Systematic variation of drop size and radar‐rainfall relations

Abstract

Abstract. Time histories of the characteristics of the drop size distribution of surface disdrometer measurements collected at Kapingamarangi Atoll were partitioned for several storms using rain rate R, reflectivity factor Z, and median diameter of the distribution of water content D 0. This partitioning produced physically based systematic variations of the drop size distribution (DSD) and Z-R relations in accord with the precipitation types viewed simultaneously by a collocated radar wind profiler. These variations encompass the complete range of scatter around the mean Z-R relations previously reported by Tokay and Short [ 1996] for convective and stratiform rain and demonstrate that the scatter is not random. The systematic time or space variations are also consistent with the structure of mesoscale convective complexes with a sequence of convective, transition, and stratiforrn rain described by various authors. There is a distinct inverse relation between the coefficient A and the exponent of the Z-R relations which has been obscured in prior work because of the lack of proper discrimination of the rain types. Contrary to previous practice it is evident that there is also a distinct difference in the DSD and the Z-R relations between the initial convective and the trailing transition zones. The previously reported Z-R relation for convective rain is primarily representative of the transition rain that was included in the convective class. The failure of present algorithms to distinguish between the initial convective and the trailing transition rains causes an erroneous apportionment of the diabatic heating and cooling and defeats the primary intent of discriminating stratiform from convective rains.

Tropical Rain Classification and Estimation of Rain From Z-R (Reflectivity-Rain Rate) Relationships

“…Battan [3] presented a list with 69 different Z-R relationships for different climatic conditions in different parts of the world. There can be dramatic changes in Z-R law parameters within an individual storm as well as between storms [6][7][8][9][10]. These changes are clearly identified with the physical processes acting to form the rain event.…”

“…Tokay and Short [6] observed a significant change in the gamma parameter, the intercept parameter, N 0 , during the transition from convective to stratiform rain. Later, many researchers reported the existence of a transition region between the convective and stratiform regimes [7][8][9][10]. Bringi et al [11] used a simple scheme to separate stratiform and convective rain types based on the standard deviation of rain rate over 5 consecutive DSD samples.…”

“…Many studies have demonstrated that stratiform rain is characterized by larger raindrop diameters relative to convective type rain for the same liquid water content [6,7]. In recent years [6][7][8][9], variations in the gamma DSD parameters are used for rain type classification.…”

“…These changes are clearly identified with the physical processes acting to form the rain event. In the past, research work was done to improve the accuracy of Z-R relations by classifying the rain into different types [6][7][8][9][10].…”

“…In recent years [6][7][8][9], variations in the gamma DSD parameters are used for rain type classification. Tokay and Short [6] observed a significant change in the gamma parameter, the intercept parameter, N 0 , during the transition from convective to stratiform rain.…”