Numerical Simulation of the Long-Range Propagation of Gravity Wave Packets at High Latitudes

We use a 2-D, nonlinear, time-dependent numerical model to simulate the propagation of wave packets under average high latitude, winter conditions. We investigate the ability of waves to propagate large horizontal distances, depending on their direction of propagation relative to the average modeled ambient winds. Wave sources were specified to represent the following: (1) the most common wave parameters inferred from observations of Nielsen et al. (2009) ((18 km λᵪ , 7.5 min period), (2) waves consistent with the average phase speed observed (40 m/s) but outlying horizontal wavelength and period values (40 km λᵪ , 17 min period), and (3) waves which would be subject to strong ducting as suggested by Snively et al. (2013) (25 km λᵪ , 6.7 min period). We find that wave energy density was sustained over large horizontal distances for waves ducted in the stratosphere. Waves traveling against winds in the upper stratosphere/lower mesosphere are more likely to be effectively ducted in the stratosphere and travel large horizontal distances, while waves which escape in the form of leakage are more likely to be freely propagating above 80 km altitude. Waves propagating principally in the direction of the stratopause winds are subject to weaker stratospheric ducting and thus increased leakage of wave energy density from the stratosphere. However, these waves are more likely to be subject to reflection and ducting at altitudes above 80 km based upon the average winds chosen. The wave periods that persist at late times in both the stratosphere and the mesosphere and lower thermosphere (MLT) range from 6.8 to 8 min for cases (1) and (3). Shorter-period waves tend to become trapped in the stratosphere, while longer-period waves can dissipate in the thermosphere with little reflection or trapping. It is suggested that the most common scenario is of partial ducting, where waves are observed in the airglow after they leak out of the stratosphere, especially at large horizontal distances from the source. Stratospheric ducting and associated leakage can contribute to a periodic and horizontally distributed forcing of the MLT

Primary Versus Secondary Gravity Wave Responses at F-Region Heights Generated by a Convective Source

A 2D nonlinear, compressible model is used to simulate the acoustic-gravity wave (AGW, i.e., encompassing the spectrum of acoustic and gravity waves) response to a thunderstorm squall-line type source. We investigate the primary and secondary neutral AGW response in the thermosphere, consistent with waves that can couple to the F-region ionospheric plasma, and manifest as Traveling Ionospheric Disturbances (TIDs). We find that primary waves at z = 240 km altitude have wavelengths and phase speeds in the range 170–270 km, and 180–320 m/s, respectively. The secondary waves generated have wavelengths ranging from ∼100 to 600 km, and phase speeds from 300 to 630 m/s. While there is overlap in the wave spectra, we find that the secondary waves (i.e., those that have been nonlinearly transformed or generated secondarily/subsequently from the primary wave) generally have faster phases than the primary waves. We also assess the notion that waves with fast phase speeds (that exceed proposed theoretical upper limits on passing from the mesosphere to thermosphere) observed at F-region heights must be secondary waves, for example, those generated in situ by wave breaking in the lower thermosphere, rather than directly propagating primary waves from their sources. We find that primary waves with phase speeds greater than this proposed upper limit can tunnel through a deep portion of the lower/middle atmosphere and emerge as propagating waves in the thermosphere. Therefore, comparing a TID\u27s/GWs phase speed with this upper limit is not a robust method of identifying whether an observed TID originates from a primary versus secondary AGW

Thermospheric Dissipation of Upward Propagating Gravity Wave Packets

We use a nonlinear, fully compressible, two-dimensional numerical model to study the effects of dissipation on gravity wave packet spectra in the thermosphere. Numerical simulations are performed to excite gravity wave packets using either a time-dependent vertical body forcing at the bottom boundary or a specified initial wave perturbation. Three simulation case studies are performed to excite (1) a steady state monochromatic wave, (2) a spectrally broad wave packet, and (3) a quasi-monochromatic wave packet. In addition, we analyze (4) an initial condition simulation with an isothermal background. We find that, in cases where the wave is not continually forced, the dominant vertical wavelength decreases in time, predominantly due to a combination of refraction from the thermosphere and dissipation of the packets’ high frequency components as they quickly reach high altitude. In the continually forced steady state case, the dominant vertical wavelength remains constant once initial transients have passed. The vertical wavelength in all simulations increases with altitude above the dissipation altitude (the point at which dissipation effects are greater than the wave amplitude growth caused by decreasing background density) at any fixed time. However, a shift to smaller vertical wavelength values in time is clearly exhibited as high-frequency, long vertical wavelength components reach high altitudes and dissipate first, to be replaced by slower waves of shorter vertical wavelength. Results suggest that the dispersion of a packet significantly determines its spectral evolution in the dissipative thermosphere

The Dynamics of Nonlinear Atmospheric Acoustic-Gravity Waves Generated by Tsunamis Over Realistic Bathymetry

The investigation of atmospheric tsunamigenic acoustic and gravity wave (TAGW) dynamics, from the ocean surface to the thermosphere, is performed through the numerical computations of the 3D compressible nonlinear Navier-Stokes equations. Tsunami propagation is first simulated using a nonlinear shallow water model, which incorporates instantaneous or temporal evolutions of initial tsunami distributions (ITD). Ocean surface dynamics are then imposed as a boundary condition to excite TAGWs into the atmosphere from the ground level. We perform a case study of a large tsunami associated with the 2011 M9.1 Tohuku-Oki earthquake and parametric studies with simplified and demonstrative bathymetry and ITD. Our results demonstrate that TAGW propagation, controlled by the atmospheric state, can evolve nonlinearly and lead to wave self-acceleration effects and instabilities, followed by the excitation of secondary acoustic and gravity waves (SAGWs), spanning a broad frequency range. The variations of the ocean depth result in a change of tsunami characteristics and subsequent tilt of the TAGW packet, as the wave\u27s intrinsic frequency spectrum is varied. In addition, focusing of tsunamis and their interactions with seamounts and islands may result in localized enhancements of TAGWs, which further indicates the crucial role of bathymetry variations. Along with SAGWs, leading long-period phases of the TAGW packet propagate ahead of the tsunami wavefront and thus can be observed prior to the tsunami arrival. Our modeling results suggest that TAGWs from large tsunamis can drive detectable and quantifiable perturbations in the upper atmosphere under a wide range of scenarios and uncover new challenges and opportunities for their observations

