Simulations of Multi-bands in the Comma Head of Northeast U.S. Winter Storms

Mesoscale precipitation structures within Northeast U.S. (NEUS) winter storms result in heterogeneous spatial and temporal snowfall throughout the region during any one particular storm. There have been several successful modeling studies of single-banded snowbands in the comma head, but no organized effort to investigate the ability of the mesoscale models to properly simulate multi-banded events. Multi-bands are defined as > 3 fine scale (5–20 km width) bands with periodic spacing and similar spatial orientation, with intensities > 5 dBZ over the background reflectivity maintained for at least 1 h. The goals of this study are to understand some of the ambient conditions (stability, lift, and forcing) that favor different multi-band band spacing, duration, and mergers across the region from NYC to Boston, and to address whether high resolution simulations from the Weather Research and Forecasting (WRF) model can realistically simulate multi-bands using different initial conditions and physics.

The bands analyzed are within the comma head of extratropical cyclones, which includes 24 cool season storms from October through March 2008-2015. Observations of storm structure from the NEUS coastal WSR-88D radars and Microwave Rain Radar at Stony Brook, NY (north-central Long Island) are used to group the more convective cases into 3 different types: 3-4 well-defined multi-bands within 150 km, finer-scale bands (5+ bands over 150 km), and convective plumes (generating cells) aloft, but no bands at lower levels. A few cases from each type are simulated for 9-24 h down to 1.33-km grid spacing using 4 different analyses (RUC/RAP, NAM, GFS, NARR). Using the “best” IC member from each case, the physics space is explored using different planetary boundary layer (PBL) and microphysical (MP) parameterizations. The “best” IC case is also simulated down to 444 m grid spacing.

Preliminary results suggest that the WRF often struggles to realistically simulate the intensity, spacing, and speed of multi-bands even at 444 m grid spacing. Whether WRF produces realistic bands is strongly related to the different initial and lateral boundary conditions used. The magnitude and duration of individual bands are most directly tied to the MP scheme used. The stability, moisture, and frontogenetical forcing are analyzed for these cases, and the WRF is further verified using radiosonde observations and hourly RUC/RAP analyses. The Northeast U.S. blizzard of 26-27 December 2010 is an exemplary case of multi-banding, and will be shown as a detailed example of the sensitivity of the WRF bands to different ICs and physics.