Distribution of Rainfall Parameters near the Coasts of France and Senegal

Abstract

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.