Drop Shapes and Fall Speeds in Rain: Two Contrasting Examples

Abstract

AbstractTwo rain events are analyzed using two collocated 2D-video disdrometers (2DVD) and a C-band polarimetric radar at 15-km distance. Both events had moderate-to-intense rainfall rates, but the second event had an embedded convective line. For the first event, the fall speed distribution for a given drop diameter interval showed a narrow and symmetric distribution with a mode at the expected value; the second event produced a wider distribution with a significant skewness toward lower fall speeds. The “slower” drops in the second event were detected while the convective line was directly over the 2DVD site. Drop shape information from the two 2DVD instruments showed that, during the passage of the convection line, around 30%–40% of the drops did not have an axis of rotational symmetry, whereas for event 1, it was only 5%. The implications are that for event 1 the dominant mode of drop oscillation is the axisymmetric mode, and that within the convective line of event 2 other fundamental modes were frequent. The radar data for the second event were analyzed in terms of the self-consistency among the radar-measured quantities. The Kdp/Zh versus Zdr variations within the line convection were not consistent with the corresponding variation determined from the scattering calculations using the measured 1-min drop size distributions and using the “reference” drop shapes. Also found were low ρhv regions within the line convection that were considerably lower than the scattering calculations. These findings are consistent with the asymmetric oscillation modes inferred from the 2DVD measurements for event 2 (probably collision induced) within the convective line.