Simulating the Transition from Freezing Rain to Ice Pellets during WINTRE-MIX: The Role of Secondary Ice Production

Accurately forecasting the precipitation-type (p-type) transition from freezing rain (FZRA) to ice pellets (PLs) is challenging. Meanwhile, factors leading to this transition have not been well studied. In this study, we use the predicted particle properties (P3) microphysics scheme with predicted liquid fraction in the Weather Research and Forecasting (WRF) Model to simulate the FZRA-to-PL transition observed during intensive observing period 4 (IOP4) of the Winter Precipitation Type Research Multiscale Experiment (WINTRE-MIX) field campaign. Station and mobile radar observations, manual p-type reports, and radiosonde data are used to evaluate the model performance in simulating the FZRA-to-PL transition. The FZRA-to-PL transition was observed as the depth of the surface-based subfreezing layer was increasing, while small ice crystals were photographed at the surface. A control simulation captures the overall evolution of surface p-type but has a 0.5–1-h delay in the FZRA-to-PL transition, probably due to a warm bias in the subfreezing layer. In contrast, a simulation wherein the secondary ice production (SIP) process is removed has no FZRA-to-PL transition. Further comparison between these two simulations shows that ice crystals produced from SIP substantially increase the efficiency of collection of rain by ice particles that freeze supercooled rain into PL in the subfreezing layer, accelerating the FZRA-to-PL transition. Without SIP, PL might be produced by immersion freezing of rain, but its efficiency is too low to initiate an FZRA-to-PL transition.