Showing papers by "David C. Fritts published in 1994"

Gravity wave breaking in two and three dimensions: 2. Three‐dimensional evolution and instability structure

Abstract: A companion paper by Andreassen et al. (this issue) introduced and used a nonlinear, compressible, spectral collocation code to address the relative evolutions of two-dimensional motions obtained in two- and three-dimensional simulations of gravity wave breaking. That study illustrated the effects of instability on the wave field and mean flow evolution and suggested that two-dimensional models are unable to fully describe the physics of the wave breaking process. The present paper examines in detail the structure, evolution, and energetics of the three-dimensional motions accounting for wave instability as well as their associated transports of momentum and heat. It is found that this instability comprises counterrotating vortices which evolve very rapidly within the convectively unstable region of a breaking wave. Instability scales are selected based on wave geometry and vortices are elongated in the streamwise direction (horizontal wavenumber in the spanwise direction) and result in the rapid collapse of superadiabatic regions within the wave field. The resulting spectra show clearly the transition from gravity wave forcing of harmonics of the incident wave to instability onset and evolution. Fluxes of momentum and heat by the instability reveal the manner in which the gravity wave amplitude is constrained and the influences of instability on the wave transports of these quantities. The breakdown of the instability structure and its evolution toward isotropic small-scale structure is the subject of the companion paper by Isler et al. (this issue).

Gravity wave breaking in two and three dimensions: 1. Model description and comparison of two‐dimensional evolutions

Abstract: A nonlinear, compressible, spectral collocation code is employed to examine gravity wave breaking in two and three spatial dimensions. Two-dimensional results exhibit a structure consistent with previous efforts and suggest wave instability occurs via convective rolls aligned normal to the gravity wave motion (uniform in the spanwise direction). Three-dimensional results demonstrate, in contrast, that the preferred mode of instability is a series of counterrotating vortices oriented along the gravity wave motion, elongated in the streamwise direction, and confined to the region of convective instability within the wave field. Comparison of the two-dimensional results (averaged spanwise) for both two- and three-dimensional simulations reveals that vortex generation contributes to much more rapid wave field evolution and decay, with rapid restoration of near-adiabatic lapse rates and stronger constraints on wave energy and momentum fluxes. These results also demonstrate that two-dimensional models are unable to describe properly the physics or the consequences of the wave breaking process, at least for the flow parameters examined in this study. The evolution and structure of the three-dimensional instability, its influences on the gravity wave field, and the subsequent transition to quasi-isotropic small-scale motions are the subjects of companion papers by Fritts et al. (this issue) and Isler et al. (this issue).

Mean Motions and Tidal and Two-Day Structure and Variability in the Mesosphere and Lower Thermosphere over Hawaii

Abstract: An overview of the motion field and an analysis of the tidal and 2-day wave motions observed in the mesosphere and lower thermosphere over the central Pacific from 1 October 1990 through 19 August 1992 is presented. Characteristics and interactions of motions at lower and higher frequencies will be addressed elsewhere. Wind measurements were obtained with an MF radar operating on Kauai, Hawaii (22 deg N, 160 deg W), using the partial reflection drift technique. Results presented in this paper reveal a zonal mean motion reflecting the mesopause semiannual oscillation (MSAO) observed at more equatorial latitudes from approximately January to July, coinciding with the period during which the MSAO and the annual cycle of the zonal mean wind at higher latitudes are in phase. Eastward and westward maxima are 55 m/s below 80 km and 45 m/s near 85 km during the first year, with maxima of 57 and 53 m/s during the second year and evidence of substantial interannual variability. The second MSAO cycle is greatly suppressed in the Hawaiian data due to the reversal of the correlation between this and the annual cycle at higher latitudes from approximately July to December and because the second cycle is weaker climatologically at equatorial latitudes. Significant planetary wave activity is observed during periods of mean eastward motions, and tidal and 2-day motions are found to be large and variable. The maximum diurnal tides were observed during October and November 1990, and February, March, April, July, and August of 1991 and 1992. Maximum 2-day amplitudes occurred during February, July, and August of 1991 and 1992. Significantly, the large diurnal amplitude maximum noted during November 1990 failed to appear the following year, while the February 2-day amplitude maximum declined somewhat in 1992.

Gravity wave breaking in two and three dimensions: 3. Vortex breakdown and transition to isotropy

Abstract: Companion papers by Andreassen et al. (this issue) and Fritts et al. (this issue) introduced a nonlinear, compressible, spectral collocation code and applied it to studies of gravity wave breaking in two and three dimensions. The former showed the two simulations to differ dramatically in the mode of instability and in its implications for the wave and mean flow evolutions. The latter considered in detail the structure and energetics of the instability and its influences via eddy transports of momentum and heat. This paper addresses the instability structure and evolution at late times, focusing specifically on secondary instability, vortex breakdown, and the transition to isotropic structure. These results exhibit several distinct behaviors, depending on the local environment. In the presence of weak environmental shears, vortex breakdown occurs through mutual interactions which cause a gradual nonlinear evolution toward smaller scales of motion. Where wave and mean shears are strong, vortex breakdown is accelerated by dynamical instability processes at small scales which modulate strongly the vortex structures due to wave instability. Spectral results suggest that our simulation has described the transition from two-dimensional laminar wave motions to three-dimensional isotropic small-scale structure.

Three-dimensional evolution of Kelvin-Helmholtz billows in stratified compressible flow

Abstract: The authors present results of an initial study of Kelvin-Helmholtz instability in a stratified shear flow using a three-dimensional, nonlinear, compressible, spectral collocation model. The simulation was performed at an intermediate Reynolds number and significantly extends previous studies of homogeneous and stratified shear flows. The authors` major findings include a secondary instability oriented along the two dimensional velocity field, with counter-rotating vortices arising due to both buoyancy and shear sources of eddy kinetic energy within the billow structures. The vortices occupy the outer regions of the billows and contribute significant eddy transport of momentum and potential temperature within the billows. The resulting KH evolution differs in important respects from the corresponding two-dimensional or unstratified flows. 33 refs., 3 figs.