The Initiation and Organization of Convective Cells atop a Cold-Air Outflow Boundary

Abstract

On 13 July 1986 a cold-air outflow from thunderstorms over Illinois and Missouri propagated through the MIST (Microburst and Severe Thunderstorm) network over northern Alabama. The study of this outflow is important since the gust front was solely responsible for the initiation of numerous convective cells. Previous studies have documented the initiation of convection due to colliding gust fronts. In addition, there was a pronounced mesoscale organization of the cells atop the outflow boundary. This was most likely due to a combination of Kelvin?Helmholtz (K?H) and internal gravity (IG) wave activity. In contrast to previous cases, the K?H wave crests were oriented nearly perpendicular to the gust front within the analysis area. The resulting intersections between the circulations associated with the K?H waves and the gust front produced favorable locations for the initiation of convection. Subsequently, the convective cells remained along the updraft side of the K?H wave circulations as they propagated back relative to the gust front. In addition, the gust front induced IG waves that were oriented parallel to the gust front. The enhanced upward motions associated with the IG waves resulted in a periodic arrangement of the convective cells along the updraft side of the K?H waves. The combined motion of the K?H and IG waves was consistent with the cell movement atop the cold-air outflow.