Analytical Approach to Critical Diameters in Raindrop Size Distribution in Durban, South Africa

Adequate information of the raindrop size distribution is very significant for the prediction and evaluation of attenuation signal due to rain. In this study, an analytical approach is adopted to determine the peak diameter  where the specific rain attenuation is maxima in Durban (29º52'S, 30º58'E), South Africa; using the spherical raindrop shape at temperature T = 20ºC. The overall rainfall attenuation is computed by integrating over all the drop sizes and determine the differential change in the attenuation as observed over a fixed diameter interval, (= 0.1 mm). The critical diameters are the range of diameters where the rain attenuation is highly predominant, which constitutes the surface area under the curve and along the abscissa regions. The critical diameters are seen to coalesce around the peak diameter, at which the maximum attenuation occurs. The maximum specific rain attenuation peaks at the diameter It was observed that the peak diameter is frequency dependent while the parameters, µ, the mean and σ the standard deviation which determines the width of the distribution are found to be region-dependent. The peak attenuation for the stratiform rainfall type varies between 0.8 ≤ D ≤ 1.5 mm whereas for the convective rainfall, the specific rain attenuation peaks between 1.4 ≤ D ≤ 2.7 mm at all frequencies. A proper knowledge of the rainfall attenuation characteristics is useful for proper planning and for the purpose of link budget analysis by operators in this particular region.