A Case-study of Probable Secondary Ice Production in a Wintertime Storm during WINTRE-MIX

Forecasting winter precipitation type presents a great challenge. Predictions of hydrometeor type are incredibly sensitive to a variety of environmental parameters that are difficult to measure due in part to their small-scale variation. This leads to increased inaccuracies and uncertainties associated with forecasts and model simulations of winter weather events. Uncertainties associated with cloud and precipitation processes, including, under certain conditions, the role of secondary ice production (SIP) in winter storms further complicate issues related to winter weather prediction and modeling.

The Winter Precipitation Type Research Multi-Scale Experiment (WINTRE-MIX) field campaign was conducted over the St. Laurence and Champlain river valleys in northeastern New York, USA and southern Quebec, Canada in February and March of 2022. One of the goals of WINTRE-MIX is to better understand how micro-scale processes influence precipitation evolution leading to differences in precipitation type at the surface. To that end, an extensive set of in situ and remotely sensed measurements from both airborne and ground-based assets produced a rich data set from mixed-phase precipitation systems in a region with complex terrain. The National Research Council of Canada’s (NRC) Convair-580 atmospheric research aircraft collected measurements of cloud, precipitation, and atmospheric conditions during nine research flights. Detailed in situ measurements of cloud and precipitation properties were made by a Cloud Droplet Probe (CDP-2), a Two-Dimensional Stereo Probe (2DS), and a High-Volume Particle Spectrometer (HVPS-3). Vertically-pointing airborne Doppler radars operating at X- and W-Band supplied vertical context for these measurements. Ground-based X- and C-Band radars operated by the Flexible Array of Radars and Mesonets (FARM) Facility provided additional data for a variety of polarimetric radar measurements.

Flight 7 occurred on the morning of March 7^th, 2022. Multiple transects were flown along a wide band of warm-frontal precipitation. During the initial aircraft ascent to cloud top, in situ observations showed the cloud to be composed almost entirely of supercooled liquid at temperatures as low as -15 ºC. Additionally, the 2DS probe captured images of large supercooled drizzle within the mid-levels of the cloud. While descending through the cloud, a sudden transition to ice at low-levels was observed in a region with temperatures ranging between -7 and -4 ºC. The large concentration of ice at cloud base in the (presumed) absence of ice-nucleating particles (INP) or primary ice at higher altitudes suggest a prominent secondary ice production (SIP) process active in this cloud. In this study we present measurements from the instruments described above to better understand the process of SIP in this cloud system.