On the Characteristics of Liquid Core Pellets during a Winter Storm

Forecasting the precise type and amount of precipitation during winter storms is challenging because these precipitations strongly depend on environmental conditions and microphysical processes. For example, both freezing rain and ice pellets form when falling through a melting layer (T>0°C) aloft and a subfreezing layer (T<0°C) below it. In the case of partial refreezing of liquid or mixed phase particles in the subfreezing layer, liquid core pellets can be produced. These particles are composed of a liquid water core surrounded by an ice shell. If liquid core pellets break upon impact with the surface, their liquid core might produce a coat of ice, which can lead to hazardous conditions. The goal of this study is to characterize liquid core pellets observed at the surface during a winter storm. To do so, data from the 2022 Winter Precipitation Type Research Multiscale Experiment (WINTRE-MIX) field campaign, held in southern Quebec (Canada), were used. Theoretical calculations based on the freezing time equation and using observed vertical temperature profiles were also conducted. The thickness of the refreezing layer varied from 448 m to 599 m, which is lower than the climatology (770 m). During this 22-23 February 2022 event, the average diameter of the liquid core pellets observed was 0.8 mm, with a range of 0.31 mm to 2.88 mm. The theoretical calculations suggested that the refreezing of supercooled drops was initiated at elevations close to the surface, which were at altitudes lower than 100 m above the surface. Given the thickness of the ice shell measured and the size of the particles, the liquid water available for icing on structure corresponds, on average, to 48% of the particles’ volume. These findings highlight the importance of precisely measuring and simulating the refreezing layer and the microphysical parameters during winter storms to improve the representation of winter precipitation types in atmospheric models.