Numerical Simulation of the Mesa-β Scale Structure and Evolution of the 1977 Johnstown Flood. Part III. Internal Gravity Waves and the Squall Line

Abstract

The interaction between internal gravity waves and a squall line that developed early in the evolution of the 1977 Johnston flood event is studied based on available surface observations and a three-dimensional model simulation of the flood-related mesoscale convective systems (MCSs). Several experimental simulators are carried out to investigate the mechanisms whereby gravity waves form and obtain energy. Both observations and model simulators of the wave/convection interaction fit certain theories of gravity wave propagation. Following the formation of the squall line, subsequent deep convection typically initiates behind a pressure trough associated with the lint and ahead of or along the axis of the trailing ridge. The zero contours of vertical motion correspond closely to the axis of the surface pressure trough. Positive potential temperature perturbations correspond with descending motion occurring ahead of the trough while negative perturbations occur with increasing ascending motion towards the approaching ridge axis. Model airflow trajectories show that the simulated gravity wave surface pressure perturbations (with amplitudes of about 1 mb) correspond to vertical parcel displacements of more than 30 mb. The model simulations indicate that the gravity waves am initiated by a super-geostrophic low-level jet with strong horizontal wind shear over an area where an explosive convective development occurs, and then are enhanced by intense convection. The waves propagate at a speed significantly faster than a meso-α scale quasi-geostrophic wave that is partly responsible for the initial explosive development and that later plays a key role in controlling the evolution of a mesoscale convective complex (MCC). The fag moving gravity waves help the squall line accelerate eastward and separate from a trailing area of convection that later develops into the MCC. It appears that the waves and the squall line interact with each other constructively prior to the squall line's mature stage. Specifically, the line of deep convection seems to provide the waves with energy through enhancing mass convergence/divergence in a deep layer and acting as an ?obstacle? to the sheared flow. The waves tend to help organize convective elements into a line structure and turn the line a little clockwise. After the squall line moves into a convectively less favorable environment, it slows down, whereas the accompanying gravity waves continue their eastward movement. Then the convection and gravity waves gradually become out of phase and interact with each other destructively. Because of the absence of low-level inversions and critical levels to duct the wave propagation, the gravity waves quickly diminish as they move away from the energy source region. Free-wave experimental simulations show many wave characteristics similar to the control simulation, indicating that the gravity waves determine the orientation, propagation and structure of the squall line. A sea breeze circulation and mountain waves associated with the Appalachians also occur in the model simulation, but do not seem to have a significant effect on the evolution of the daytime deep convection. The results indicate that physical interaction between deep convection and internal gravity waves can be simulated by numerical models if a compatible grid resolution, proper model physics and good initial conditions are incorporated. In particular, the apparent relationship between the gravity waves and the squall ling suggests that preserving the components of layered internal gravity waves in the model initial conditions may be very important for successful model prediction of the timing and location of wave-related MCSs.