Drop-size distribution characteristics in tropical mesoscale convective systems

Drop-size distribution characteristics were retrieved in eight tropical mes oscale convective systems (MCS) using a dual-frequency (UHF and VHF) wind p rofiler technique. The MCSs occurred near Darwin, Australia, during the 199 3/94 wet season and were representative of the monsoon (oceanic) regime. Th e retrieved drop-size parameters were compared with corresponding rain gaug e and disdrometer data, and it was found that there was good agreement betw een the measurements, lending credence to the profiler retrievals of drop-s ize distribution parameters. The profiler data for each MCS were partitione d into a three-tier classification scheme (i.e., convective, mixed convecti ve-stratiform, and stratiform) based on a modified version of Williams et a l. to isolate the salient microphysical characteristics in different precip itation types. The resulting analysis allowed for an examination of the dro p-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 characteris tics. Although there was significant overlap in the range of many of the pa rameter distributions, the mean profiles were distinct. In the stratiform r egion, there was minimal vertical structure for all of the drop-size distri bution parameters. This result suggests an equilibrium between depletion (e .g., evaporation) and growth (e.g., coalescence) over the height range exam ined. In contrast, the convective parameter distributions showed a more com plicated 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 = AR(B) were developed for each precipitation category as a function of height using linear regres sions to the profiler retrievals of R and Z in log space. Similar to findin gs from previous studies, the rainfall decreased for a given reflectivity a s the precipitation type changed from convective to stratiform. This result primarily was due to the fact that the coefficient A in the best-fit strat iform ZR was approximately a factor of 2 greater than the convective A at a ll heights. The coefficient A generally increased downward with height in e ach category; the exponent B showed a small decrease (stratiform), almost n o 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, ra nging from about 15% to over 80%.