David C. Fritts

Stratified shear turbulence: Evolution and statistics

The highest-resolution 3D direct numerical simulations to date of Kelvin-Helmholtz instability are reported. The ensuing turbulence spans a broad range of spatial scales and exhibits Kolmogorov spectra in horizontal planes. Non-Gaussian statistics are observed, with highly intermittent entrainment zones at the edges of the resulting shear layer. Profiles of the local gradient-Richardson number are presented and shown to remain less than 1/4 throughout the entire evolution of the turbulence.

Turbulence statistics of a Kelvin-Helmholtz billow event observed in the night-time boundary layer during the Cooperative Atmosphere-Surface Exchange Study field program

Abstract An apparent shear flow instability occurred in the stably stratified night-time boundary layer on 6 October 1999 over the Cooperative Atmosphere–Surface Exchange Study (CASES-99) site in southeast Kansas. This instability promoted a train of billows which appeared to be in different stages of evolution. Data were collected by sonic anemometers and a high-frequency thermocouple array distributed on a 60xa0m tower at the site, and a high resolution Doppler lidar (HRDL), situated close to the tower. Data from these instruments were used to analyze the characteristics of the instability and the billow event. The instability occurred in a layer characterized by a minimum Richardson number Ri∼0.13, and where an inflection in the background wind profile was also documented. The billows, which translated over the site for approximately 30xa0min, were approximately L∼320xa0m in length and, after billow evolution they were contained in a layer depth H∼30xa0m. Their maximum amplitude, determined by HRDL data, occurred at a height of 56xa0m. Billow overturns, responsible for mixing of heat and momentum, and high-frequency intermittent turbulence produce kurtosis values above the Gaussian value of 3, particularly in the lower part of the active layer.

An estimate of strong local body forcing and gravity wave radiation based on OH airglow and meteor radar observations

[1]xa0Airglow measurements of gravity wave and smaller-scale flow features, used together with other measurements of larger-scale winds, provide a unique ability to quantify gravity wave dynamics at mesopause altitudes. We consider here an event observed with an OH airglow imager and the meteor radar at the MU Observatory in Japan. This was a wave breaking event of unusually large amplitude and momentum flux. Our hypothesis is that such events are relatively common, and that the resulting local forcing of the mean flow represents a vigorous source of secondary gravity waves that penetrate well into the thermosphere. Our analysis suggests a gravity wave momentum flux of ∼900 m2s−2, far larger than estimated by other techniques, and a mean flow acceleration of ∼80 ms−1 in less than an hour. We also estimate the scales and frequencies of the secondary waves resulting from this local body forcing.

Numerical modeling of initially turbulent wakes with net momentum

This paper presents results from the first fully three-dimensional direct numerical simulations of initially turbulent wakes with net momentum in unstratified and density stratified fluids. The initial conditions contain a super-position of an initially axisymmetric mean streamwise velocity profile plus a spectrally specified fluctuation velocity field with initially incoherent phases to model initial turbulence. To provide evidence in favor of their validity, we compare results from these simulations with previous measurements behind towed bodies in wind tunnels and towing tanks, and also compare with theories of turbulent wakes. Comparisons with laboratory flow experiments provide agreement, both with statistical quantities and vortex structures and evolution. We subsequently investigate open questions by analysis of the fully three-dimensional flow. Coherent vortices in stratified wakes have their origins in the vortex geometry of the mean wake flow, and do not require stratification or coherent seedin...

Gravity Wave Influences in the Thermosphere and Ionosphere: Observations and Recent Modeling

Observational and theoretical studies have suggested gravity wave propagation and influences in the thermosphere and ionosphere for half a century. Gravity waves contribute, or are believed to contribute, to a variety of neutral and electrodynamic phenomena ranging from vertical coupling, energy and momentum transport and deposition, neutral perturbations and accelerations, traveling ionospheric disturbances, ionospheric irregularities, and plasma instabilities under quiet conditions to strong coupling from high to low latitudes and accompanying electrodynamics under storm-time conditions. Our goals here are to briefly review what has been learned to date, to illustrate some of the more recent results indicative of gravity wave effects, and to identify some aspects of neutral dynamics not previously considered that we expect may also have significant influences on neutral dynamics and electrodynamics in the thermosphere and ionosphere.

