Secondary Ice Production Improves Forecasts of Explicit Freezing Rain

Freezing rain and ice pellets are high-impact weather elements and are associated with numerous hazards due to the accumulation of ice on surfaces. Thus, forecasts of winter precipitation types are important to public transportation, safety and electrical supply. Projected changes in a warmer climate also depend on how well these precipitation types are represented in models. In the Canadian numerical weather precipitation systems, forecasts of explicit freezing rain predicted by the Predicted Particle Properties (P3) microphysics scheme are often overestimated during the cold season when in reality snow occurs. A case study of freezing rain and ice pellets over eastern North America is simulated to illustrate that the overestimation of freezing rain is improved when a configuration of P3 with two ice categories is used. The reduction of excessive freezing rain results from the collection of supercooled drops with ice particles formed by secondary ice production process. A statistical analysis of 40 winter cases confirms this systematic result and forecast skill scores, such as the frequency bias index, are improved as a consequence of the reduction in freezing rain. This study demonstrates that the representation of secondary ice production is important in explicit simulations and forecasts of winter precipitation.