Environmental Conditions Associated With Different Snow Band Structures within Northeast U.S. Winter Storms

Mesoscale precipitation bands within Northeast U.S. winter storms result in heterogeneous spatial and temporal storm total snowfall. Several studies have provided subjective analysis of precipitation bands with a focus on larger, meso-α scale bands. Smaller meso-β to meso-γ scale structures within winter storms are less well understood. This study examines 110 cyclones that tracked near the Northeast U.S. Coast over the 19 cool-seasons (October - April) of 1996/97 through 2015/16. Composite radar products were created to provide 2 km AGL data from 6 operational radars in the region from Delaware to Maine. The Developmental Testbed Center (DTC) Model Evaluation Tools (MET) Method for Object-Based Diagnostic Evaluation (MODE) tool was used to objectively identify individual precipitation structures in the composite radar data field. MODE was run on the radar composites using several sets of thresholds in order to provide uncertainty bounds on feature classification. The distributions of sizes of objectively-determined bands relative to the center of the associated cyclone are provided. Precipitation structures are classified as bands if their aspect ratio (shorter axis / longer axis) is ≤ 0.5. Bands are further classified into primary bands which are defined as having lengths ≥ 250 km and mid-sized bands that have lengths < 250 km. Preliminary results support that there are at least five different modes of precipitation banding in coastal Northeast U.S. winter storms as follows: (1) large primary band only, (2) multi-banded (no primary band, but mid-sized bands), (3) both single and multi-banded at the same time, (4) cellular, (5) stratiform only (no bands). 

Using observed soundings from 6 sounding locations (KGYX, KALB, KCHH, KOKX, KIAD, KWAL) and reanalysis data (CFSR), differences in horizontal moisture fluxes, available instability (gravitational, potential, conditional), depth of any instability layer, depth and strength of stable layer at low-levels that may serve as a wave duct for any gravity waves and associated bands, vertical shear strength and depth, and depth and amount of  moisture and dry layers aloft will be examined for different band types. Preliminary results support that the existence of a large, primary band may be the result of a more stable environment with greater large-scale forcing. Conversely, sets of smaller multi-bands may be the result of either a shallow stable layer within which gravity waves propagate and generate vertical motions that may result in precipitation or a more unstable environment suitable for convection to initiate and deform into bands. Storms exhibiting both a large, primary band and smaller multi-bands at the same time are hypothesized to be related to the maturity of the cyclone and its associated air streams. A few cases are examined in more depth to highlight the differences in precipitation modes among storms and within the lifetime of individual storms.