The Role of Low-Level Convergence and Latent Heating in a Simulation of Observed Squall Line Formation

Abstract

The effects of mesoscale forcing and diabatic heating on the development of convective systems have been investigated using a simplified numerical model to simulate the squall line and the convective system preceding it that occurred over Texas and Oklahoma on 10? 11 April 1979. A simulation run without including latent but showed both systems to be initiated and maintained by convergence produced by larger-scale forcing. The first cloud system formed downwind of the convergence zone that was produced by the confluence of airstreams along a dryline. A cloud front approaching from the west then merged with this dryline, destroying its horizontal gradients through diffusive effects and replacing it with a frontal convergence line that was alinged with the low-level flow. This new configuration was then favorable for the formation of the squall line that developed in the simulation. When latent heat was included the continuous cloud in the first convective system broke down into isolated cells which moved downstream from the convergence zone. In the non-latent heat case, the primary mechanism for providing moisture to this cloud was vertical diffusion from the moist surface layer. When latent heat was added, vertical advection within cell updraft provided a more efficient means to supply moisture to the convective system. In the simulated squall line, latent heat release produced a deeper cloud system while intensifying and maintaining the low-level convergence. However, unlike the earlier system, the squall line did not break into convective cells when latent but was included in the simulation.