Density Currents in Shear Flows: Effects of Rigid Lid and Cold-pool Internal Circulation, and Application to Squall Line Dynamics

In this paper, the idealized density current models developed in previous studies that contain a rigid lid are evaluated. It is shown, through numerical experiments, that a realistic stable layer placed at the level of the rigid lid acts very much like a rigid lid, in confining the environmental flow to the levels below and in controlling the behavior of density currents that propagate in the flow. When this stable layer is replaced by a neutrally stable layer, the behavior of the density current is very different. These results support the hypothesis that in the atmosphere, the tropopause or a tropospheric inversion layer acts much like a rigid lid in the idealized density current models, and the solutions of these models are applicable to organized quasi-two-dimensional convective systems such as squall lines in the atmosphere. The effects of cold pool internal circulations on density current behavior are also examined. When a single circulation is present initially inside the cold pool, the direction of circulation that gives rise to a smaller shear across the density interface leads to a smoother interface and a much more steady density current head. Large-amplitude eddies develop along the interface when the direction of circulation is reversed. When two layers of shear are initially present inside the cold pool, the flow pattern inside the cold pool tends to be dominated by the circulation of the lower layer. In this case, the behavior of the density interface and density current head depends on more than just the initial cross-interface shear. The overall flow pattern in the density current in which the cold pool circulation contains rearward flows at the ground level bears a close similarity with that found in mature squall line systems, as shown by comparisons with a simulated squall line. When the cold pool in a squall line is defined in terms of the equivalent potential temperature, the solutions of idealized density currents in sheared flows become more readily applicable to squall lines.