Numerical Simulation of the Meso-Beta Scale Structure and Evolution of the 1977 Johnstown Flood. Part 3. Internal Gravity Waves and Squall Line

摘要

The interaction between internal gravity waves and a squall line that developed early in the evolution of the 1977 Johnstown 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 simulations are carried out to investigate the mechanisms whereby gravity waves form and obtain energy. Both observations and model simulations 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 line 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. 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 line 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. Reprints. (JHD)