Evolution of Specific Differential Phase in Squall Lines and Corresponding Lightning Channels

Abstract

Dual-polarimetric radar products have been used in observing changes and persistence of thunderstorm electric fields in relation to lightning discharges. One such product, specific differential phase (KDP), is valuable for its ability to detect the change in particle orientation. Negative KDP values above the freezing level indicate ice crystals are oriented vertically beyond 45 degrees in response to an electric field. The relationship between negative KDP to electric fields and the evolution of negative KDP values through the life cycle of thunderstorms has not been previously well documented. In this study, one of the Shared Mobile Atmospheric Research and Teaching (SMART) radars was used to sample a small Florida squall line (2012) and a large Oklahoma squall line (2016). Data collected from the Florida event was overlaid with local lightning mapping array (LMA) data. The resulting composites were used to compare lightning channel positions to polarimetric signatures, and to study the evolution of those signatures through the life cycle of the squall line. A charge analysis was performed to examine the locations of charge regions in relation to the polarimetric ice-alignment signatures for the Florida squall line. Polarimetric signatures from the Oklahoma squall line were compared to those found in the Florida squall line. In both cases, a persistent, strongly-negative KDP region was observed above the freezing level on the stratiform side of the reflectivity maximum. This negative KDP region was elongated and sloped downward from the convective region into the stratiform region during later stages of the stratiform region development. A second region of negative KDP also existed on the forward side of the reflectivity maximum associated with mature convective cells, but fluctuated in strength frequently. In the Florida case, LMA radiation points for a given flash tended to follow contours of zero-KDP and would initiate around one of the negative KDP regions. A charge analysis of the flashes found that the negative KDP region tended to be below the positive charge region and above the negative charge region. Given that the location of the negative KDP region in relation to the lightning channels, it can be concluded that radar could be used to monitor the electrification of thunderstorms. However, the application is limited by the scan speed. The use of phased-array technology would be necessary to attempt to predict individual intracloud flashes.