Systematic variation of drop size and radar-rainfall relations

Time histories of the characteristics of the drop size distribution of surf ace disdrometer measurements collected at Kapingamarangi Atoll were partiti oned for several storms using rain rate R, reflectivity factor Z, and media n diameter of the distribution of water content D-0. This partitioning prod uced physically based systematic variations of the drop size distribution ( DSD) and Z-R relations in accord with the precipitation types view-ed simul taneously by a collocated radar wind profiler. These variations encompass t he complete range of scatter around the mean Z-R relations previously repor ted by Tokay, and Short [1996] for convective and stratiform rain and demon strate that the scatter is not random. The systematic time or space variati ons are also consistent with the structure of mesoscale convective complexe s with a sequence of convective, transition, and stratiform rain described by various authors. There is a distinct inverse relation between the coeffi cient A and the exponent of the Z-R relations which has been obscured in pr ior work because of the lack of proper discrimination of the rain types. Co ntrary to previous practice it is evident that there is also a distinct dif ference in the DSD and the Z-R relations between the initial convective and the trailing transition zones. The previously reported Z-R relation for co nvective rain is primarily representative of the transition rain that was i ncluded in the convective class. The failure of present algorithms to disti nguish between the initial convective and the trailing transition rains cau ses an erroneous apportionment of the diabatic heating and cooling and defe ats the primary intent of discriminating stratiform from convective rains.