The role of conveyor belts in organizing processes associated with heavy banded snowfall

Extensive research on heavy banded snowfall associated with cyclogenetic events has revealed that the development of a trough of warm air aloft (trowal), mid-level frontogenesis, and a region of reduced equivalent potential vorticity are critical in the formation of mesoscale snow bands. The present research involves the investigation of the roles of the warm, cold, and dry conveyor belts as an organizing construct within which to view these features and processes. Simulations of two snowstorms were run using the Penn State-NCAR MM5 model in order to compute trajectories during the 24 h leading up to the snow event. The Air Resources Laboratory's HYSPLIT trajectory model is used with model data to generate ensembles of atmospheric trajectories representative of the three major conveyor belts. These trajectories are used to demonstrate that the interactions between these conveyor belts produce mesoscale regions of reduced stability, deep moisture, and lift.

Two extratropical cyclones (ETC) of varying intensity were simulated using the MM5 model. The first event occurred on 26-27 November 2001 over central and southwestern Minnesota where snowfall exceeded 20 inches. The central pressure of this weak ETC never fell below 998 hPa, after it had reached the occluded stage. In contrast, the second case involved rapid cyclogenesis during 9-10 November 1998. In this case, a record surface low pressure of 964 hPa was observed at Duluth International Airport. Nearly 5 hours of thundersnow were reported in the vicinity of Sioux Falls, South Dakota during the peak of the storm. The investigation of the airstreams associated with these two ETCs has revealed that as the dry conveyor belt overruns the warm conveyor, a region of small positive to negative equivalent potential vorticity is generated. As the warm, moist air associated with the warm conveyor belt advanced poleward a region of mid-level frontogenesis formed within the comma head of the ETC. Furthermore, the warm conveyor belt contributes to the deep moisture seen in the comma head required for enhanced dendritic crystal formation. Trajectory analysis also reveals that air parcels associated with the cold conveyor belt are restricted to the lower troposphere and turn cyclonically around the ETC with modest upward vertical motion. In contrast to classic views of the cold conveyor belt, the present research indicates no evidence of parcels ascending to the mid-troposphere or turning anticycloncially. It will be shown that within the framework of these three conveyor belts, both the spatial and temporal evolution of the features and processes associated with heavy banded snowfall are more easily visualized.