Gravity wave propagation in a frontal environment.

Abstract

The purpose of this dissertation is to further explore results obtained by Gall, Williams, and Clark (1988) in a series of numerical frontogenesis experiments. In the course of those simulations, gravity waves were produced which grew in intensity as the fronts (one at each of the two horizontal planes bounding the model domain) increased in strength. Waves formed ahead (upwind) of and directly above the surface front, but not behind it, which suggests a connection with the banded cloud patterns often seen ahead of cold fronts (Fig. 1). In addition, structures resembling standing waves were observed in the "wedge" between the frontal surface and the ground. The question of the asymmetrical distribution of waves relative to the surface front was investigated using the method of ray-tracing. The frontal fields which served as the mean state were obtained from the anelastic model developed by T. L. Clark at NCAR. Hypothetical sources of various frequencies were place in the vicinity of both frontal zones and the energy followed through the fluid. It was found that the energy density of those waves moving toward the center of the model domain (which includes waves moving behind the surface front) decreased with time. This decrease was due mainly to geometrical spreading of the rays and the loss of energy to the mean flow via the action of Reynolds stresses. By contrast, waves moving away from the center of the domain (including waves propagating ahead of the surface front) gained energy from the mean flow and appeared to experience a much smaller ray divergence. The single most important factor in producing this asymmetry was found to be vertical shear of the frontogenesis-induced cross-front circulation. The first attempts to analyze the waves underneath the surface front using linear theory produced a surprising result: If waves impinge upon a horizontal surface at a certain angle a reflected wave of infinite amplitude is produced. It is suggested that this may be a mechanism responsible for the buildup of energy beneath the front (rather than multiple reflections between the front and the ground).