Enhanced Layers of Spectrum Width within Northeast U.S. Coastal Storms and Their Relationship with Precipitation Structures

Within United States East Coast winter storms there are small-scale precipitation features and flow perturbations (e.g., eddies) not observed by operational radars that may be important to the processes within the cloud. Radars operating at higher temporal and spatial resolution help reveal more detailed and transient structures within these storms. The Ka-Band Scanning Polarimetric Radar (KASPR) at Stony Brook University is ideally suited to sample short-lived mesoscale features. KASPR’s highly sensitive dual polarimetry measurements are combined with a 1s sampling rate and spatial resolutions of 15 m and 45 m for the vertical profiles and PPI/RHI scans, respectively. KASPR was operating from 2017-2023, in part with the NASA Investigation of Microphysics and Precipitation of Atlantic Coast-Threatening Snowstorms (IMPACTS) field campaign in 2020 & 2022. During winter storms, KASPR often observes horizontal layers of enhanced spectrum width (SW). The origin of these layers of enhanced SW (SWLs) is uncertain, but they may have origins in turbulent wind shear layers and vertical gradients in microphysical growth and fallout. SWLs frequently occur at 2-3 km AGL, and are often accompanied by a sharp increase in reflectivity below. We hypothesize that the most prominent SWLs are produced by wind shear within the warm advection and veering wind region of the cyclone comma head. This work aims to understand the temporal and spatial dimensions of SWLs, the processes that favor SWL development, and how SWLs may alter the precipitation growth, including sharp vertical gradients in reflectivity, cell production, and fallstreaks.

A two-dimensional convolution-based feature recognition algorithm was developed to automatically identify SWLs from KASPR PPI scans, RHI scans, and vertically pointing profiles. Over 50, 130, and 178 hours of cool-season KASPR PPI, RHI, and vertical scans, respectively, were used from the 2017 - 2023 winter seasons. The basic characteristics of SWLs, such as their frequency, altitude, thickness, and duration are explored. Velocity Azimuth Display (VAD) data, generated from the KASPR PPI scans, is used to investigate the relationship between vertical wind shear and SWL development and evolution. In addition to reflectivity gradients, dual polarimetric data from PPI and RHI scans provide additional insight into the microphysical processes occurring in regions of SWLs. Sounding data from the National Weather Service in Upton, NY (OKX) and from Stony Brook, NY, as well as reanalysis data from the Rapid Refresh (RAP) is used to provide context about the environmental conditions.

This presentation will first highlight the climatological characteristics of these SWLs using KASPR data from 2017-2021. The majority of SWLs identified are thin and transient, with more than half of SWLs being <150 m thick and persisting for <30s. Only ~12% of SWLs are >250 m thick and persist >30s. Vertical wind shear was found to be 50% greater in regions where a SWL was present compared to regions where no SWL was present. This presentation will then focus on the more prominent SWLs that have thicknesses > 250 m with durations of tens of minutes, as these are often accompanied by more variations of the precipitation features (gradients, fallstreaks,. etc) which suggests the turbulence within these layers may be modifying the precipitation processes. These SWLs are most often horizontally linear; however, in a few cases they take on a wave-like appearance, are sloped, or have magnitude fluctuations.