An Observational Study of Atmospheric Bore Formation from Colliding Density Currents

Abstract

Observations from east-central Florida during the Convection and Precipitation/Electrification (CaPE) experiment are used to investigate the factors that influence atmospheric bore formation from colliding density currents. Ten cases involving the collision of a gust front with a sea-breeze front are analyzed with Doppler radar and sounding and surface mesonet data. The gust fronts in these collisions were generally deeper, denser, and faster propagating than their sea-breeze-front counterparts. Seven of the 10 cases produced dual boundaries that moved away from each other in a relative sense after the collision. Post-collision boundaries moving in the same direction and oriented similarly to the pre-collision gust front occurred in all 10 cases. They transported mass in the manner of a density current in six cases, while the others behaved more like bores or bore–density current hybrids, as they were characterized by little or no mass transport. Similarity of gust-front and sea-breeze-front propagation speed prior to the collision was the best indicator of bore character in post-collision boundaries induced by gust fronts. Sea-breeze-front-induced post-collision boundaries occurred in the seven cases in which the pre-collision sea-breeze front was nonstationary. These post-collision boundaries exhibited no purely mass transport behavior and were all categorized as bores or bore/density current hybrids. The potential ability to predict dual zones of convergence emanating from boundary collisions as demonstrated in this study may be of value to convective nowcasting in Florida and perhaps other locales, as 70% of the observed post-collision boundaries initiated new convection or enhanced existing convection.