Preliminary Analysis of Dual-Polarization Signatures Associated with Lightning Cessation in Multicellular Convection

Past studies have attempted to generate lightning cessation algorithms based on radar signatures associated with thunderstorm activity. These studies have resulted in varying degrees of success, focusing primarily on isolated convective events, and none of the algorithms developed can be used in an operational setting. If a successful algorithm is to be developed, foundational knowledge of the dual-polarization radar signatures associated with varying types of precipitation systems before, during and after lightning cessation needs to be developed. This study aims to bridge that gap.

Dual-polarization radar data, gathered from the Hytop, Alabama S-band Weather Surveillance Radar – 1988 Doppler (WSR-88D) radar, are used to infer the kinematic and microphysical properties of multicellular convection as it approaches lightning cessation. Cases are determined by identifying multicellular storms that exhibit a last flash observed by the North Alabama Lightning Mapping Array (NALMA) within 125 km of the Hytop radar. Once identified, the convective and non-convective regions of the storm are identified using horizontal reflectivity data.

Using dual-polarization variables, hydrometeor identification (HID) is carried out for each classified region of the storm. Microphysical and inferred charging characteristics of the multicellular storms leading up to, at the time of, and after the last flash are inferred by using the dual-polarization variables from Hytop data and the associated HID results. It is widely accepted that non-inductive charging (NIC) is the main charging mechanism within convective regions of thunderstorms, with graupel being a necessary component for this charge process. As such, graupel mass/volume calculations are done within the charging region (-10°C to -40°C) of the convective regions and analyzed in conjunction with vertical profiles of reflectivity to infer the presence of NIC.

The source of charging within non-convective regions is less clear. Past studies have shown that most lightning flashes within the stratiform regions of mesoscale convective systems (MCS) initiate in a convective cell and propagate into the stratiform region, including in and around a brightband or other enhancement in reflectivity structure. A few flashes do initiate locally in the stratiform region above an active brightband region. While the dynamics associated with multicellular storms and MCS’s is undoubtedly different, it is plausible that similar mechanisms could occur within a multicellular system. The presence of a brightband is determined in the non-convective regions using vertical profiles of reflectivity and compared to the location of the lightning initiation points. Non-precipitation ice mass calculations are done within the non-convective regions as well. If a consistent relationship, or set of trends, can be established in the radar data, then it may be possible to develop a cessation algorithm that can be used in an operational setting for a wider variety of precipitation systems, including all portions of multicellular storms.