Search

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[sup -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 m[sub R] , the mean climatic value of R (around 5–6 mm h[sup -1] ). Because most of the sub-Saharan West African sites have a similar m[sub R] , 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. [ABSTRACT FROM AUTHOR]

Because of a lack of data, the structural characteristics of rain fields over the sea are poorly documented. Coastal radars offer an opportunity to observe the distribution of rain parameters at the land–sea transition. In this study, two datasets on rain fields collected over the Atlantic coast: one at midlatitude, in a westerly general atmospheric circulation, that is, onshore, in the southwest of France, the other at tropical latitude, in an easterly general atmospheric circulation, that is, offshore, in the west of Senegal (Africa), are analyzed. In the two areas, the rain volume, or cumulative rainfall, is found to be markedly larger over land than over sea. However this difference is due mainly to a higher rain occurrence and duration over land than over sea. The mean rain rate, when raining, is almost the same over land and over sea at midlatitude and at tropical latitude. In addition, the mean rain rate is found to be constant through rain fields in which strong gradients of cumulative rainfall are observed [when no other kind of forcing (not considered in this work), notably orographic, is present]. Thus, for example, in Senegal a meridional change of the annual cumulative rainfall from 300 to 1200 mm over 400 km is not associated with any mean rain-rate variation. In a similar way, the parameters of the probability density function of the rain rate and the distribution of the statistical variation coefficient are not influenced by the gradient of the cumulative rainfall. The studied rain fields are thus approximately ergodic. These results strengthen the validity of the probability density function of rain rate as a climatic characteristic of rain fields and the reliability of the statistical method of estimating the average rainfall by area integrals over large space scales and timescales. The variation coefficient of the rain rate is found to be constant and close to 2.24 over sea and over land for mid- and tropical latitudes. [ABSTRACT FROM AUTHOR]