MEASUREMENT OF APPROXIMATE RAINDROP SIZE BY MICROWAVE ATTENUATION

Abstract

It is not possible to deduce the size of raindrops from the average intensity of the radar signal produced by the precipitation region. However, if it is assumed that all the drops are of the same size, a relation between the diameter of the drops and the number per unit volume may be obtained. When measurable relative attenuation between radar waves of different lengths occurs, a different relation between drop size and concentration may be derived therefrom. The combination of these two types of measurement yields a single solution for the size and concentration of the drops. The solution can be considered as an approximation to the size of the largest drops present in the precipitation region, since the smaller drops contribute only slightly to the radar echo. Measurable attenuation of 3-cm waves may be expected to occur under two conditions: (a) heavy rain with large drops, and (b) moderately heavy rain with fairly uniform characteristics over a large area. Measurable attenuation of l-cm waves can be expected even with light rain.