Size sorting of raindrops in convective storms is an important microphysical process that can result in localized significantly altered drop size distributions (DSDs). Regions of pronounced size sorting provide information about airflow patterns within the storm. Observing these areas is not possible with conventional single polarization radars, which give little information about the DSD within the observed sampling volume. However, with the advent of dual-polarization radars, measurements at orthogonal polarizations allow estimating median drop sizes and thus observing DSD characteristics. Such observations can be used to infer the locations of ongoing size sorting in the storm.

This study examines several kinematic mechanisms of size sorting that occur in many convective thunderstorms. These mechanisms are analyzed through observations of convective storms (including mesoscale convective systems and supercell thunderstorms) as well as simple numerical models. The size sorting mechanisms discussed in this paper include differential sedimentation, updrafts and downdrafts, strong rotation, and vertical veering of the wind. The latter mechanism results in a low-level signature in Z_DR known as the Z_DR arc. Through a simple, explicit microphysics numerical model we show that the Z_DR arc is proportional to the storm-relative environmental helicity (SREH) and that it might indicate storm severity and potential for tornadogenesis. With the anticipated polarimetric upgrade of the NEXRAD WSR-88D network in mind, operational applications and the importance of such microphysical observations are discussed.