A Comparison of Layered Lightning and Ice Processes in Stratiform Precipitation

This project focused on stratiform lightning and the hydrometeor growth mechanisms that occur within that region of a Mesoscale Convective System (MCS). Three key hypotheses were the basis of this project: 1) ice particles are typically formed by aggregation, 2) charged regions can be formed within clouds by ice particles of various sizes colliding with each other and then seperating via gravitational sedimentation, and 3) lightning preferentially propagates though charged regions of cloud. To investigate the role of stratiform ice microphysical processes on lightning production in MCS's, this project studied inferring hydrometeor types and their growth mechanisms from dual-polarization radar data, and then comparing the placement of stratiform lightning layers with the placement of different hydrometeor types. Three MCS's were chosen based on their proximity to the KHTX radar in Hytop, AL. From those three cases, one flash each was chosen based on where in the stratiform region it occurred and its proximity to the North Alabama Lightning Mapping Array sensors. Radar cross sections of horizontal reflectivity (Z_H), differential reflectivity (Z_DR), and correleation coefficient (CC) were manually and visually analyzed from the melting layer (around 2.5km MSL) to the top of storms, and vertical profiles were made for each variable. Hydrometeor types and their respective growth mechanisms were then inferred using dual-polarization radar. The altitudes of the flash layers were compared to the growth mechanism layers, and any commonalty between the cases was recorded.

Based on the vertical profiles created, ice crystals and deposition typically occurred above 8km MSL, aggregates and aggregation occurred between 4-8km MSL, and rain mixed with wet aggregates occurred between 2.5-4km MSL. Each lightning flash produced two to the three distinct layers. The three flashes chosen produced two lightning layers at 4km, one at 2.5km, one at 6km, one at 7km, and one at 10.5km (all heights in MSL). Based on these findings, it was concluded that the lightning tended to layer either where aggregation was the dominant growth mechanism or in the melting layer of all three cases. Both of these regions contained hydrometeor types of mixed sizes that could have been conducive to charge build up via ice particle collisions or other growth mechanisms, which supports the three hypotheses that this project was based on. Future work could study the polarity of charge regions and if there is a pattern as to where they form within the stratiform region.