Kinematics, microphysics, and lightning during STEPS: A summary of results

A summary of radar and lightning observations from four storms observed during the Severe Thunderstorm Electrification and Precipitation Study (STEPS) field campaign will be presented. The four cases include a negative CG-producing multicellular storm on 19 June 2000, a supercell on 3 June 2000 that produced no CG lightning of either polarity, a predominantly positive CG-producing (PPCG) supercell on 29 June 2000, and a PPCG multicellular storm on 22 June 2000. Data from multiple Doppler radars have been synthesized to calculate three-dimensional wind fields, polarimetric radar variables have been combined with thermodynamic soundings to estimate hydrometeor types throughout the echo volumes, and Lightning Mapping Array (LMA) data have been sorted into flashes and studied to determine the flash rates and charge structure for several hours of each storm's lifetime. The purpose of this study is to determine what features are unique for storms that produce PPCG lightning, and attempt to reveal the processes that lead to this behavior.

The results indicate that PPCG storms tend to have larger updrafts (both wider and larger in volume), which is consistent with previous studies. Large updrafts and enhanced vertical vorticity also play an important role in the production of large hail. Furthermore, low-level negative charge (below a larger region of positive charge) was observed in the cases that produced positive CG lightning, which may be the impetus needed for the flash to come to ground. This lower negative charge, in essence, represents the lowest charge layer of an inverted tripolar charge structure. The charge structures observed during the production of negative CG lightning were a normal tripole (with negative charge situated between upper and lower positive charge layers) on 19 June and an inverted dipole (with negative charge above positive) in the anvil on 22 June. Cloud-to-ground flash rates (of either polarity) decreased when either the lower charge layer of the corresponding tripolar structure was absent, or when the low-level charge layer exhibited an enhanced number of LMA sources, in which case intra-cloud (IC) discharges seemed to be preferred between the two lowest charge layers of the tripole.