Confirmation of the Existence of Super-Terminal Raindrops

Abstract:

Impact disdrometers generally assume that raindrops fall at their terminal velocities. Recently, observations have been made that suggest that a substantial fraction of sub-millimeter raindrops fall at speeds exceeding their expected terminal velocities. The prevalence of these super-terminal drops and their microphysical importance (if any) is still uncertain.

Here, the existence of such super-terminal velocities for sub-millimeter drops is confirmed with the use of 22 optical disdrometers of two different types (21 laser precipitation monitors and a single 2-dimensional video disdrometer). A substantial fraction of sub-millimeter drops are seen to exceed their theoretical terminal fall velocities by at least 30%. Super-terminal fractions seem to be reasonably spatially uniform within a single rain event, but can vary substantially from storm to storm. Analysis of the 2-dimensional video disdrometer data convincingly demonstrates that the detected super-terminal drops are not due to drop fragmentation on the detector.