Observations of gravity wave breakdown into ripples associated with dynamical instabilities (original) (raw)

Wave breaking signatures in OH airglow and sodium densities and temperatures: 1. Airglow imaging, Na lidar, and MF radar observations

Journal of Geophysical Research, 1997

The Collaborative Observations Regarding the Nightglow (CORN) campaign took place at the Urbana Atmospheric Observatory during September 1992. The instrumentation included, among others, the Aerospace Corporation narrowband nightglow CCD camera, which observes the OH Meinel (6-2) band (hereafter designated OH) and the O2 atmospheric (0-1) band (hereafter designated 02) nightglow emissions; .... ,•,,.•.Ly/L,•..•Lu• lidar; and the University of Illinois MF radar. Here we report on observations of small-scale (below 10-km horizontal wavelength) structures in the OH airglow images obtained with the CCD camera. These small-scale structures were aligned perpendicular to the motion of 30-to 50-km horizontal wavelength waves, which had observed periods of about 10-20 min. The small-scale structures were present for about 20 min and appear to be associated with an overturned or breaking atmospheric gravity wave as observed by the lidar. The breaking wave had a horizontal wavelength of between 500 and 1500 km, a vertical wavelength of about 6 km, and an observed period of between 4 and 6 hours. The motion of this larger-scale wave was in the same direction as the •30-to 50-km waves. While such small-scale structures have been observed before, and have been previously described as ripple-type wave structures [Taylor and Hapgood, 1990], these observations are the first which can associate their occurrence with independent evidence of wave breaking. The characteristics of the observed smallscale structures are similar to the vortices generated during wave breakdown in three dimensions in simulations described in Part 2 of this study [Fritts et al., this issue]. The results of this study support the idea that ripple type wave structures we observe are these vortices generated by convective instabilities rather than structures generated by dynamical instabilities. 1. Illinois at Urbana-Champaign. Copyright 1997 by the American Geophysical Union. Paper number 96JD02619. 0148-0227/97/96JD-02619509.00 b, c]. Much of the work described in the literature up to now has focused on descriptions of the wave characteristics and speculation about the sources of these ripple structures. Until recently, the best explanation for these ripple structures was that they are dynamical instabilities due to short-lived velocity shears generated in situ by the chance combination of wind and wave motions [Taylor and Hapgood, 1990; Taylor et al., 1995a].

Evidence of a gravity wave breaking event and the estimation of the wave characteristics from sodium lidar observation over Fort Collins, CO (41°N, 105°W)

Geophysical Research Letters, 2007

On the night of December 3rd, 2004 (UT day 338), we observed a significant acceleration of horizontal wind near 100 km between 0900 and 0915 UT accompanied by a temperature cooling at the same altitude and warming below it. The Lomb spectrum analysis of the raw dataset revealed that a gravity wave with 1.5 hr period was significant between 0500 and 0900 UT, but blurred after 0900 UT, suggesting the transfer of wave energy and momentum from wave field to mean flow. Most likely, this observed phenomenon is due to the breaking of an upward propagating gravity wave with an apparent period of $1.5 hr. Using linear saturation theory and assuming a monochromatic wave packet, we estimated the characteristics of breaking gravity wave, eddy diffusion coefficient, and a simple relation between Prandtl number and turbulence localization measure when the wave is breaking, from the experimentally determined heating rate, horizontal wind acceleration, and background temperature and winds.

Propagation and Breaking at High Altitudes of Gravity Waves Excited by Tropospheric Forcing

Journal of the Atmospheric Sciences, 1996

A hi--resolution (544 X 80 X 291 grid points at Δx = 380 m) numerical solution of a three dimensional gravity wave model shows spectacular trends at mesopause altitudes in an evolving wavefield that is forced from tropospheric levels. The lower boundary forcing approximates the effect of a narrow, 2D squall line; with characteristic zonal and time scales of 2 km and 1 h, respectively. The maximum forcing occurred at 2 h and at 45 km (the zonal domain ranged from --100 ≤ x ≤ 100 km). To save computer resources, the problem was executed in 2D until 120 minutes of the physical time. At 120 minutes, the 3D domain was created by repeating the solution in the spanwise direction y, and seeding the buoyancy field with a small amplitude (1% of the basic state) white noise. This seeding procedure was found to be satisfactory based upon an earlier comparative study at lower resolution Δx = 625 m). The full 3D solution was computed up to a physical time of 180 minutes. Additional details of the computation, run on a massively parallel 512 processor Cray T3E, may be found in . The basic state was one of uniform zonal wind (uo = --32 ms --1 ) and stability (Brunt--Vaisalla frequency N = 0.020 s --1 ); and constant density scale height H = 6.63 km). Through a dispersive mechanism this basic state and forcing favor the development of a monochromatic (with fundamental zonal wavenumber ko ~ 0.40 km --1 ), 2D primary (convective) instability with near unit aspect ratio . Just prior to the onset of wave overturning, higher harmonics of the fundamental become excited, and within several Brunt--Vaisalla periods, significant energy occurs at all wavenumbers, k ≥ ko . Subharmonic excitations also occur prior and subsequent to the primary instability such that significant energy is located at k ≤ ko. Ultimately, vigorous wavebreaking produces

