Processes Governing Precipitation Variability in Winter Extratropical Cyclones: Results from the Recent IMPACTS Field Campaign

Winter storms present major challenges to numerical modeling due to the variable nature of the intensity, distribution, and longevity of snowfall across the storm life cycle. The processes that control snowfall variability occur on multiple scales, from synoptic to mesoscale to microscale, and how these processes synergistically interact in these storms are poorly understood. High temporal and spatial resolution observations in diverse winter storms are not available from operational data networks alone, necessitating research quality measurements from field campaigns. The NASA-funded Earth Venture Suborbital-3 Investigation of Microphysics and Precipitation for Atlantic Coast-Threatening Snowstorms (IMPACTS) took place during three winter seasons (2020, 2022, and 2023) to address this need. IMPACTS flew two research aircraft and sampled 35 Midwest and Northeast US winter storms. The high-altitude, satellite-simulating ER-2 aircraft was equipped with passive and active remote sensing instruments that are the same or similar to those flown on current satellite platforms. The in situ P-3 aircraft was equipped with multiple probes to document flight-level microphysical properties and environmental conditions at high temporal resolutions and launched dropsondes to gain thermodynamic profiles of temperature, winds and humidity when sampling over the ocean. The two aircraft were flown in coordination so that the remote sensing measurements from the ER-2 can be directly related to the microphysical and environmental measurements from the P-3. Additional ground-based resources included mobile and fixed radars, additional National Weather Service soundings, mobile sounding teams, and the New York State Mesonet surface data network. The overall IMPACTS goals are to characterize the temporal and spatial variability of snowfall intensity, rate, and duration, understand the dynamical, thermodynamical, and microphysical processes controlling this variability, and improve remote sensing and numerical modeling of snowfall. The IMPACTS dataset represents one of the most comprehensive and wide-encompassing documentation of precipitation processes within Northeast and Midwest storms. This presentation describes results from studies capitalizing on over 21 hours of coordinated radar remote sensing and the in situ microphysics measurements obtained during IMPACTS. In each study, the ability to interpret the radar reflectivity and/or radar Doppler velocity fields in terms of microphysical processes is made possible through these coordinated observations. Through simulations using a particle-based precipitation model constrained by IMPACTS observations, vertical evolution of a particle population by deposition, aggregation, and riming is explored. It was found that the Doppler velocity was uniquely sensitive to particle evolution during riming and can thus become a useful diagnostic indicator of when the riming process is dominant. In another study, the in-situ observations shed light on the radar signatures and microphysical processes occurring in the -20°C to -10°C temperature layer, often referred to as the dendritic growth layer. Initial results indicate that within this layer there are times when the radar profiles have an increase of the vertical gradient of reflectivity that is coincident with a region of larger particles, which were found to be predominantly side-plane particles as opposed to dendritic shaped particles. Lastly, the dual-frequency ratio (DFR) derived from the Ku- and Ka-band reflectivity of the HIWRAP radar on the ER-2 was analyzed and the microphysical properties within regions of prominently higher DFR at the altitude and location of the in situ P-3 were compared to regions of lower DFR for all coordinated flights. In regions of enhanced DFR, the mass weighted mean diameter was 66% larger, the effective density was 73% smaller, the normalized intercept parameter was 123% smaller, and the ice water content was 46% higher compared to in regions of lower DFR. These results are consistent with the aggregation process occurring within regions of high DFR. All of the aforementioned studies above address the IMPACTS science goals, especially improving remote sensing of snowfall processes from satellite-borne instruments.