On the Uses of Polarimetric Radar to Study Thunderstorm Electrification: Current Techniques and Potential Future Applications

Advances in data collection and analysis techniques have made polarimetric radar a powerful tool in studying thunderstorm electrification. One of the chief advantages of polarimetric radar is its ability to discriminate between different hydrometeor types, in a bulk sense. This has allowed the collection of important microphysical information relevant to cloud electricity, such as the presence and amount of graupel, hail, snow, and vertically aligned ice. This is important in observational studies as well as cloud electrification model validation. After a brief synopsis of polarimetric radar principles and its past applications in atmospheric electricity, recent analyses from the Severe Thunderstorm Electrification and Precipitation Study (STEPS) will be reviewed, to further demonstrate the utility of the microphysical information provided by polarimetric radar. In STEPS it was demonstrated that the strong relationship between graupel and lightning flash rate holds even when considering detrended residuals of the respective time series. Furthermore, STEPS analyses reveal that changes in the distribution of graupel correspond to changes in the observed charge distribution of storms as they evolve. Other major STEPS results, relevant to polarimteric radar, will be reviewed as well. Future applications of polarimetric radar will be examined. Among these are increased scanning rate through the use of improved signal processing, simultaneous transmit and receive of different polarizations, and phased-array technologies. This allows more rapid updates of microphysical data, an important consideration given the high temporal resolution data already provided by advanced 3-D lightning mappers. The planned polarimetric upgrade of NEXRAD radars, coupled with the increasing numbers of operational lightning mapping networks in the U.S., will allow more cases to be examined with potentially research-quality data, as these instruments will act as essentially ongoing field projects. Indeed, such networks could allow true statistical analysis, instead of the current case study model. In addition, higher frequency (C, X, W-band, etc.) polarimetric radars are smaller, cheaper, and more portable. Planned networks of these radars, such as CASA, may provide microphysical information at higher temporal and spatial resolution. They may also provide these information at lower reflectivities than is currently possible with S-band radars like CHILL, S-Pol, and NEXRAD. Improvements in fuzzy-logic hydrometeor identification techniques, resulting from ongoing validation efforts, promise more hydrometeor categories with better accuracy. Other present and future applications also will be reviewed. In summary, polarimetric radar has proven excellent utility in electrification studies, and the datasets - in terms of quantity, quality, and diversity - should improve rapidly in the coming years.