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

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 towar...