Short-Term Convective Mode Evolution along Synoptic Boundaries

This paper investigates the relationships between short-term convective mode evolution, the orientations of vertical shear and mean wind vectors with respect to the initiating synoptic boundary, the motion of the boundary, and the role of forcing for ascent. The dominant mode of storms (linear, mixed mode, and discrete) was noted 3 h after convective initiation along cold fronts, drylines, or prefrontal troughs. Various shear and mean wind vector orientations relative to the boundary were calculated near the time of initiation. Results indicate a statistical correlation between storm mode at 3 h, the normal components of cloud-layer and deep-layer shear vectors, the boundary-relative mean cloud-layer wind vector, and the type of initiating boundary. Thunderstorms, most of which were initially discrete, tended to evolve more quickly into lines or mixed modes when the normal components of the shear vectors and boundary-relative mean cloud-layer wind vectors were small. There was a tendency for storms to remain discrete for larger normal shear and mean wind components. Smaller normal components of mean cloud-layer wind were associated with a greater likelihood that storms would remain within the zone of linear forcing along the boundary for longer time periods, thereby increasing the potential for upscale linear growth. The residence time of storms along the boundary is also dependent on the speed of the boundary. It was found that the boundary-relative normal component of the mean cloud-layer wind better discriminates between mode types than does simply the ground-relative normal component. The influence of mesoscale forcing for ascent and type of boundary on mode evolution was also investigated. As expected, it was found that the magnitude and nature of the forcing play a role in how storms evolve. For instance, strong linear low-level convergence often contributes to rapid upscale linear growth, especially if the boundary motion relative to the mean cloud-layer wind prevents storms from moving away from the boundary shortly after initiation. In summary, results from this study indicate that, for storms initiated along a synoptic boundary, convective mode evolution is modulated primarily by the residence time of storms within the zone of linear forcing, the nature and magnitude of linear forcing, and secondarily by the normal component of the cloud-layer shear.