P-type Processes and Predictability: A Case Study from the Winter Precipitation Type Research Multiscale Experiment (WINTRE-MIX)

During near-freezing surface conditions, diverse precipitation types (p-types) are possible, including rain, drizzle, freezing rain, freezing drizzle, wet snow, ice pellets, and snow. Near-freezing precipitation affects wide swaths of the United States and Canada, impacting aviation, road transportation, power generation and distribution, winter recreation, ecology, and hydrology. Fundamental challenges remain in observing, diagnosing, simulating, and forecasting near-freezing p-types, particularly during transitions and within complex terrain.

Motivated by these challenges, the field phase of the Winter Precipitation Type Research Multi-scale Experiment (WINTRE-MIX) was conducted from 1 February – 15 March 2022 to better understand how multi-scale processes influence the variability and predictability of p-type and amount under near-freezing surface conditions. WINTRE-MIX took place near the US / Canadian border, in northern New York and southern Quebec, a region with plentiful near-freezing precipitation influenced by complex terrain. During WINTRE-MIX, existing advanced mesonets in New York and Quebec were complemented by deployment of: (1) surface instruments, (2) the National Research Council Convair-580 research aircraft with W- and X-band Doppler radars and in situ cloud and aerosol instrumentation, (3) two University of Illinois X-band dual-polarization Doppler radars and one dual-polarization Doppler C-band radar, and (4) teams collecting manual hydrometeor observations and radiosonde measurements. Eleven intensive observing periods were coordinated.

Initial results will be presented from WINTRE-MIX intensive observing period 5 (IOP5), which took place on 22-23 February 2022. Rain, freezing drizzle, freezing rain, ice pellets, and snow were observed across the study domain. Synoptic-scale lower-tropospheric warm air advection and mesoscale terrain-channeled sub-freezing flow in the Saint Lawrence valley strongly influenced thermodynamic profiles and surface p-types, facilitating the occurrence of ice pellets in the center of the St. Lawrence Valley and freezing rain in southern Quebec and northern New York. All of the operational mesoscale models from the High-Resolution Ensemble Forecast (HREF) system struggled to forecast p-types accurately during IOP5. In particular, in the southern St. Lawrence Valley, low-level cold air was observed to be only a few hundred meters deep and model forecasts exhibited large warm biases. Analyses of airborne in situ probes, mobile radars, and other WINTRE-MIX field observations are used to highlight the role of mesoscale processes in determining the observed variability of p-types in this event.