Finite amplitude gravity waves in the Venus atmosphere generated by surface topography

A two-dimensional, fully nonlinear, nonhydrostatic, gravity wave model is used to study the evolution of gravity waves generated near the surface of Venus. The model extends from near the surface to well above the cloud layers. Waves are forced by applying a vertical wind at the bottom boundary. The boundary vertical wind is determined by the product of the horizontal wind and the gradient of the surface height. When wave amplitudes are small, the near-surface horizontal wind is the zonally averaged basic-state zonal wind, and the length scales of the forcing that results are characteristic of the surface height variation. When the forcing becomes larger and wave amplitudes affect the near-surface horizontal wind field, the forcing spectrum becomes more complicated, and a spectrum of waves is generated that is not a direct reflection of the spectrum of the surface height variation. Model spatial resolution required depends on the amplitude of forcing; for very nonlinear cases considered, vertical resolution was 250 m, and horizontal resolution was slightly greater than 1 km. For smaller forcing amplitudes, spatial resolution was much coarser, being 1 km in the vertical and about 10 km in the horizontal. Background static stability and mean wind are typical of those observed in the Venus atmosphere

The neutral wind “flywheel” as a source of quiet‐time, polar‐cap currents

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94751/1/grl2850.pd

Wave-modified Mean Exothermic Heating in the Mesopause Region

We employ a model of wave-driven OH nightglow fluctuations to calculate the effects of gravity waves on the chemical exothermic heating due to reactions involving odd hydrogen and odd oxygen species in the mesopause region. Using a model based on time means and deviations from those means, it is demonstrated that gravity waves contribute to the time-average exothermic heating. The effect can be significant because the fractional fluctuations in minor species density can be substantially greater than the fractional fluctuation of the major gas density. Our calculations reveal that the waves mitigate the exothermic heating, demonstrating their potential importance in the heat budget of the mesopause region

Group Velocity and Energy Flux in the Thermosphere: Limits on the Validity of Group Velocity in a Viscous Atmosphere

The response to wave forcing of finite duration comprises a transient forerunner and the steady state signal (or simply the signal). It is the latter that carries information on the spectral content of the forcing, and the signal velocity is the velocity at which wave energy flows. To the extent that group velocity is a good measure of the energy flow velocity, the ray‐tracing formalism is a valid description of signal propagation. We have examined vertical group velocities as a measure of vertical energy flow velocity for gravity and acoustic waves propagating into the dissipative lower thermosphere. We find that the effects of dissipation on gravity waves can cause group velocity to become a meaningless measure of the energy flow velocity. When certain terms originating in the diffusion of heat and momentum are neglected, the validity of group velocity can be extended to F region altitudes. For acoustic waves, group velocity can be a good measure of energy flow velocity throughout the lower thermosphere because acoustic waves are far less subject to dissipation

Gravity Wave Propagation in a Diffusively Separated Gas: Effects on the Total Gas

We present a full-wave model that simulates acoustic-gravity wave propagation in a binary-gas mixture of atomic oxygen and molecular nitrogen, including molecular viscosity and thermal conductivity appropriately partitioned between the two gases. Compositional effects include the collisional transfer of heat and momentum by mutual diffusion between the two gases. An important result of compositional effects is that the velocity and temperature summed over species can be significantly different from the results of one-gas models with the same height dependent mean molecular weight (M(z)). We compare the results of our binary-gas model to two one-gas full-wave models: one where M is fixed and fluctuations of M (M′) are zero and the other where M is conserved following parcel displacement (whence M′ is nonzero). The former is the usual approach and is equivalent to assuming that mutual diffusion acts instantaneously to restore composition to its ambient value. In all cases we considered, the single gas model results obtained assuming that M is conserved following parcels gave significantly better agreement with the binary-gas model. This implies that compositional effects may be included in one-gas models by simply adding a conservation equation for M and for the specific gas at constant pressure, which depends on M

Gravity Wave Ducting in the Upper Mesosphere and Lower Thermosphere Duct System

We report on a numerical study of gravity wave propagation in a pair of ducts located in a region where dramatic changes in the airglow most likely associated with ducted wave trains are observed. We examine ducting in an upper mesosphere inversion (INV) and an always present lower thermosphere stable layer (LTD) for a range of phase speeds and horizontal wavelengths characteristic of ducting events. We analyze the propagation and modal structure of ducted waves for backgrounds with increasing realism, starting with a climatological temperature profile where only the LTD is present. In succession, we add the INV based on the work of Smith et al. (2003), climatological winds, and winds in the upper mesosphere based on the work of Smith et al. (2003). We examine ducting for phase speeds between 40 and 100 m s¯¹ and horizontal wavelengths between 20 and 60 km. We find that without winds, only the LTD supports ducting of waves forced from below. When observed winds and temperatures are included, strong ducting is evident in both regions. For waves forced from below, the strongest ducted modes are those with slower phase speeds, and of these the third gravest agree reasonably well with the observed phase speeds and wavelengths, indicating that the observations are consistent with linear ducted waves. For waves forced in the INV, we find an intense and strongly dominant fundamental mode. This is a fast mode having phase speeds ~100 m s¯¹ for a horizontal wavelength of 30 km in the INV and much faster in the LTD. That the fundamental is not seen in Smith et al.’s (2003) observations indicates that the waves were forced from below and that the lowest mode was blocked by an evanescent barrier below the INV. Our results show that the two ducts communicate: the upward extensions of waves ducted in the INV are seen in the LTD. This is particularly significant in the case of in situ forcing, where the fundamentals combine to give amplification exceeding a factor of 10 in the LTD