The nonlinear mechanism of gravity wave generation by meteorological motions in the atmosphere

Journal of Atmospheric and Terrestrial …, 1995

Using asymptotic expansions of the hydrodynamic equations in the Rossby number and the method of multiple time scales, we derive approximate expressions for the inhomogeneous "forcing" terms which describe the continuous generation of inertio-gravity waves by quasi-geostrophic motions. As a result of numerical modelling applied to the evolution of tropospheric meso-and macro-scale wave sources, the values of these forcing terms are estimated. A three-dimensional numerical simulation of wave propagation from a mesometeorological tropospheric eddy into the upper atmosphere was done to estimate the gravity wave response to the sources described. The results of the calculations show that the most part of the wave energy propagates quasi-horizontally carried by two-dimensional inertio-gravity waves. At the same time, a part of the energy is transported into the upper atmosphere by internal-gravity waves which can create regions of wave disturbance in the upper atmosphere at considerable distances from the source site. The amplitudes of these waves increase with increasing intensity and decreasing time scales of the wave sources and can reach the values observed in the upper atmosphere.

On the possibility of in situ shear excitation of vortical perturbations and their coupling with short-period gravity waves by airglow and ionosphere observations

Journal of Atmospheric and Solar-Terrestrial Physics, 2002

The essential part of in situ shear excited vortical perturbations (shear waves) at mesosphere-thermosphere heights is considered by use of airglow, from Abastumani (42:8 • E, 41:8 • N), and ionospheric, from Tbilisi (44:8 • E 41:7 • N), observations. One of the characteristic processes for excitation of vortical perturbations (shear waves) is a phenomenon of its possible transformation into short-period atmospheric gravity waves (GWs) with a frequency close to the Brunt-V ais al a frequency (BVf). More convenient condition for shear excitation of vortical perturbations and their transformation into short-period GWs at mesosphere-thermosphere heights appears on magnetically disturbed days and during the mid-latitude springtime transition. Nighttime observations of intensities of the hydroxyl OH(8-3) band, the oxygen green 557.7 and red 630:0 nm lines from Abastumani show a possible presence of shear excited vortical perturbations and an accumulation of wave energy by short-period GWs with frequencies close to BVf on magnetically disturbed days and during springtime transition. The amplitude ampliÿcation of short-period gravity waves is considered as a possible indicator of regional and planetary scale changes of the horizontal wind in the upper atmosphere.

Wave breaking signatures in sodium densities and OH nightglow: 2. Simulation of wave and instability structures

Journal of Geophysical Research, 1997

Measurements of atmospheric structure and dynamics near the mesopause were performed using a sodium lidar, an MF radar, and a nightglow CCD camera during the CORN campaign performed in central Illinois during September 1992. The major features of the observed structure on September 27/28 include a low-frequency, large-scale wave accounting for persistent overturning of the temperature and sodium density fields, superposed higher-frequency motions, smallscale transient ripples in the nightglow images suggestive of instability structures, and large-scale wind shear near the height of apparent instability. We describe four simulations of wave breaking with a three-dimensional model designed to assist in the interpretation of these observations. Two simulations address the instability of a low-frequency wave in a background shear flow with and without higher-frequency modulation. These show higher-frequency motions to be important in assigning the spatial and temporal scales of instability structures. Two other simulations examine the instabilities accompanying a convectively unstable inertia-gravity wave with and without higher-frequency modulation without mean shear. These show the instability structure to remain aligned in the direction of wave propagation, with only weak influences by the high-frequency motion. Our results suggest that instability due to a superposition of waves accounts best for the nightglow features observed during the CORN campaign and that streamwise convective instabilities observed due to wave breaking at higher intrinsic frequencies continue to dominate instability structure for internal waves for which inertial effects are important.

