Abstract
Mid-latitude squall lines are most often observed in the warm sector of a developing or incipient baroclinic wave somewhat in advance of, and parallel to, the surface cold front. An important, yet poorly understood problem in mesoscale dynamics concerns the manner in which the deep convection in such systems is organized. Here we test the idea that the process of frontogenesis may lead naturally to the generation of a packet of internal gravity waves whose group velocity is normal to the frontal surface. We hypothesize that this wave packet may in turn be responsible for providing the convergence necessary to initiate severe convection along its path of propagation. For the most part, the idea is developed analytically within the context of a highly simplified atmospheric model. We employ the non-linear, cross-front geostrophy solutions of Hoskins and Bretherton (1972) to determine the temporal evolution of the unbalanced ageostrophic flow in the region of high baroclinicity. These unbalance terms are then used to force a correction to the Hoskins-Bretherton solution, a correction which consists of a packet of internal waves. A model calculation is performed to determine the magnitude and form of the pressure fluctuations associated with the wave field. This calculation suggests the existence of a local region of horizontal convergence at a distance of approximately 75–125 km into the warm air in advance of the front.