Table: Spatial Extents of the Numerical Domains

The spatial extents of the numerical domains

Numerical and statistical evidence for long-range ducted gravity wave propagation over Halley, Antarctica

Abundant short‒period, small‒scale gravity waves have been identified in the mesosphere and lower thermosphere over Halley, Antarctica, via ground‒based airglow image data. Although many are observed as freely propagating at the heights of the airglow layers, new results under modeled conditions reveal that a significant fraction of these waves may be subject to reflections at altitudes above and below. The waves may at times be trapped within broad thermal ducts, spanning from the tropopause or stratopause to the base of the thermosphere (∼140 km), which may facilitate long‒range propagation (∼1000s of km) under favorable wind conditions

Evidence for Horizontal Blocking and Reflection of a Small-Scale Gravity Wave in the Mesosphere

The variations of the horizontal phase velocity of an internal gravity wave, generated by wave “blocking” or “reflection” due to an inhomogeneous wind field, have been predicted theoretically and numerically investigated but had yet to be captured experimentally. In this paper, through a collaborative observation campaign using a sodium (Na) Temperature/Wind lidar and a collocated Advanced Mesospheric Temperature Mapper (AMTM) at Utah State University (USU), we report the first potential evidence of such a unique gravity wave process. The study shows that a small-scale wave, captured by the AMTM, with initial observed horizontal phase velocity of 37 ± 5 m/s toward the northwest direction, experienced a large and increasing headwind as it was propagating in the AMTM field of view. This resulted in significant deceleration along its initial traveling direction, and it became quasi-stationary before it was “reflected” to the opposite direction at later time. The USU Na lidar measured the horizontal wind and temperature during the event, when the wave was found traveling within a temperature inversion layer and experiencing an increasing headwind relative to the wave. The wind agrees well with the expected value for wave blocking suggested by the wave tracing theory, implying the existence of a large horizontal wind gradient that night near the OH layer altitudes. The study indicates the critical role of horizontal winds and their horizontal gradients in determining propagation in vertical and horizontal directions

A Single-Step Route to Robust and Fluorine-Free Superhydrophobic Coatings via Aerosol-Assisted Chemical Vapor Deposition

Robust fluorine-free superhydrophobic films were produced from a mixture of two fatty acids (stearic acid and palmitic acid), SiO2 nanoparticles, and polydimethylsiloxane. These simple and nontoxic compounds were deposited via aerosol-assisted chemical vapor deposition to provide the rough topography required for superhydrophobicity, formed through island growth of the aggregates. The optimum conditions for well-adhered superhydrophobic films produced films with a highly textured morphology, which possessed a water contact angle of 162 ± 2° and a sliding angle of <5°. Superhydrophobicity was maintained after ultraviolet exposure (14 days at 365 nm), heat treatment (5 h at 300 °C and 5 h at 400 °C), 300 tape peel cycles, and exposure to ethanol and toluene (5 h each)

Fabrication of robust superhydrophobic surfaces via aerosol-assisted CVD and thermo-triggered healing of superhydrophobicity by recovery of roughness structures

Artificial self-healing superhydrophobic surfaces have become a new research hotspot because of their recoverable non-wetting performance and practical perspective. In this paper, a superhydrophobic surface was fabricated by aerosol-assisted layer-by-layer chemical vapor deposition (AA-LbL-CVD) of epoxy resins and PDMS polymer films. The obtained samples still showed superhydrophobicity even after long-term exposure to different pH solutions and UV light irradiation as well as great mechanical stability against sandpaper abrasion and double-sided tape peeling. Importantly, due to the shape memory effect of the polymer films, the as-prepared samples could recover the previously crushed micro–nano structures upon heat treatment to make the surface superhydrophobic, showing thermo-triggered healing of superhydrophobicity

Secondary gravity wave generation over New Zealand during the DEEPWAVE campaign

Multiple events during the Deep Propagating Gravity Wave Experiment measurement program revealed mountain wave (MW) breaking at multiple altitudes over the Southern Island of New Zealand. These events were measured during several research flights from the National Science Foundation/National Center for Atmospheric Research Gulfstream V aircraft, utilizing a Rayleigh lidar, an Na lidar, and an Advanced Mesospheric Temperature Mapper simultaneously. A flight on 29 June 2014 observed MWs with horizontal wavelengths of ~80_120ækm breaking in the stratosphere from ~10 to 50ækm altitude. A flight on 13 July 2014 observed a horizontal wavelength of ~200_240ækm MW extending from 20 to 90ækm in altitude before breaking. Data from these flights show evidence for secondary gravity wave (SGW) generation near the breaking regions. The horizontal wavelengths of these SGWs are smaller than those of the breaking MWs, indicating a nonlinear generation mechanism. These observations reveal some of the complexities associated with MW breaking and the implications this can have on momentum fluxes accompanying SGWs over MW breaking regions. ©2017. American Geophysical Union. All Rights Reserved