Raindrop Size Distribution and Sampling Size Errors

Abstract The standard deviation of the rain intensity R and the reflectivity factor Z are derived theoretically for R and Z values which are calculated from measured distributions of raindrop size. The derivation is based on the assumption that the distribution of raindrop size follows a negative exponential law. The Poisson distribution is assumed for the number of the counted drops with diameters between D and D+dD. We found that a large sample is necessary to get a good estimate of R and Z; for example, in a widespread rain with a rainfall rate of 1 mm hr−1, a filter paper with an area of ˜1 m2 must be exposed during 1 sec to obtain, with a probability of 68%, a Z value which deviates less than 20% from the mean.