Vertical motions observed in mesoscale winter precipitation bands during PLOWS

A primary scientific objective of the Profiling of Winter Storms (PLOWS) project is to determine the forcing for the mesoscale precipitation bands that often occur in the wraparound deformation zone of mid-latitude cyclones northwest of the cyclone center. Several explanations for the forcing of the bands have been hypothesized, including gravity waves, conditional symmetric instability, and potential instability. One key to delineating between these mechanisms is the measurement of vertical air velocity.

This study utilizes data taken by the University of Alabama-Huntsville's Mobile Integrated Profiling System wind profiler to determine the vertical air motions within the wraparound deformation zone of a cyclone observed on 11 and 12 February 2009 during PLOWS. Vertical air motions are derived from the profiler's Doppler spectra using a technique called the lower bound method and PLOWS rawinsonde wind, temperature, and aircraft microphysical observations. The magnitudes and spatial distributions of the vertical motion are used to deduce the physical processes leading to heavy precipitation in the wraparound region of the storm. For the 11-12 February 2009 case, the calculated vertical air velocities in the wraparound bands are up to 5 ms^-1, with the largest values along the upper level front near the boundary between the deformation zone and the dry slot. The vertical motions were well beyond the theoretical limits expected for the release of conditional symmetric instability and are consistent with potential instability release.