A Model of a Tropical Squall Line Boundary Layer Wake

Abstract

A composite analysis has recently been made of the boundary layer associated with the squall line that moved through the GATE ship array on 12 September 1974. That observational study has motivated a modeling investigation of the recovery of the squall boundary layer. The zero-order model of the growth of an unstable boundary layer, as modified by Lilly, and the general structure entrainment model developed by Deardorff are used in order to simulate the wake.recovery and to make more explicit the factors influencing the evolution of the mixed layer. A procedure is developed for obtaining the two-dimensional fields of mixed layer depth, specific humidity and dry static energy by formulating the model equations relative to the squall system in natural coordinates. The results of this study confirm what others have shown, namely, that in the squall wake, mixed layer growth is inhibited mainly by the downward vertical velocity at the top of the mixed layer. Our analysis indicates a large stability above the mixed layer inversion which also plays a significant role in inhibiting growth. The asymmetrical structure of the mixed layer height behind the squall is well simulated, and reasonable agreement is obtained between predicted and observed thermodynamic fields. Large negative gradients of specific humidity atop the inversion appear.important in explaining the mixed layer drying for this squall line. Modeling results reveal that precipitation evaporation in the squall wake has a significant effect on the mixed layer height and thermodynamic variables.