Thursday, 2 August 2001
FINE-SCALE STRUCTURE AND DYNAMICS OF A HEAVY SNOWBAND
The mesoscale structure and dynamical forcing of a heavy snowband that occurred on December 10, 1997 over Michigan in the northwest quadrant of an extratropical cyclone is presented. Cross-band wind and thermodynamic analyses derived from airborne ELDORA radar, WSR 88-D Level II data, and high resolution dropsonde analyses are combined with analyses from high-resolution MM5 modeling experiments to deduce the key mechanisms forcing vertical motion within the snowstorm. The observations reveal in unprecedented detail the fine-scale structure and sharp transition zones near the trowal and dry slot boundaries, the structure of the low-level jet, and the relationship between the heavy snowfall and vertical motions and these features. Analysis of the vertical motions associated with quasi-geostrophic forcing will be compared with estimates of vertical motions associated with mesoscale forcing. This latter forcing appears to be associated with gravity waves, which in data and simulations, propagate northward from the leading edge of the dry slot across the trough of warm layer aloft. Estimates of vertical motion associated with these apparent wave motions, obtained from analysis of dropsonde wind measurements, will be compared with those obtained from high resolution model simulations.