Attenuation-rain rate power-law relation and critical diameters from drop size distribution measurements in Durban

The role of critical raindrop diameters on the specific rainfall attenuation in Durban (29o52'S, 30o 58'E), South Africa and the power law relations of the specific rainfall attenuation, rain rate and the critical diameters are investigated. The total specific rainfall attenuation is calculated by integrating over all the raindrop sizes and the differential change in the attenuation is determined over a given range of raindrop sizes. The gamma distribution model with the shape parameter of 2 is employed to estimate the parameters required to investigate the drop sizes which produce a major contribution to the total specific rainfall attenuation for the selected rain rate values. Five rain rates: 1.41, 9.91, 14.21, 44.52 and 77.70 mm/hr are selected for the purpose of analysis over the measured raindrop size distribution. For all the rain rate values, the maximum contribution to the rainfall attenuation is created by the raindrop diameters not exceeding 2.0 mm at frequencies of 10-100 GHz in Durban. The rain attenuation increases with increased frequencies and the highest rain rate produces the highest specific rainfall attenuation. The total percentage fraction formed by drops in the diameter range 0.5mm ≤ D ≤ 2.5 mm and 1.0 mm ≤ D ≤ 3.0 mm are found to be most critical for the specific rain attenuation for the stratiform and convective rainfall types. The derived coefficients of the power law relations differ slightly from the ITU-R due to parameters used in the analysis.