High-resolution observations of the rapid development and decay of the 29 June 2003 squall line during BAMEX

This study presents unique observations with high spatial and temporal resolution of the complete life cycle of a small and short-lived bow-shaped squall line that developed over north-central Kansas on 29 June 2003 during the Bow Echo and Mesoscale Convective Vortex Experiment. Airborne quad-Doppler radar data yielded detailed measurements of the rear inflow jet, as well as dynamic pressure field retrievals. These observations offer an unprecedented opportunity to examine the forcings responsible for the development, evolution, and descent of the rear inflow jet that formed in the stratiform region behind the 29 June mesoscale convective system. The strengthening of the rear inflow was concurrent with the formation of the bow, as well as with the formation of a region of lower pressure beneath the rearward tilted updraft. Our analysis shows that the rear inflow was primarily forced by the circulation between the bookend vortices at earlier times, and later by the combination of the bookend vortices and the horizontal pressure gradient resulting from variations in the vertical buoyancy distribution.

As the rear inflow descended within the trailing stratiform region, diabatic cooling from sublimation, melting, and evaporation occurred, as precipitation fell through dry air associated with the rear inflow jet. Microphysical observations of ice falling through dry air, acquired during a spiral aircraft descent in the trailing stratiform region, are unique due to their position immediately behind the apex of the bow during its early stage. Ice was observed at temperatures as warm as +7°C, indicating the important role of sublimation in the diabatic cooling at this time and location, and allowing for the downward forcing of the rear inflow from negative buoyancy.