The importance of spatial variability in the generation of secondary gravity waves from local body forces

[1]xa0We hypothesized earlier that the zonal mean body force required to close the mesospheric jets is sporadic in time, and is composed of a large number of spatially and temporally localized body forces. To explore the effects of such localization, we randomly generate a series of localized, 3D body forces in the mesosphere which create a mean acceleration of ∼100 m s−1 day−1 over this forcing volume. Secondary waves are also generated, and because they have large vertical scales, phase speeds, and vertical group velocities, they may induce important variabilities in the lower thermosphere where they dissipate. We find that the secondary waves from spatially smoothed body forces have much smaller momentum fluxes, frequencies, and vertical group velocities. Thus, global models having coarse resolution may be missing a significant source of sporadic wave drag and its effect throughout the middle atmosphere and lower thermosphere.

Comparisons of predicted bore evolutions by the Benjamin!Davis!Ono and Navier!Stokes equations for idealized mesopause thermal ducts

[1]xa0Numerical simulations are performed employing two numerical models to contrast nonlinear bore evolutions predicted by the Benjamin-Davis-Ono (BDO) equation with evolutions described by the Navier-Stokes (NS) equations. The first model is a simple one-dimensional solver of the BDO equation; the second describes the nonlinear two-dimensional dynamics of the NS equations. Both utilize the Boussinesq approximation. Owing to their simpler, horizontally isotropic nature, only isolated thermal ducts are considered in this study. Simulations assume an initial long-wave perturbation and address the influences of perturbation amplitude and wavelength, viscosity, and nonzero background stability on the resulting evolutions. Results indicate that the BDO equation provides reasonable predictions of bore character and evolution for conditions that satisfy its underlying assumptions. BDO predictions fail to describe bore character and evolution in cases where either the initial perturbations or the thermal environment differs significantly from BDO assumptions. Predictions employing the NS equations will thus provide more realistic guidance in the interpretation and understanding of bores observed in the mesopause region for general environments.

Application of direct and large-eddy simulation methods to late wakes of submerged bodies

Late wakes of submerged bodies are simulated in support of the ONR mechanics and energy conversion turbulence program. Direct numerical simulations (DNS) is used to examine the dynamics, energetics, and structure of wakes evolving to late times. Particular emphasis is placed on quantifying the structural differences between wakes of towed and self-propelled bodies and on quantifying the influences of environmental shear and stratification. The DNS results clearly show that wakes of towed bodies lead to larger-scale coherent structures at late times. Stratification tends to confine the wake in the vertical direction and provides a mechanism for the radiation of internal waves. Shear is found to interrupt the upscale cascade of vorticity. In addition to DNS, large-eddy simulation (LES) is considered as a means of accessing higher Reynolds numbers and of enabling broader parameter studies. As a first step in this direction, the LES approach is validated by repeating prior DNS on much coarser meshes. A parameter-free dynamic subgrid-scale model is used for this purpose and good results for low-order statistics are obtained on a mesh that is a factor of 4 coarser in each direction as compared with the DNS. When the effect of the increased time step is taken into account, this represents a factor of 4/sup 4/ = 256 reduction in computational expense. Thus LES can greatly reduce the computer time needed for wake studies and can allow significant increases in Reynolds number.

Numerical simulation of late wakes in stratified and sheared flows

We employ high-resolution DNS methods to examine the dynamics, energetics, and structure of submarine wakes evolving to late times. Because of the computational focus of the project, our efforts are twofold: both computational efficiency and numerical accuracy. Thus our codes are highly optimized on the DoD computational platforms, scale linearly with increasing CPUs, and employ methods to achieve high computational efficiency. Emphasis in the wake simulations is placed on quantifying the differences between wakes of towed and self-propelled bodies and the influences of environmental shear and stratification. Results indicate that both source and environmental conditions can profoundly impact wake structure and evolution. Wakes of towed bodies lead to larger-scale coherent structures at late times, stratification confines wakes vertically and enables internal wave radiation, and shear organizes wake vorticity and interrupts the upscale cascade occurring in its absence.