Using dual-polarization radar and crowdsourced mPING reports to investigate hydrometeor refreezing

Operational S-band polarimetric WSR-88D radar observations from the 2013-14 and 2014-15 winter seasons are used to investigate a hydrometeor refreezing signature indicative of ice pellet formation. The signature is seen as a pronounced enhancement in Z_DR below the melting layer and within a region of Z_H decreasing towards the ground. A slight enhancement in K_DP and decrease in ρ_hv are also observed in some cases. These signatures reveal the presence of unique microphysical processes associated with hydrometeor refreezing. Several cases have been chosen to characterize this signature, the environment in which it appears, and to explore its underlying microphysics.

Quasi-vertical profiles (QVPs) of the polarimetric radar variables are constructed for each event. QVP evolution has revealed the persistence of this signature during prolonged ice pellet cases. Surface precipitation type reports provided by the Meteorological Phenomena Identification Near the Ground (mPING) project accompanying the QVPs show agreement between the polarimetric signatures and ice pellet reports. Interestingly, ice pellet to freezing rain changeover events are characterized by a monotonic descent of the radar-observed refreezing layer. The time of this change in precipitation type corresponds to the time at which the refreezing layer intersects the surface. Thermodynamic profiles of the atmosphere throughout ice pellet and changeover events will provide insight into the thermodynamic environment in which hydrometeors are refreezing, and how the environment changes the observed characteristics of the refreezing signature.