Microphysical evolution within winter snow storms over Long Island, NY

Predicting the magnitude and location of heavy snow bands is a challenging forecast problem over the Eastern U.S. There has been recent progress in our knowledge of the dynamical evolution of these snow bands, but only limited research on the microphysics within East Coast winter storms. Only limited comparisons of snow habit and degree of riming have been made between observations and the microphysics of mesoscale models using bulk microphysical parameterizations. The goal of this research is to determine the microphysical evolution and the factors controlling it during several winter storms over Long Island, NY during the 2009-2010 winter season. This research compares the Weather Research and Forecast (WRF) model simulations of snow size distribution, riming degree, fall speeds, and snow density with surface observations at Stony Brook, NY. A vertically-pointing Ku-band radar was used to observe the vertical profile of reflectivity and Doppler velocity of the winter storms 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. One snow event on 19-20 December 2009 produced widespread snowfall (48.3 cm) over Long Island and included a heavy snow band. The dominant crystal types varied throughout the event. During the early stages of the event, dendritic and cold-type crystals were dominant with some riming. As precipitation areal coverage increased, a transition to more needles and columns was observed. During snow band maturity, dendritic and stellar crystals were dominant, with moderate riming, suggestive of more intense vertical motions aloft. A transition to sector and plates-like crystals occurred as the band moved out 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. In addition, the microphysical observations will be compared to model output from a few different WRF microphysical schemes.