Characteristics of the raindrop size distribution in tropical continental squall lines observed in Darwin, Australia

Disdrometer data measured during the passage of tropical continental squall lines in Darwin, Australia, are analyzed to study characteristics of raind rop size distribution (DSD). Fifteen continental squall lines were selected for the DSD analysis. An observed squall line was partitioned into three r egions based on radar reflectivity pattern, namely, convective line, strati form, and reflectivity trough. A convective line was further partitioned in to the convective center, leading edge, and trailing edge using a threshold rain rate of 20 mm h(-1). Statistics of modified gamma DSD parameters obta ined by a least squares fitting method show distinct differences between th e convective-center and the stratiform regions; the shape of DSD for the co nvective center is convex upward, but it is more exponential for the strati form region; the intercept parameter N-0 of the modified gamma function for the convective center and the reflectivity trough tends to be larger than that for the stratiform region, also. The observed drop size distributions are normalized to remove the effect of differences in rainfall rate. Gamma distributions then are least squares fitted to the normalized DSD data to s how distinct differences between the convective-center and the stratiform r egions; the characteristics of the trailing-edge and reflectivity-trough re gions are equivalent to those of the convective center. DSD changes associa ted with the rainwater content variations are calculated using the obtained normalized gamma DSD function and the observed D-0-M relationship. The sim ulation demonstrates that the stratiform region is characterized by a large r drop spectrum (i.e., the maximum drop diameter and the median volume diam eter are larger for the stratiform region than the convective center and th e reflectivity trough for DSD with the same rainwater content). The Waldvog el "N-0 jump'' is clearly shown, and the large drop spectrum for the strati form region suggests the importance of the aggregation mechanism above the melting level in the stratiform region. The difference in the DSD for the c onvective-center and the stratiform regions causes systematic differences i n Z-R relationships (Z = AR(b)). A larger value for coefficient A in the st ratiform region is found, but values of A and b change case by case; an inv erse relationship between A and b (A = 10(3.22) b (-6.25)) is found for rai nfall in the convective-center and the trailing-edge regions.