The Dynamical Structure of Squall-Line Type Thunderstorms

Abstract

The structure of squall-line type thunderstorms is investigated with special emphasis on the upshear sloping updraft which is often observed in these storms. To aid in the investigation, a new version of the convective Richardson number is formed which adds the negative buoyancy due to liquid water loading to the thermal buoyancy of the lifted parcel in the calculation of the buoyant energy. This modified convective Richardson number is easily calculated using a pre-storm sounding and has predictive value for thunderstorm type. While it is widely accepted that the updraft's upshear slope is caused by the ascending parcels partially conserving their horizontal momentum toward the rear of the storm, this paper presents a new theory for the production of this slope. In this theory, the vorticity production due to the liquid water distribution leads to an upshear slope of the updraft/downdraft interface. A stable slope is reached when the loading mechanism is balanced by the forces exerted by the environmental shear. A two-dimensional numerical model is developed which is shown to be capable of reproducing many of the observed features of squall-line type thunderstorms, including the upshear sloping updraft. The internal structure of these storms is investigated with both Boussinesq and anelastic versions of the model. Through comparative simulations, the importance of liquid water loading and evaporative cooling is demonstrated.