A Numerical Investigation of the Organization and Interaction of the Convective and Stratiform Regions of Tropical Squall Lines

Abstract

A set of 13 two-dimensional numerical simulations based on the 22 and 23 June Soundings from the ConvectionProfonde Tropicale in 1981 (COPT81) experiment in West Africa is used to study the organization and interactionof the convective and stratiform regions of squall-line-type convective systems. The initial wind profiles arecharacterized by the African easterly jet (AEJ) and the tropical easterly jet (TEJ) located at about 3.5 km and14 km, respectively. The physical processes that generate and maintain the mesoscale inflow at the rear of squall-line-type mesoscaleconvective systems are thereby examined. Horizontal potential temperature gradients generated by a combinationof 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 consequenceof these processes. The convective activity induces a significant upscale influence; ahead of the system the AEJstrength is reduced and the TEJ is enhanced while in the rear the TU is reduced. The velocity perturbationbelow 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 variedto 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 seriesof transient convective cells traveling backwards relative to the cold pool at about 10 m s1. In many of thesimulations, the inflow to the convective-scale downdraft originates ahead of the line, crosses between thetransient cells and contributes to the maintenance of the cold pool and rotor. However, a significant proportionof the cold-pool mass can originate from the midlevel Stratiform region, demonstrating that the longevity ofthe 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 onlyafter 3-4 h 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-levelconvergence (instead of a density current) organizes the development of new cells. Later stages are typified bya transition to a system having a lowered rear inflow, decreased convective depth, intensity and slope. Thisbehavior is most pronounced for a strong AEJ. The system-scale organization is examined by using Lagrangian conservation properties. First, passive traceranalyses quantify the relative importance of individual transports. Second, the vorticity field is analyzed byusing a nonlinear steady state conservation theorem that, despite being applied to a system containing transientconvective cells, adequately represents the persistent nature of the vorticity dynamics and demomtes thestrong interaction between the convective and stratiform regions. It is demonstrated that the vorticity structure in the COPT81 lines is much more complex than a balancebetween the inflow shear and the vorticity generated in the vicinity of the cold pool mainly because the system-scale (convective and stratiform) baroclinic vorticity generation cannot be neglected. Due to the form of the initial wind profiles, the simulations are directed at tropical squall lines. Nevertheless,important characteristics of midlatitude lines are also evident, such as the transient cellular convective activityand the strong rear inflow. The latter is shown to have an important effect on the convective region by enhancingthe low-level convergence and the mass in the cold pool, thereby promoting a direct scale interaction. In addition,the ambient shear modulates the structure and the vorticity dynamics organizes the entire structure. Generalphysical properties of squall lines are therefore demonstrated.