Polarimetric Radar–Based Investigation of Microphysics in Dendritic and Needle Temperature Aggregation Zones during NASA IMPACTS

Microphysical processes that determine cloud particle habits, sizes, and concentrations can be inferred by specific polarimetric radar signatures, providing new perspectives to inform NWP models about hydrometeor types and their distributions in winter storms. In particular, areas of enhanced K_DP sometimes occur in regions of aggregates within the dendritic growth zone (DGZ) and needle temperature zone (NTZ). Further, copolar correlation coefficient (CC) reductions are sometimes found in the DGZ. These polarimetric signatures are of interest because they suggest the presence of highly nonspherical particles among the otherwise isotropically scattering aggregates. In this study, in situ observations of cloud particle populations collected during the NASA Investigation of Microphysics and Precipitation for Atlantic Coast-Threatening Snowstorms (IMPACTS) field campaign are used to address debates in the previous literature about these signatures’ origins. In the regions of aggregates, cloud particle imager (CPI) cloud particle images show a variety of particle habits that contribute to enhancing K_DP or reducing CC, suggesting these polarimetric signatures may result from a combination of different particle types. Particle size, concentration, and aspect ratio data from the two-dimensional stereo (2DS) and high-volume precipitation spectrometer (HVPS) probes determined that the K_DP enhancement in the NTZ is produced by ice particles of 0.9–7.0-mm size, along with nonspherical aggregates with a mean aspect ratio, 0.6 at sizes between 4.6 and 20.0 mm. Increased K_DP in the DGZ is attributed to ice particles of sizes 0.4–6.0 mm. Additionally, asymmetric particles at sizes 3.0–10.0 mm are evident in the DGZ. The DGZ’s CC reduction is attributed to ice particles with sizes 1.4–5.0 mm, while CPI images show a wide range of particle types including irregularly shaped crystals that would scatter anisotropically.