Numerical Simulation of a Snowfall Event with Mesoscale Snowbands to the Northeast and Northwest of the Surface Low: March 30th 2009

Mesoscale snow banding has most often been observed in the northwest quadrant of a surface low. The atypical event presented here features banded snowfall in both the northwest and northeast quadrants of a developing cyclone. This type of banded snowfall evolution was rare in a parent study of the distribution of banded snowfall relative to the surface cyclone over a 5 year period. 24-hour snowfall totals exceeded a foot of snow in southern North Dakota in the northeast quadrant, with similar snowfall totals in eastern South Dakota in the northwest quadrant over the next 24 hours. Using RUC model data, the basic processes that lead to mesoscale banding were observed to evolve from the northeast to the northwest of the cyclone. These processes include the reduction of stability observed through negative saturated equivalent potential vorticity, and large and mesoscale lift seen as isentropic upglide and midtropospheric frontogenesis. The evolution of midtropospheric deformation is of particular interest in this event, as the transition of the upper-level trough from an open wave to a closed circulation determines the orientation of the dilatation axis, and thus influences the magnitude and location of frontogenesis. As the spatial and temporal resolution of the RUC model can be coarse relative to some of the features important in the evolution of the events, a WRF (Weather Research and Forecasting) model run of the event has been completed. The ability of the WRF model to represent the physical processes relevant to the event is assessed. Output from the model is used to calculate trajectories in order to analyze how the warm, cold, and dry conveyor belts came together to produce the unique evolution of the banded snowfall.