Hydrometeor Characterization of Lightning Regions Relative to Downdraft Processes

The broad-scale relationship between lightning and mixed phase updraft processes has been resolved to the extent that it may be used to assess a thunderstorm’s ability to exhibit severe phenomena, including hail, tornadoes, and strong non-tornadic surface winds. In particular, the lightning jump algorithm was developed to utilize the relationship between rapid increases in total lightning flash rate, or lightning jumps, and subsequent severe phenomena for nowcasting applications. Though strong, direct relationships exist between mixed phase updraft properties and lightning production, physical processes linking total lightning behavior, thunderstorm kinematics, and downdraft-related severe phenomena are more complex. Specifically, the spatial and microphysical relationships of lightning activity (including rapid increase and abatement) and enhanced downdraft behavior require further study.

The updraft, lightning, and downdraft-related processes are thought to be related by hydrometeor type, concentration, and distribution within the mixed-phase region of thunderstorms. Broadly, thunderstorm electrification is thought to primarily result from the non-inductive charging mechanism by which ice crystals and graupel particles become charged during collisions in the presence of supercooled liquid water. In addition to facilitating gravitation sedimentation of charged ice crystals and graupel particles for electric field development prior to lightning initiation, the hypothesized role of the updraft is to continually supply the mixed-phase constituents that participate in this process. The downdraft then serves as a sink for mixed phase precipitation where microphysical processes generate negative buoyancy through melting, evaporation, and water loading. However, the variability of the relationship between these processes in space and time has not been well-resolved, particularly at sub-storm scales.

Ongoing work seeks to clarify roles of specific hydrometeor types in lightning production and downdraft enhancement, building up on results documented in the literature concerning microphysical and kinematic thunderstorm processes related to lightning and the lightning jump. Using a small set of supercell thunderstorms, spatial and temporal relationships of mixed-phase hydrometeor field growth and evolution are examined in detail with respect to trends in lightning behavior. This analysis focuses on periods of enhanced lightning activity defined by objectively-identified lightning jumps and subsequent marked decreases in lightning flash rate suggested to be associated with downdraft development. Properties of hydrometeor fields and their evolution in this analysis are assessed through use of polarimetric Doppler radar, hydrometeor identification (HID), and multi-Doppler analysis when available. The use of an HID algorithm in case study analysis plays a key role in characterizing the hydrometeor fields of interest with respect to lightning and downdraft processes. Specifically, fuzzy logic-based HID directly contributes to quantification of thunderstorm precipitation and non-precipitation ice volumes and mass. As part of the study, sensitivity testing of the implementation of an HID algorithm is performed to evaluate the impacts of the weighting terms and overall contributions of individual polarimetric variables in results. Results utilizing S-band and C-band radar data are also compared as an initial assessment of how their combination may be leveraged to provide added value in HID solutions.