One-gas Models with Height-dependent Mean Molecular Weight: Effects on Gravity Wave Propagation

Many models of the thermosphere employ the one-gas approximation where the governing equations apply only to the total gas and the physical properties of the gas that depend on composition (mean molecular weight and specific heats) are height-dependent. It is further assumed that the physical properties of the gas are locally constant; thus motion-induced perturbations are nil. However, motion in a diffusively separated atmosphere perturbs local values of mean molecular weight and specific heats. These motion-induced changes are opposed by mutual diffusion of the constituent gases, which attempts to restore diffusive equilibrium. Assuming that composition is locally constant is equivalent to assuming that diffusion instantaneously damps the changes that winds attempt to produce. This is the limit of fast diffusion. In the limit of slow diffusion, gas properties are constant (conserved) following the motion but are perturbed locally by advection. An analysis of the static stability shows that composition effects significantly change the static stability, with greater changes for the slow-diffusion limit than for the fast-diffusion limit. We have used a one-gas full-wave model to examine the effects of wave-perturbed composition on gravity waves propagating through the lower thermosphere. We have augmented the conventional system (fixed gas properties) with predictive equations for composition-dependent gas properties. These equations include vertical advection and mutual diffusion. The latter is included in parameterized form as second-order scale-dependent diffusion. We have found that the fast diffusion implied by locally fixed properties has a significant effect on the dynamics. Predicted temperatures are larger for locally fixed composition than for conserved composition. The simulations with parameterized mutual diffusion gave results that are much closer to the results for conserved gas properties than for fixed properties. We found that the divergence between the fast and slow limits was greatest for fast waves and for colder thermospheres. This is because the propagation characteristics of fast waves are sensitive to changes in the static stability and because compositional gradients are stronger for colder thermospheres. We conclude that future models that use the one-gas approximation for fast waves in the lower thermosphere should include, at minimum, the simplification of conserved rather than fixed properties, especially for colder thermospheres

Acoustic Waves Generated by Gusty Flow over Hilly Terrain

We examine the generation of acoustic waves by gusty flow over hilly terrain. We use simple theoretical models of the interaction between terrain and eddies and a linear model of acoustic-gravity wave propagation. The calculations presented here suggest that over a dense array of geographically extensive sources orographically generated vertically propagating acoustic waves can be a significant cause of thermospheric heating. This heating may account in good part for the thermospheric hot spot near the Andes reported by Meriwether et al. (1996, 1997)

Propagation of Tsunami-Driven Gravity Waves into the Thermosphere and Ionosphere

Recent observations have revealed large F-region electron density perturbations (~100%) and total electron content (TEC) perturbations (~30%) that appear to be correlated with tsunamis. The characteristic speed and horizontal wavelength of the disturbances are ~200 m/s and ~400 km. We describe numerical simulations using our spectral full-wave model (SFWM) of the upward propagation of a spectrum of gravity waves forced by a tsunami, and the interaction of these waves with the F-region ionosphere. The SFWM describes the propagation of linear, steady-state acoustic-gravity waves in a nonisothermal atmosphere with the inclusion of eddy and molecular diffusion of heat and momentum, ion drag, Coriolis force, and height-dependent mean winds. The tsunami is modeled as a deformation of our model lower boundary traveling at the shallow water wave speed of 200 m/s with a maximum vertical displacement of 50 cm and described by a modified Airy function in the horizontal direction. The derived vertical velocity spectrum at the surface describes the forcing at the lower boundary of the SFWM. A steady-state 1-D ionospheric perturbation model is used to calculate the electron density and TEC perturbations. The molecular diffusion strongly damps the waves in the topside (\u3e300-km altitude) ionosphere. In spite of this, the F-region response is large, with vertical displacements of ~2 to 5 km and electron density perturbations of ~100%. Mean winds have a profound effect on the ability of the waves to propagate into the F-region ionosphere

Comparison of Theories for Gravity Wave Induced Fluctuations in Airglow Emissions

A comparison is undertaken of theories for the gravity wave induced fluctuations in the intensity of airglow emissions and the associated temperature of the source region. The comparison is made in terms of Krassovsky\u27s ratio ηE for a vertically extended emission region (ηE is the ratio of the vertically integrated normalized intensity perturbation to the vertically integrated normalized intensity-weighted temperature perturbation). It is shown that the formulas for ηE in the works by Tarasick and Hines (1990) and Schubert et al. (1991) are in agreement for the case of an inviscid atmosphere. The calculation of ηE using the theory of Tarasick and Hines (1990) requires determination of their function χ; we show that χ is simply related to the “single-level” Krassovsky\u27s ratio η of Schubert et al. (1991). The general relationship between χ and η is applied to a simple chemical-dynamical model of the O2 atmospheric airglow and the altitude dependence of these quantities is evaluated for nonsteady state chemistry. Though the Tarasick and Hines (1990) formula for ηE does not explicitly depend on the scale heights of the minor constituents involved in airglow chemistry, ηE implicitly depends upon these scale heights through its dependences on chemical production and loss contained in χ. We demonstrate this dependence of ηE for the OH nightglow on atomic oxygen scale height by direct numerical evaluation of ηE; in this case the dependence originates in the chemical production of perturbed ozone