Characterizing Variations of Cloud Microphysical Properties with Respect to Position in Snowbands and Small-Scale Vertical Motion: A Case Study of 23 January 2023 Storm during IMPACTS

Winter storms in the Northeastern United States often feature heavy localized precipitation in the form of linear snowbands. Current weather models have a difficult time discerning the strength and location of these features. To gain further insight into the processes that lead to the formation of snowbands, the NASA Investigation of Microphysics and Precipitation for Atlantic Coast Threatening Snowstorms (IMPACTS) observed these features in many different storms. Using a low-altitude P-3 and a high-altitude ER-2 aircraft, the IMPACTS campaign was able to collect extensive in-situ data on cloud microphysical properties and utilized remote sensing instruments to collect high resolution radar and LIDAR data.

The 23 January 2023 storm observed during the IMPACTS campaign featured a stationary and long-lived snowband that was sampled by both aircraft during five coordinated flight legs, with three legs parallel and two legs perpendicular to the band. These observations allow for a characterization of how cloud microphysical properties, such as the liquid and ice water content (LWC and IWC), ice and liquid particle number concentration (N_i and N_l), median mass diameter of particles larger than 500 µm (D_mm>500), and cloud phase, vary with position within the snowband and in the presence of small-scale vertical motions like those in convective generating cells.

To derive cloud properties, a cloud phase identification algorithm was first developed so that each 1 s observation (~120 m of P-3 flight distance) was characterized as ice-, liquid- or mixed-phase. Size distributions for liquid and ice-phase particles are then computed, as are the bulk properties such as N_i, N_l, LWC, IWC, and D_mm>500. Using the multi-radar multi-sensor product and the radars aboard the ER-2, the collected data are separated into a large-scale convective, a stratiform, and a high-reflectivity snowband region. Vertical air velocity data (w) from the Turbulent Air Motion Measurement System (TAMMS) is used to separate all the measurements into either updrafts, downdrafts, or calmer regions with w between -0.5 and 1.0 m s^-1. Thus, the variation of microphysical properties over small and large spatial scales can be determined. The results in this study found that mixed and liquid phase clouds are found in 35% and 65% of samples taken in strong vertical motions and the large-scale convective region, respectively. This is upwards of 20% higher than samples outside of these regions. The 23 January 2023 storm had higher LWC, upwards of 0.06 g m^-3, in the small-scale updrafts than in other areas, and the warmer convective regions of the storm contained N_l greater than the stratiform region by a factor of 4. Additionally, the storm displays particle sizes that are larger in the snowband with a median D_mm>500 of 1700 µm vs 1100-1300 µm outside the band. Such information can be useful to determine how particles grow in winter storms and can help characterize the cause of the higher reflectivity observed in snowbands.