ground-based radar involves hypotheses about the raindrop size distribution (DSD). A universal knowledge of DSD characteristics is needed. A 4-yr dataset collected with a disdrometer at Dakar, Senegal, on the Atlantic coast of West Africa is used to analyze the DSD at the end of the continental trajectory of Sahelian squall lines. The DSDs are stratified in eight rain-rate classes and are fitted to analytical distributions. The shape of the averaged DSDs is found to be very similar from one year to the next. From rain rates R higher than about 20 mm h?1, the slope of the DSDs tends toward a constant value. The coefficients of the Z?R relation, between the radar reflectivity factor Z and R, are different for convective and stratiform parts of the squall lines. However, because the Z?R relations for convective rain intersect the relation for stratiform rain for high rates, it is suggested that using a single Z?R relation enables correct representation of the whole Z and R range of variation in West Africa. The coefficients of this single Z?R are close to that of the stratiform relation and to that observed in other West African areas. The conditional probability density function of rain rate, P(R), is of lognormal shape and also is very stable year after year. The coefficient of variation of R is found to be constant and close to 2.24, the value observed at many other sites. From P(R), the linear coefficient S(τ) of the relation that links the area-averaged rain rate to the fractional area where the rain rate is higher than the threshold τ is computed and is found to be very stable for the values of τ close to mR, the mean climatic value of R (around 5?6 mm h?1). Because most of the sub-Saharan West African sites have a similar mR, comparison shows that S(τ) is homogeneous over this area. This result suggests that S(τ) can be used with confidence for average rainfall estimation over a climatically homogeneous region.