Modeling of raindrop size distributions from multiwavelength rain attenuation measurements

Abstract

Techniques for modeling the distribution of raindrop sizes, in terms of the lognormal and modified gamma distribution, from multiwavelength rain attenuation measurements at millimeter and infrared wave bands have been demonstrated. In order to obtain the three-parameter lognormal distribution, three experimental measurements are required, which can be attenuations at two frequencies and rain rate. Three measurements are used to form three equations, which are treated as nonlinear and are solved using a numerical technique to obtain the distribution parameters. The gamma distribution is treated as a two-parameter distribution, taking a fixed value of 2 for the index. The two parameters of the gamma distribution are obtained from measurements of rain rate and infrared attenuation following the technique described by Maitra and Gibbins [1995]. From an analysis of two rain events it is found that the measured attenuations at millimeter wavelengths show somewhat better agreement with the calculated attenuations obtained from the lognormal distribution than from the modified gamma distribution. The gamma model gives higher number densities for small drops than those given by the lognormal model.