Gravity current and undular bore structures observed within a cold frontal zone and their role in the process of triggering severe convection

Dr. Steve Koch,

Chief, Forecast Research Division,

NOAA Forecast Systems Laboratory, Boulder CO

This study utilized WSR-88D and research Doppler radars, a boundary layer wind profiler, microwave radiometer, mesonetwork, mobile CLASS soundings, and theory to understand the convection initiation process associated with a prefrontal bore wave created by the intrusion of a gravity current-like cold front into a shallow, surface-based stable layer.  A line of severe thunderstorms erupted behind the bore front.  A low-level jetlet accompanying the bore provided an efficient wave trapping mechanism, due to wind curvature effects on the Scorer parameter profile, as well as the effects of low-level shear on the mass convergence field. 
Computations of the effects of the bore and subsequent gravity current passage on the ambient atmosphere show that the strong bore-induced lifting was insufficient to trigger the storms; rather, it was the dual lifting provided by the bore and the gravity current that made it possible for parcels to reach their level of free convection.  These results suggest that lifting sufficient to trigger deep convection must be sufficiently deep or have long duration.