A Numerical investigation of the organization and interaction of the convective and stratiform regions of tropical squall lines

A set of 13 two-dimensional numerical simulations based on the June 22-23 soundings from the Convection Profonde Tropicale in 1981 (COPT81) experiment in West Africa is used to study the organization and interaction of the convective and stratiform regions of squall-line-type convective systems. The initial wind profiles are characterized by the African easterly jet (AEJ) and the tropical easterly jet (TEJ) located at similar to 3.5 and 14 km, respectively. The physical processes that generate and maintain the mesoscale inflow at the rear of squall-line-type mesoscale convective systems are thereby examined. Horizontal potential temperature gradients generated by a combination of latent heat release in the convective region and unsaturated mesoscale descent, both modulated by evaporation, cause a horizontal pressure gradient and generate horizontal, line-parallel vorticity. The rear inflow is a consequence of these processes. The convective activity induces a significant upscale influence; ahead of the system, the AEJ strength is reduced and the TEJ is enhanced, while in the rear the TEJ is reduced. The velocity perturbation below 4 km, associated with the rear inflow, is the most marked signature in the horizontal momentum change. The effects of ice physics are examined by using a simple parameterization, and the intensity of the AEJ is varied to test its effect on the rear inflow and the longevity of the convective system. Generally, there is an extensive rotor circulation in the cold pool, and the convective region consists of a series of transient convective cells traveling backward, relative to the cold pool, at similar to 10 m sec super(-) super(1) . In many of the simulations, the inflow to the convective-scale downdraft originates ahead of the line, crosses between the transient cells, and contributes to the maintenance of the cold pool and rotor. However, a significant proportion of the cold-pool mass can originate from the midlevel stratiform region, demonstrating that the longevity of the convective system is influenced by a judicious combination of convective and mesoscale processes. The density current mechanism for maintaining the convective region of the squall line is dominant only after 3-4 hr of simulation, while in the initial few hours, the low-level inflow advects through the cooling region. With certain wind profiles, this behavior persists throughout the lifetime of the system and a wavelike, low-level convergence (instead of a densi