Complex Aggregates within Coastal Northeast US Snow Storms

The characteristics of snowflakes observed at the surface provide information to make inferences about the dynamic and thermodynamic structures within winter, extratropical cyclones. A multi-angle snowflake camera (MASC) has collected images of snowflakes in freefall, from multiple angles simultaneously, since January 2015 at Stony Brook, NY on the north shore of central Long Island. These images provide information to assess the size, habit(s), degree of riming, and many other properties of the snowflakes in situ. To date, the MASC has captured more than one million images at Stony Brook.

Observations from the MASC have revealed new insights on the diversity and complexity of snow aggregates. At Stony Brook, aggregates are a common if not predominant mode of snowfall. In contrast to idealized clusters of dendrites, these aggregates show a wide diversity of components. We frequently observe aggregates composed of multiple crystal habits. Furthermore, aggregate components are often not rimed uniformly. We sometimes see aggregates composed of rimed and unrimed crystals. We also observe partially melted aggregates and aggregates that have partially melted and then refrozen.

This diversity reflects complex microphysical pathways. Aggregates are composed of crystals originating and growing at different times and places within the cyclone. Three dimensional winds and layers of higher turbulence within a storm produce differences in snow advection, growth, and riming; factors that affect not just the characteristics of snow at the surface but also the diversity of the characteristics and habits observed at the surface.

Prior work has used a Lagrangian framework relative to the low-pressure center to relate the spatial patterns of snow habit and degrees of riming to temperature and vertical motion patterns within the northwest and northeast sectors of developing and mature cyclones. We also used a Lagrangian framework to provide insights on storm processes and structures that influence microphysical processes and how these influences change as a storm evolves. We coupled MASC observations with operational scanning radar (WSR-88D) data and vertically pointing radar (Metek Micro Rain Radar) data to put MASC observations into context with the three dimensional storm environment.

MASC observations of large numbers of flakes and the derived information on the distributions of a variety of snow characteristics should inform efforts to retrieve snow mass flux using remote sensing. These data also provide a benchmark for evaluating model microphysics schemes.

Figure 1. Images of a complex snow aggregate captured simultaneously from multiple angles, 30° apart, by a MASC at Stony Brook, NY on 9 January 2015 at 14:00:58 UTC. The maximum dimension of the snowflake is approximately 11 mm.