Drop-Size Distribution Characteristics in Tropical Mesoscale Convective Systems

Abstract

Drop-size distribution characteristics were retrieved in eight tropical mesoscale convective systems (MCS) using a dual-frequency (UHF and VHF) wind profiler technique. The MCSs occurred near Darwin, Australia, during the 1993/94 wet season and were representative of the monsoon (oceanic) regime. The retrieved drop-size parameters were compared with corresponding rain gauge and disdrometer data, and it was found that there was good agreement between the measurements, lending credence to the profiler retrievals of drop-size distribution parameters. The profiler data for each MCS were partitioned into a three-tier classification scheme (i.e., convective, mixed convective?stratiform, and stratiform) based on a modified version of Williams et al to isolate the salient microphysical characteristics in different precipitation types. The resulting analysis allowed for an examination of the drop-size distribution parameters in each category for a height range of about 2.1 km in each MCS. In general, the distributions of all of the retrieved parameters showed the most variability in convection and the least in stratiform, with the mixed convective?stratiform category usually displaying intermediate characteristics. Although there was significant overlap in the range of many of the parameter distributions, the mean profiles were distinct. In the stratiform region, there was minimal vertical structure for all of the drop-size distribution parameters. This result suggests an equilibrium between depletion (e.g., evaporation) and growth (e.g., coalescence) over the height range examined. In contrast, the convective parameter distributions showed a more complicated structure, probably as a consequence of the complex microphysical processes occurring in the convective precipitation category. Reflectivity?rainfall (Z?R) relations of the form Z = ARB were developed for each precipitation category as a function of height using linear regressions to the profiler retrievals of R and Z in log space. Similar to findings from previous studies, the rainfall decreased for a given reflectivity as the precipitation type changed from convective to stratiform. This result primarily was due to the fact that the coefficient A in the best-fit stratiform Z?R was approximately a factor of 2 greater than the convective A at all heights. The coefficient A generally increased downward with height in each category; the exponent B showed a small decrease (stratiform), almost no change (convective), or a slight increase (mixed convective?stratiform). Consequently, the amount by which convective rain rate exceeded stratiform (for a given reflectivity) varied significantly as a function of height, ranging from about 15% to over 80%.