Multimodal Ice Crystal Size Distributions in Atlantic Coast Snowstorms: Results from IMPACTS 2020

In January and February 2020, 2022, and 2023, the Investigation of Microphysics and Precipitation for Atlantic Coast Threatening Snowstorms (IMPACTS) was conducted out of Wallops Island, Virginia to study the banding of precipitation in snowstorms. The NASA Airborne Science Program’s P-3 Orion (N426NA) research aircraft flew inside snow bands between −15°C and −5°C to study the microphysics of these bands. A team of ground forecasters monitored and directed the P-3 aircraft’s location relative to the band structure identified on nearby WSR-88D (NEXRAD) radars, while the NASA ER-2 (809) remote sensing aircraft with a High-Altitude Imaging Wind and Rain Airborne Profiler (HIWRAP) and Doppler radar (EXRAD) installed flew above the P-3 to monitor reflectivity and ensure the P-3 sampled areas of reflectivity enhancement. Installed on the P-3 were a Water Isotope System for Precipitation and Entrainment Research (WISPER) probe measuring total ice water content (IWC), a King probe measuring liquid water content (LWC), a turbulent air motion measurement system (TAMMS) measuring temperature, a Rosemount icing detector (RICE) identifying supercooled water presence, a Cloud Droplet Probe (CDP) identifying liquid water presence, a Two-Dimensional Stereo (2D-S) probe and a High Volume Precipitation Spectrometer (HVPS-3) measuring particle and habit size distributions (PSDs), and a Particle Habit Imaging and Polar Scattering (PHIPS) probe identifying particle habits. For the storms in 2020, a phase identification algorithm using data from multiple probes is used to characterize each 1 s time period as ice-phase, liquid-phase, or mixed-phase. For ice- and mixed-phase clouds, a previously developed fitting routine that automatically determines whether a unimodal, bimodal, or trimodal gamma distribution best fits an observed PSD is used to determine the characteristics of PSDs (e.g., presence of unimodal, bimodal, or trimodal distributions, and gamma fit parameters of each of these distributions characterized by volume of equally realizable parameters to account for variability and uncertainty). Derived fit parameters are compared against those obtained in previous studies characterizing winter snowstorms and other meteorological conditions. Bimodal PSDs were subcategorized as either bimodal1 or bimodal2 by whether maximum dimension D ≈ 0.15 mm or D ≈ 1 mm separated the two modes. The variations in PSD characteristics and bulk properties (median mass diameter, IWC) are determined as functions of temperature, IWC, reflectivity, and habits. Implications for the understanding of processes occurring in snow bands are discussed.