Regime Analysis of Critical Raindrop Diameters for Rainfall Attenuation in Southern Africa

Abstract

IntroductionRain has been identified as one of the major and important parameters affecting the propagation of signals in the microwave (3-30 GHz) and millimeter (30-300 GHz) wave bands. Other rain factors such as the canting angle, drop size and raindrop shapes also have profound effects on waves propagating in these bands at extremely high frequencies [1]. The specific rainfall attenuation is often predicted from three parameters, which are; the frequency, rain rate and polarization, where the population of the raindrops is represented by the single parameter, rainfall rate [2]. A good knowledge of the drop size distribution (DSD) is very essential in the estimation of the rainfall attenuation at these radio frequency bands because it governs all the microwave and rainfall integral relations. The modeling of the DSD varies from one climate to another. Drop size distribution modeling in temperate region; characterized by moderate rainfall is well suitable with models such as proposed by Marshall and Palmer [3], Laws and Parsons [4] and the negative exponential model of gamma [5].The modeling of DSD in the tropical region is most suitable with the globally accepted Ajayi and Olsen [6] lognormal model. In Durban, South Africa, a reasonable number of works have been carried out on rainfall attenuation and DSD [7][8][9][10][11][12] establishing the suitability of the lognormal and gamma models for DSD modeling in the region. Similarly, various approaches and models have been adopted by some researchers across the globe to investigate the particular contributions of certain raindrop diameters to the specific rain attenuation [13][14][15][16][17]. The influence of critical raindrop diameters on the specific rainfall attenuation in Durban, South Africa using various rainfall regimes is analyzed in this work. Different rain rate values representing drizzle (below 5mm/h), widespread (5-20 mm/h) shower (20-50 mm/h) and thunderstorm (above 50 mm/h) as classified according to [6,18] are selected for the purpose of analysis over the measured raindrop size distribution. The three-parameter lognormal and gamma DSD models with shape parameter of 2 as determined in [12] for Durban, South Africa are used to represent the measured DSD, N(D).