S123 Snowflake Mixtures in Coastal Northeast United States Winter Storms

Sunday, 7 January 2018
Exhibit Hall 5 (ACC) (Austin, Texas)
Levi Lovell, North Carolina State University, Raleigh, NC; and S. E. Yuter, M. A. Miller, and E. Scott

Heavy snow and ice from cold-season extratropical cyclones in the northeastern United States can shut down major cities for extended periods of time. Snow and mixed-phase precipitation accumulation are influenced by many characteristics including the number and sizes of particles, particle crystal shape, degree of riming, and density.

We use a vertically pointing Micro Rain Radar (MRR) and a multi angle snowflake camera (MASC) to examine storm structure and surface precipitation at Stony Brook University on Long Island in New York. Our data show that snowflake mixtures within these winter storms are diverse and complex. Heavily rimed particles (i.e. graupel) can occur in weakly forced environments and unrimed snowflakes can occur near the low center. Degree of riming varies among snowflakes falling simultaneously as well as within individual snowflake aggregates.

Snow particles that fall to the surface at the same place at the same time often originate from different places within the storm, likely form at different times, and take different paths to the surface. Turbulent eddies, updrafts, and advective processes circulate, disperse, and transport particles. Numerous microphysical pathways contribute to the diversity and complexity of snowflake mixtures observed at the surface.

The radar data reveal generating cells and fall streaks marked by narrow bands of locally increased reflectivity and spectral width. Generating cells originate from instability near cloud-top. Increased reflectivity is typically associated with increased snow rate, but variations in several snowflake characteristics, including aggregation and whether snow is partially melted, can affect reflectivity without changing mass.

Previous work has shown that multiple habits frequently co-occur at the surface. Our data confirm previous work and show that habit and degree of riming can also vary among individual aggregates. This distinction comes from methodology differences as previous work involved collection and observation of snowflake samples on glass plates while our method allows for observation of snowflakes in freefall so aggregate structure is preserved.

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