A preliminary survey of DSD measurements collected during VORTEX2

A key understanding drawn from the VORTEX campaign was the sensitivity of supercell behavior, and in particular tornadic potential, to low-level thermodynamic characteristics of the storm's cold pool. Specifically, among storms with low-level mesocyclones significant tornadic activity was more likely when the thermodynamic properties of the storm's cold pools had relatively small departures from the background low-level environment. Unfortunately, data available from the VORTEX experiment were not adequate to fully address downdraft forcing mechanisms and the thermodynamic patterns of the cold pools associated with those downdrafts. A significant element of uncertainty was in the impact of water loading and drop size distribution (DSD) variability on surface cold pool characteristics.

As a component of the VORTEX2 data collection effort, in situ DSD measurements were made using a small set of low-cost laser disdrometers mounted to rapidly deployable weather stations. Numerous samples, particularly within hook appendage regions of supercell storms, were collected during the 2009 and 2010 seasons. Statistics from collected observations will be presented and compared with common microphysical model representations of near-surface precipitation with an eye toward representativeness, and with a view toward characterizing the range of DSD variations across the supercell spectrum. Results will also be discussed in the context of reconstruction of observed VORTEX2 storm events through data assimilation methods with appropriate representation of thermodynamic tendencies within rainy downdrafts.