Environmental Conditions Promoting Snowbands in 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 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 reflectivity objects 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 ≥ 200 km and mid-sized bands that have lengths < 200 km. Through a combination of objective and 6-hourly subjective classification of structures within the comma head of each case, four modes of precipitation banding in coastal Northeast U.S. winter storms were identified as follows: (1) large primary band only (SINGLE), (2) multi-banded (no primary band, but mid-sized bands, MULTI), (3) both single and multi-banded at the same time (BOTH), (4) non-banded (NONE). The most common mode of banding was BOTH and the least common was SINGLE with 107 and 5 events, respectively. MULTI events (35) was comparable to the number of cases of NONE (46). Events were further sub-classified into being associated with either a developing or mature cyclone. MULTI events were equally likely between both cyclone phases but most often found in the northeast quadrant relative to the cyclone center, whereas BOTH and SINGLE were more likely with mature cyclones and found in the northwest quadrant. NONE was more likely to occur with developing cyclones in the eastern quadrants.

Vertical profiles and cyclone-relative composites calculated from the Climate Forecast System Reanalysis (CFSR) and Climate Forecast System version 2 (CFSv2) were analyzed for all events to compare the environments resulting in the various modes of banding or non-banding. Vertical profiles showed little difference in temperature and available moisture given the common conditions within the comma head of a cyclone. Cyclone-relative composites were analyzed for each classification broken down by cyclone maturity. SINGLE and BOTH were associated with deeper cyclones than MULTI and NONE. There is a larger amplitude trough present in the 700 hPa and 500 hPa geopotential height fields for SINGLE, MULTI, and BOTH that is not evident in the nearly zonal height fields for NONE. A few cases are examined in more depth to highlight the differences in precipitation modes among storms. The SINGLE and BOTH cases exhibited bands in the NW quadrant in an area of enhanced 700-hPa frontogenesis. The MULTI case was one of warm frontal bands that formed poleward of weak mid-level forcing but the NONE case had no such mid-level forcing to concentrate the precipitation into bands. Subtle variations in mesoscale forcing and instability in addition to the differences in larger-scale flow patterns may result in the different types of bands (i.e., multi-bands or single bands).