An observed and simulated cloud microphysical perspective of snowbands within coastal Northeast U.S. winter storms

Precipitation bands are important mesoscale features within the comma head of an extratropical cyclone. There has been recent work investigating the structure and dynamical evolution of these bands, but only limited work putting the precipitation evolution in the context of the observed microphysics. Furthermore, only limited comparisons of snow habit, density, fallspeed, and degree of riming within winter storms have been made between observations and mesoscale models using bulk microphysical parameterizations. This presentation will put the observed microphysics within the context of rapidly evolving winter storms over Long Island, NY during the 2009-2011 winter seasons. High resolution (~1 km grid spacing) Weather Research and Forecasting (WRF) model simulations will be compared with the observed snow size distribution, riming degree, fall speeds, and snow density observations at Stony Brook, NY on Long Island. A vertically-pointing Ku-band radar was used to observe the vertical profile of reflectivity and Doppler velocity of the precipitation features as they passed overhead. A PARSIVEL disdrometer was used to obtain particle sizes, fall speeds, and number concentration. A stereo microscope and camera were used to observe the snow crystal habit and degree of riming. Snow crystal identification followed Magono and Lee (1966). Snow depth and snow density were also measured.

The 19-20 December 2009 blizzard produced widespread snowfall (48.3 cm) over Long Island and included a heavy snow band. The dominant crystal types varied throughout the event as the ambient conditions and mesoscale forcing for ascent changed. During the early stages of the event, dendritic and cold-type crystals were dominant with light riming. As precipitation areal coverage increased before snowband development, a transition to more needles and columns was observed. During snowband maturity, dendritic and stellar crystals were dominant, with moderate riming, suggestive of more intense vertical motions aloft. A transition to plates-like crystals and no riming occurred as the band moved east of the observation area and the event concluded. The snow-liquid ratios varied by almost a factor of two during the event from 7:1 early in the event to 13:1 during the snow band. These microphysical changes will be described in the context of the evolving ambient conditions and mid-level frontogenesis within the comma head as the cyclone developed. Although the WRF at 1.33-km grid spacing was able to realistically simulate the primary snowband over Long Island, there were large microphysical differences among the various microphysical schemes, and the many WRF schemes failed to match the observations for riming, fallspeed, and snow number concentrations. The 19-20 December microphysical evolution will also be briefly compared with other winter storm events with different ambient conditions.