A mechanism for convective initiation in advance of squall lines

On 21 June 2003, a squall line traversed Oklahoma during the nighttime hours, continually spawning new convective cells ~10-50 km ahead of the line and its associated gust front. The new cells were sufficiently close to the line to suspect a causal relationship between the new and existing convection, but also far enough away to exclude gust front lifting as an initiation mechanism. Furthermore, the new convection aligned in lines that made an acute angle to the oncoming storm. Thus, there are two tasks to perform: to demonstrate and action-at-a-distance mechanism involving the existing convection, and to explain the orientation of the new cell lines relative to the existing squall line.

Previous work on this and similar cases have identified convectively-excited gravity waves as the action-at-a-distance forcing. These waves can be trapped beneath the forward anvil, and spark the development of shallow cumulus clouds in the lower troposphere as they propagate ahead of the squall line. The organizing mechanism responsible for the orientation of the new cell lines is believed to be horizontal convective rolls that were present during the previous afternoon. The rolls decayed after sunset, but presumably left behind bands of moisture perturbations corresponding to the roll updraft and downdrafts.

We demonstrate the viability of this two-pronged mechanism with the help of idealized modeling. We create and control rolls and their associated moisture bands by introducing a momentum source in the boundary layer, and then permit a squall line to develop and evolve into a mature storm. Environmental conditions such as the vertical shear are manipulated to demonstrate the conditions under which gravity wave trapping is possible. The interaction between these waves and the moisture bands is examined for further insights into the important convective initiation question.