Characterizing rain drop size distributions in supercell hook echoes: results from VORTEX2

Knowledge of rain drop size distributions (DSDs) in severe convective storms has been limited due to the difficulty and low coverage of reliable in situ measurements. The form and parameters of the rain DSD have been shown in many recent modeling studies to strongly impact the behavior of these storms, particularly in the strength and evolution of the cold pool. These impacts in turn have important implications for substorm-scale phenomena, including tornadoes in supercell thunderstorms. In this study, we present results of the comparison of several in situ disdrometer measurements in the hook echo appendages of a subset of supercell thunderstorms intercepted during VORTEX2 (2009-2010), with a goal of comparing with radar observations. The emphasis will be on the dataset collected in the 9 June 2009 Greensburg, KS weakly tornadic supercell. Parameters of the widely used exponential and gamma distributions are derived and compared with common assumptions used in cloud-resolving model microphysics parameterizations. Preliminary results indicate good agreement between disdrometer-derived radar reflectivity and observed S-band radar reflectivity, when the wind speeds at the disdrometer site are relatively low. Derived magnitudes of the intercept parameter [O(10⁶ m^-4)] for the assumed exponential DSD are consistent with previous measurements in warm-season Plains convection, but it is also shown that the DSD varies widely in time and space within the hook echo environment, providing many challenges for future field programs, microphysics parameterizations, and numerical simulations.