Influence of the local environment on 2 June 1995 supercell cloud-to-ground lightning polarity

Radar and cloud-to-ground (CG) lightning characteristics were documented for 17 supercells that occurred across the West Texas Panhandle on 2 June 1995. Most of the supercells that crossed a pre-existing mesoscale outflow boundary increased in updraft speed (inferred from maximum echo top of 40 dBZ reflectivity) and mesocyclone rotation (inferred from the NSSL Mesocyclone Detection Algorithm "strength rank"). Many storms also increased in their production of positive cloud-to-ground lightning, cyclic mesocyclones, and large hail upon crossing the boundary as well. The onset of large positive flash rate occurred coincident in time with the appearance of base-scan radar reflectivity in excess of 60 dBZ (in 8/13 of cases).

As supercells crossed the pre-existing boundary, proximity soundings reveal that they encountered larger CAPE and vertical wind shear. The larger CAPE is likely due to outflow air that had modified throughout the afternoon via latent heat fluxes (driven by 3-5 hours of direct sunlight), before the supercells in question interacted with it.

Sounding analysis and numerical simulations show that the larger environmental CAPE and shear environment is supportive of larger riming accretion rates for hail/graupel at warmer sub-freezing temperatures and over a greater depth of the storm. This would support greater-magnitude positive charging of hail/graupel over a deeper layer. While the vertical electric field profile though these storms is unknown, we believe that the change in riming accretion rate could support a stronger-magnitude and deeper lower positive charge region. Such a charge structure has recently been observed in other supercells dominated by positive CG lightning during the MCS Electrification and Polarization Radar Study (MEaPRS).

These results suggest that monitoring changes in supercell CG lightning flash rate and polarity, 40 dBZ echo top, and mesocyclone strength should be useful to the forecaster in helping to pinpoint the location of boundaries and in the prediction of severe weather. Future work is needed to study whether increases in the positive CG lightning flash rate is typical for other days when supercells cross pre-existing mesoscale boundaries characterized by enhanced CAPE and vertical wind shear. We hypothesize that storms that cross relatively "fresh" outflow boundaries (characterized by smaller CAPE and cooler/ drier air) may not experience enhanced riming accretion rate and an increase in positive flashes.