Winter Weather Research in Two Acts: Weather Conditions and Messaging associated with Fatal Crashes, and a Polarimetric and Microphysical Investigation of Ice Pellets

Approximately 1000 fatalities occur annually on U.S. roadways during winter weather. Minimizing the adverse impacts of winter weather on motor vehicle safety requires: (a) identifying hazardous weather conditions leading up to and surrounding fatal crashes, and (b) effective, targeted messaging of those hazards to motorists. Weather conditions leading up to fatal crashes and the language used within Winter Weather Warnings and Advisories (WSWs) issued by the NWS are analyzed. Ongoing snowfall, freezing precipitation onset, and deteriorating weather conditions contribute to many fatalities. WSWs often both identify a road hazard (e.g., “roads will become slick”) and provide action items for motorists(e.g., “slow down and use caution while driving”); yet only a third of fatalities occur during WWAs.

A polarimetric radar signature indicative of hydrometeor refreezing has been documented during ice pellets events, yet the underlying microphysical cause remains unclear. The signature is observed when fully melted hydrometeors refreeze, thus the favored hypothesis is the preferential refreezing of small drops. However, microphysical model simulation results reveal this effect is insufficient to produce the observed signature. Simulations emulating a thicker ice shell on the bottom of a falling particle with minimal wobbling produce realistic signatures. Conversely, the signature is not observed during an event with the refreezing of partially melted hydrometeors. This suggests that the presence or absence of the signature may be used to distinguish between the refreezing of fully melted and partially melted hydrometeors.