Characteristics of the Raindrop Size Distribution in Tropical Continental Squall Lines Observed in Darwin, Australia

Abstract

Disdrometer data measured during the passage of tropical continental squall lines in Darwin, Australia, are analyzed to study characteristics of raindrop size distribution (DSD). Fifteen continental squall lines were selected for the DSD analysis. An observed squall line was partitioned into three regions based on radar reflectivity pattern, namely, convective line, stratiform, and reflectivity trough. A convective line was further partitioned into the convective center, leading edge, and trailing edge using a threshold rain rate of 20 mm h?1. Statistics of modified gamma DSD parameters obtained by a least squares fitting method show distinct differences between the convective-center and the stratiform regions; the shape of DSD for the convective center is convex upward, but it is more exponential for the stratiform region; the intercept parameter N0 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 show distinct differences between the convective-center and the stratiform regions; the characteristics of the trailing-edge and reflectivity-trough regions are equivalent to those of the convective center. DSD changes associated with the rainwater content variations are calculated using the obtained normalized gamma DSD function and the observed D0?M relationship. The simulation demonstrates that the stratiform region is characterized by a larger drop spectrum (i.e., the maximum drop diameter and the median volume diameter are larger for the stratiform region than the convective center and the reflectivity trough for DSD with the same rainwater content). The Waldvogel ?N0 jump? is clearly shown, and the large drop spectrum for the stratiform region suggests the importance of the aggregation mechanism above the melting level in the stratiform region. The difference in the DSD for the convective-center and the stratiform regions causes systematic differences in Z?R relationships (Z = ARb). A larger value for coefficient A in the stratiform region is found, but values of A and b change case by case; an inverse relationship between A and b (A = 103.22b?6.25) is found for rainfall in the convective-center and the trailing-edge regions.