Hail detection and nowcasting

This presentation starts with a review of hail detection/nowcasting. With one exception, the current operational detection and nowcasting relies on indirect inferences from storm structure, strength of diverging outflow at storm top, height of storm, reflectivity values, and so on. An exception is the three body scattering signature, which is succinctly explained.

Then the presentation turns to more direct detection methods with emphasis on the use of polarimetric radar data for detection and sizing; it is followed with a brief review of dual wavelength techniques for estimating hail size. Recent results from Joint POLarization Experiment (JPOLE) are shown in which polarimetry can discriminate sizes larger than about 2 cm. The challenge for polarimetry is to gauge more than one size. Other yet unresolved issues that will be discussed are a) orientation in fall, b) reasons for negative values of differential reflectivity in hail cores, c) bulk properties of precipitation revealed by their polarimetric signatures in and near hail cores, and d) clues within severe storms that can tell potential hail size and area of fall.

Throughout, there will be examples of measurements such as drop shedding (indirectly deduced from polarimetric variables), locations of intense rainfall in the vicinity of hail cores, and fields of polarimetric variables in different types of hail producing storms. Radar data will be supplemented with some observations by a two dimensional video imager, airborne hail spectrometer, and airborne hail impactor.