Simulating the Transition From Freezing Rain to Ice Pellets: The Role of Secondary Ice Production During WINTRE-MIX IOP4

Accurately forecasting the precipitation type (p-type) transition between freezing rain (FZRA) and ice pellets (IP) is challenging since they can occur under similar conditions. This limits our ability to predict the duration of freezing rain and the associated impacts. This study focuses on the influence of the Hallet-Mossop mechanism of secondary ice production (SIP) within the subfreezing layer on the p-type transition from FZRA to IP during intensive observational period 4 (IOP4) of the Winter Precipitation Type Research Multi-scale Experiment (WINTRE-MIX) field campaign. The storm observed during IOP4 occurred on 17-18 February 2022 and produced approximately 5 hours of FZRA and IP in the Champlain Valley of northeastern New York and southern Quebec, providing a great opportunity to investigate the potential factors that influence the transition between FZRA and IP. Station observations are used to evaluate the spatial distribution of simulated meteorological conditions and p-types at the surface. Manual p-type reports, hydrometeor macro-photos, and research soundings collected during the WINTRE-MIX field campaign are further used to identify the model bias in the surface p-type evolution at two sites in the Champlain Valley during IOP4 (DOW-US-P and DOW-CAN-S).

A control simulation is conducted using the WRF v4.2.2 model with a 1-km resolution innermost domain centered over the WINTRE-MIX study region. An HRRR-like model configuration is adopted with the Thompson aerosol-aware microphysics scheme and the MYNN planetary boundary layer (PBL) scheme. The control simulation performs well in simulating the transition from rain to FZRA but shows a delay in the transition from FZRA to IP. Analysis of microphysical process rates reveals that the rain-collecting-ice process might play an important role in the production of IP when the melting layer is relatively deep. A sensitivity test using the MYJ PBL scheme is about 2 °C colder in the sub-freezing layer around the DOW-US-P site and activates the SIP at an earlier time, leading to an earlier formation of ice pellets via the rain-collecting-ice process and an improvement in the transition from FZRA to IP. A second sensitivity test adds an additional source for the SIP process, which enhances the local production of cloud ice at the DOW-CAN-S site. As a result, the transition from FZRA to IP occurs at an earlier time and is closer to the manual observations. These results suggest that both the occurrence time and intensity of the SIP process within the subfreezing layer might modulate the transition from FZRA to IP, motivating the need for an accurate representation of SIP in numerical models used to forecast p-type in winter storms.