Airborne measurements of gravity wave breaking at the tropopause

Geophysical Research Letters, 2003

1] A breaking atmospheric gravity wave was investigated with a combination of airborne in-situ dynamical measurements and ground-based VHF radar observations. The wave was generated by flow over mountains and it was observed to break near the tropopause. The measurements reveal that the wave was overturning at the tropopause and that the initial breakdown into turbulence involved the generation of smaller oscillations with a horizontal wavelength of around 500 m. There was also evidence that the turbulence associated with the wave breaking can cause substantial mixing in the tropopause region. INDEX TERMS: 0341 Atmospheric Composition and Structure: Middle atmosphere-constituent transport and chemistry (3334); 3362

On the nonlinear evolution of wind-driven gravity waves

Physics of Fluids, 2004

We present a study of wind-driven nonlinear interfacial gravity waves using numerical simulations in two dimensions. We consider a case relevant to mixing phenomenon in astrophysical events such as novae in which the density ratio is approximately 1:10. Our physical setup follows the proposed mechanism of Miles [J. Fluid Mech. 3, 185 (1957)] for the amplification of such waves. Our results show good agreement with linear predictions for the growth of the waves. We explore how the wind strength affects the wave dynamics and the resulting mixing in the nonlinear stage. We identify two regimes of mixing, namely, the overturning and the cusp-breaking regimes. The former occurs when the wind is strong enough to overcome the gravitational potential barrier and overturn the wave. This result is in agreement with the common notion of turbulent mixing in which density gradients are increased to diffusion scales by the stretching of a series of vortices. In the latter case, mixing is the result of cusp instabilities. Although the wind is not strong enough to overturn the wave in this case, it can drive the wave up to a maximum amplitude where a singular structure at the cusp of the wave forms. Such structures are subject to various instabilities near the cusp that result in breaking the cusp. Mixing then results from these secondary instabilities and the spray-like structures that appear as a consequence of the breaking.

Wavelet analysis and the governing dynamics of a large-amplitude mesoscale gravity-wave event along the East Coast of the United States

Quarterly Journal of the Royal Meteorological Society, 2001

Detailed diagnostic analyses are performed upon a mesoscale numerical simulation of a well-observed gravity-wave event that occurred on 4 January 1994 along the East Coast of the United States. The value of using wavelet analysis to investigate the evolving gravity-wave structure and of using potential vorticity (PV) inversion to study the nature of the flow imbalance in the wave generation region is demonstrated. The cross-stream Lagrangian Rossby number, the residual in the nonlinear balance equation, and the unbalanced geopotentialheight field obtained from PV inversion are each evaluated for their usefulness in diagnosing the flow imbalance. All of these fields showed clear evidence of strong imbalance associated with a middle-to-upper tropospheric jet streak, and tropopause fold upstream of the large-amplitude gravity wave several hours before the wave became apparent at the surface. Analysis indicates that a train of gravity waves was continuously generated by geostrophic adjustment in the exit region of the unbalanced upper-level jet streak as it approached the inflection axis in the height field immediately downstream of the maximum imbalance associated with the tropopause fold. A split front in the middle troposphere, characterized by the advance of the dry conveyor belt above the warm front, was overtaken by one of these propagating waves. During this merger process, a resonant interaction resulted, which promoted the rapid amplification and scale contraction of both the incipient wave (nonlinear wave development) and the split front (frontogenesis). The gravity wave and front aloft became inseparable following this merger. The situation became even more complex within a few hours as the vertical motion enhanced by this front-wave interaction acted upon a saturated, potentially unstable layer to produce elevated moist convection. An analysis of the temporal changes in the vertical profile of wave energy flux suggests that moist convective downdraughts efficiently transported the wave energy from the midlevels downward beneath the warm-front surface, where the wave became ducted. However, pure ducting was not sufficient for maintaining and amplifying the waves; rather, wave-CISK (Conditional Instability of the Second Kind) was crucial. This complex sequence of nonlinear interactions produced a long-lived, large-amplitude gravity wave that created hazardous winter weather and disrupted society over a broad and highly populated area. Although gravity waves with similar appearance to this large-amplitude wave of depression occasionally have been seen in other strong cyclogenesis cases involving a jet streak ahead of the upper-level trough axis, it is unknown whether other such events share this same sequence of interactions.