Atmospheric Conditions and Processes Associated With Different Degrees of Snowflake Riming

Atmospheric Conditions and Processes Associated With Different Degrees of Snowflake Riming

Spencer Rhodes, Jason Endries

Dr. Sandra Yuter

Cloud Precipitation Processes and Patterns Group

Department of Marine, Earth, and Atmospheric Sciences

North Carolina State University

The sizes and structures of ice crystals span a wide range as a result of various combinations of conditions and processes within clouds. This research focuses on rimed ice particles and the environments in which they form. Part of the information needed to predict snowfall accumulation is the degree of riming ice crystals since heavily rimed particles like graupel have a higher density than lightly rimed aggregates. We use data sets obtained from the University of Utah's Multi-Angle Snowflake Camera and two NCSU vertically-pointing MicroRainRadars (MRRs) at the Alta Ski Resort in Utah. Surface meteorological observations are also recorded throughout the Alta Ski Resort. High resolution images of falling snowflakes are obtained from the Multi-Angle Snowflake Camera. These images are of sufficient detail to determine several key characteristics of each snow particle, including size, shape, orientation, fall speed and a complexity index which is related to the degree of riming. The data from the two MRRs are combined to obtain very high spatial resolution data near the surface and coarser spatial resolution data aloft of radar reflectivity, Doppler velocity and spectral width. Preliminary work has indicated that nearly all the images of snow particles obtained at Alta show that the ice is rimed to some degree. Further, the magnitude of spectral width, a proxy for turbulence, is often similar near the ground and aloft within the storm. Our work will examine particle characteristics at the surface in the context of radar-column-observed conditions and surface meteorology in order to better understand where and when riming occurs within storms. More detailed information on the environmental conditions under which various degrees of riming occur is an important step in producing more accurate snowfall forecasts.