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Abstract During freezing rain, secondary ice produced by the fragmentation of freezing drops (FFD) can initiate a chain reaction, potentially transitioning freezing rain into ice pellets. Including this process in numerical weather prediction models is challenging due to the uncertainty of this mechanism. To bridge this gap, this study aims to evaluate the efficiency of the FFD process during ice pellet precipitation using measurements collected onboard the National Research Council Canada (NRC) Convair-580 research aircraft during the 2022 Winter Precipitation Type Research Multiscale Experiment (WINTRE-MIX). Below the supercooled raindrops freezing altitude, in situ probes measured a bimodal particle size distribution. Observations from imaging and optical-array probes show that most particles smaller than 500μm in diameter were nonspherical ice crystals, in the concentration of ∼500 L⁻¹. In contrast, most particles larger than 500μm were identified as fractured ice pellets and ice pellets with bulges, which suggested the occurrence of the FFD process. A conceptual model is then developed to show that five–eight fragments of ice were produced for each freezing drop. Two existing parameterizations of the FFD process are also tested. It is shown that one parameterization would result in less ice crystals than the measured number concentration, while the second one would result in too many ice crystals. Adjustments to these parameterizations are computed based on the collected observations. This analysis will be valuable for including the FFD process into simulations of freezing rain, ice pellets, and other weather phenomena where this process plays a significant role. Significance StatementThis study presents unique measurements from a winter storm recorded by state-of-the-art instruments onboard a research aircraft at the altitude where ice pellets are formed. The collected data suggest that the freezing of a few initial raindrops at an altitude of around 250 m above the ground resulted in the production of ice crystals. These ice crystals led to the freezing of additional raindrops in a feedback loop that can be referred to as ice multiplication. This process is quantified in the current study. The results will be valuable in improving the representation of ice pellets and freezing rain in computer simulations of winter storms. 

Two airborne Doppler radars deployed on the National Research Council (NRC) Convair 580 aircraft collected data during nine research flights of the WINTRE-MIX field campaign. One of these radars operates at W-band (94.05 GHz) and is called NAW while the other operates at X-band (9.41 GHz) and is called NAX. The two radar systems in combination are called NAWX. This dataset is an enhanced subset of the companion datasets that were published by Nguyen and Wolde. Specifically, the data are separated into legs when the NRC Convair 580 was flying approximately straight and mostly at constant altitude. These are periods when Doppler velocities from the radars are most usable due to relatively stable aircraft attitude. The NAWX dataset described here is also an enhancement relative to the companion datasets in that improvements to beam pointing vectors have been implemented, allowing a more accurate navigation correction of Doppler velocities (i.e., removing aircraft motion from the Doppler velocity data). Additionally, new fields are included in the dataset such as horizontal wind corrected Doppler velocities and quality control masks to mitigate artifacts in the data (e.g., ground clutter). 

This dataset is comprised of a series of PDF flight summaries that describe the general microphysical characteristics that were recorded along each flight leg for the 9 research flight missions of the WINTRE-MIX NRC Convair-580 aircraft. Each summary includes a written overview of the IOP and orientation of performed flight legs as well as a description of the microphysical characteristics of cloud measured along each transect by the NRC Convair-580. Several figures are provided for each flight leg detailing microphysical parameters such as particle number concentration and mean size distributions measured across these periods. Particle imagery from the 2DS probe is also provided for select periods across all flight legs to demonstrate the cloud composition of the weather systems of focus for each flight. General atmospheric state statistics are also provided in these summaries, as well as any data limitations observed with the parameters provided in each summary. 

Abstract During near-0°C surface conditions, diverse precipitation types (p-types) are possible, including rain, drizzle, freezing rain, freezing drizzle, ice pellets, wet snow, snow, and snow pellets. Near-0°C 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-0°C 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 multiscale processes influence the variability and predictability of p-type and amount under near-0°C surface conditions. WINTRE-MIX took place near the US / Canadian border, in northern New York and southern Quebec, a region with plentiful near-0°C precipitation influenced by 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 X-band dual-polarization Doppler radars and a C-band dual-polarization Doppler radar from University of Illinois, and (4) teams collecting manual hydrometeor observations and radiosonde measurements. Eleven intensive observing periods (IOPs) were coordinated. Analysis of these WINTRE-MIX IOPs is illuminating how synoptic dynamics, mesoscale dynamics, and microscale processes combine to determine p-type and its predictability under near-0°C conditions. WINTRE-MIX research will contribute to improving nowcasts and forecasts of near-0°C precipitation through evaluation and refinement of observational diagnostics and numerical forecast models.