Interactions between Upper and Lower Tropospheric Gravity Waves on Squall Line Structure and Maintenance

Abstract

Using a simplified thermodynamic sounding, and variable vertical wind shear, we investigate the role of gravity waves on the structure and propagation of a simulated two-dimensional squall line. Based on an observed squall line environment, the modeled troposphere has been divided into three distinct thermodynamic layers. These consist of an absolutely stable atmospheric boundary layer (ABL), an elevated well-mixed layer, and an upper tropospheric layer of intermediate stability. We find the mixed layer to have a dual role; it has a reduced stability and thus provides abundant buoyancy for the convective scale updrafts, and it provides an ideal layer to trap mesoβ-scale (20–200 km) wave energy generated in the stable layers. The generated waves thus have a significant and lasting impact on the simulation. We also find this thermodynamic structure to be conducive to both strong surface wind perturbations and long-lived squall lines. Experiments that vary the vertical wind shear profile demonstrate that the most vigorous and long-lived squall lines arise with a deep layer of strong vertical wind shear. This result is dependent on the changes in the phase speed and magnitude of the stable layer waves that occur in the sheared versus nonsheared environments. Without flow, waves generated by an initial heat pulse split into symmetric leftward and rightward moving disturbances. Waves generated in the upper tropospheric stable layer are found to move relative to the lower tropospheric waves resulting in a decoupling of deep tropospheric vertical motion and a decrease in strength of the simulated system. With vertical wind shear, the magnitude of the simulated waves is enhanced and an opportunity for sustained coupling between the upper and lower waves exists. It is shown that the upper and lower tropospheric waves in a sheared environment account for many of the circulation features typically associated with two-dimensional squall lines. A simple mechanism for the rear-to-front middle-level jet and surface wake low is also presented.