Multi-Faceted View of Winter Precipitation: Societal Impacts, Polarimetric Radar Detection, and Microphysical Modeling of Transitional Winter Precipitation
Dana M. Tobin, PhD
Cooperative Institute for Severe and High-Impact Weather Research and Operations
Tuesday, October 12, 2021, 2:00 pm
There remain several unanswered questions related to transitional winter precipitation, ranging from the impacts that it has on society to what microphysical processes are involved with its formation. With an improved understanding of the formation and impacts of transitional winter precipitation types, it is possible to reduce adverse societal impacts in the future by improving their detection and forecasting. Precipitation has an adverse effect on motor vehicle transportation, yet no study has quantified the impacts of ice pellets or freezing precipitation. Crash risk is estimated by matching traffic crash data to nearby precipitation-type reports, resulting in a hierarchy of risk based on precipitation, with transitional winter precipitation having higher crash risk than rain or snow.
A polarimetric signature indicative of hydrometeor refreezing was recently documented, yet the underlying microphysical explanation remains unclear. The signature is characterized by a prominent enhancement in differential reflectivity (ZDR) within a layer of decreasing radar reflectivity (ZH) towards the ground. These observations were made during prolonged periods of ice pellets where hydrometeors were fully melted prior to refreezing. The favored explanation in the literature for the observed ZDR enhancement is preferential refreezing of small drops. A simple microphysical model is developed to model the refreezing of fully melted hydrometeors, and coupled with a polarimetric radar forward operator. Results demonstrate that preferential refreezing is insufficient to produce the observed signatures. In contrast, simulations emulating a thicker ice shell on the bottom of a falling particle with minimal wobbling are capable of producing realistic signatures. Conversely, no such ZDR enhancement is found using high-resolution polarimetric radar data during an event with refreezing of partially melted hydrometeors. These observations suggest that the presence or absence of a ZDR enhancement in the refreezing layer may be used to distinguish between the refreezing of fully melted and partially melted hydrometeors.
Speaker Bio: Dr. Tobin received her PhD in Meteorology and Atmospheric Science at The Pennsylvania State University in 2021 working with Dr. Matt Kumjian. Her research interests are in the realm of winter precipitation, including precipitation-type observations, polarimetric radar detection, microphysical modeling, and societal impacts. She is currently a Peter Lamb Postdoctoral Fellow at the University of Oklahoma's Cooperative Institute for Severe and High-Impact Weather Research and Operations (CIWRO; formerly known as CIMMS) working with Dr. Heather Reeves. Her current work is related to researching prevalent weather conditions leading up to fatal wintertime crashes, public-facing messaging of deadly winter weather, and the impacts of freezing precipitation on traffic volumes.