Showing papers by "David C. Fritts published in 1982"

Shear Excitation of Atmospheric Gravity Waves

Abstract: Unstable Velocity shears are a Common source of vertically propagating gravity waves in the atmosphere. However, the growth rates of unstable modes predicted by linear theory cannot always amount for their observed importance. We examine in this paper, using a numerical model, the nonlinear excitation and evolution of atmospheric gravity waves. It is found that such waves can reach large amplitudes and induce significant accelerations of the mean velocity profile, resulting in shear stabilization and jet formation. Unstable modes that are vertically propagating above and below the shear layer may, when growing in isolation, achieve a state of quasi-sustained radiation. The nonlinear excitation of vertically propagating gravity waves via the interaction of two KH modes is found to be very rapid, providing an explanation for their occurrence in the atmosphere.

The transient critical‐level interaction in a Boussinesq fluid

Abstract: A numerical model was used to examine the consequences of transience and nonlinearity in the critical-level interaction of an internal gravity wave. Wave packets in a shear flow were observed to evolve as predicted by the linear, initial-value results of Booker and Bretherton (1967) until convectively unstable layers evolved. Once formed, such layers were found to break down via a convective instability, presumably resulting in the turbulent dissipation of the incident wave packet. Several simulations were used to illustrate the transient stabilization of, and the Eulerian mean flow accelerations induced by, critical-level interactions. Other simulations were performed to examine the evolution of a wave packet in a time-dependent shear flow. The latter suggest that critical-level absorption and wave action dissipation can be either greatly accelerated or effectively eliminated depending upon the tendency of the mean velocity shear. The implications of these findings for internal gravity wave propagation in the atmosphere and the oceans are discussed.

Laboratory and numerical studies of internal gravity wave propagation in a sheared environment

Abstract: During the last two decades, internal gravity waves have been recognized as an important feature in atmospheric and oceanic dynamics. In both fluids, internal gravity waves account for much of the variability in observed velocity and density profiles and play a role in a variety of processes. However, wave propagation, and therefore wave effects, in the atmosphere and the oceans, are profoundly affected by local velocity shear. In this paper, we examine various aspects of the interaction of internal gravity waves with shear using a numerical model and a laboratory shear flow apparatus.