Sharon L. Vadas

Bio: Sharon L. Vadas is an academic researcher from Cora. The author has contributed to research in topics: Gravity wave & Thermosphere. The author has an hindex of 33, co-authored 71 publications receiving 3396 citations. Previous affiliations of Sharon L. Vadas include National Waste & Recycling Association.

Horizontal and vertical propagation and dissipation of gravity waves in the thermosphere from lower atmospheric and thermospheric sources

Abstract: [1] The dissipation of high-frequency gravity waves (GWs) in the thermosphere is primarily due to kinematic viscosity and thermal diffusivity. Recently, an anelastic GW dispersion relation was derived which includes the damping effects of kinematic viscosity and thermal diffusivity in the thermosphere and which is valid before and during dissipation. Using a ray trace model which incorporates this new dispersion relation, we explore many GW properties that result from this dispersion relation for a wide range of thermospheric temperatures. We calculate the dissipation altitudes, horizontal distances traveled, times taken, and maximum vertical wavelengths prior to dissipation in the thermosphere for a wide range of upward-propagating GWs that originate in the lower atmosphere and at several altitudes in the thermosphere. We show that the vertical wavelengths of dissipating GWs, λz(zdiss), increases exponentially with altitude, although with a smaller slope for z > 200 km. We also show how the horizontal wavelength, λH, and wave period spectra change with altitude for dissipating GWs. We find that a new dissipation condition can predict our results for λz(zdiss) very well up to altitudes of ∼500 km. We also find that a GW spectrum excited from convection shifts to increasingly larger λz and λH with altitude in the thermosphere that are not characteristic of the initial convective scales. Additionally, a lower thermospheric shear shifts this spectrum to even larger λz, consistent with observations. Finally, we show that our results agree well with observations.

Thermospheric responses to gravity waves: Influences of increasing viscosity and thermal diffusivity

Abstract: [1] A gravity wave anelastic dispersion relation is derived that includes molecular viscosity and thermal diffusivity to explore the damping of high-frequency gravity waves in the thermosphere The time dependence of the wave amplitudes and general ray trace equations are also derived In the special case that the thermal structure is isothermal and the Prandtl number (Pr) equals 1, exact linear solutions are obtained For high-frequency gravity waves with ωIr/N ≪ 1 an upward propagating gravity wave dissipates at an altitude given by ≃ z1 + H ln(ωIr/2H∣m∣3ν1), where H is the density scale height, N is the buoyancy frequency, ν1 is the viscosity at z = z1, and ωIr and m are the gravity wave intrinsic frequency and vertical wave number, respectively Thus high-frequency gravity waves with large vertical wavelengths dissipate at the highest altitudes, resulting in momentum and energy inputs extending to very high altitudes We find that the vertical wavelength of a gravity wave with an initially large vertical wavelength decreases significantly by the time it dissipates just below where it begins to reflect The effect of diffusion on a gravity wave is similar to the effect of shear in the sense that as the molecular viscosity and thermal diffusivity increase due to decreasing background density, the intrinsic frequency plus mν/H decreases and the vertical wave number increases in order to satisfy the dispersion relation for Pr = 1 We also briefly explore the results with different Prandtl numbers using numerical ray tracing Gravity waves in a Pr = 07 environment dissipate just a few kilometers below those in a Pr = 1 environment when H = 7 km, showing the utility of the analytic, Pr = 1 solutions

Generation of large-scale gravity waves and neutral winds in the thermosphere from the dissipation of convectively generated gravity waves

Abstract: phase speeds of cH � 480–510 m/s, density perturbations as large as jr 0 /r j� 3.6–5% at z = 400 km, relative [O] perturbations as large as � 2–2.5% atz = 300 km, and total electron content perturbations as large as � 8%. This transfer of momentum from local, relatively slow, small scales at the tropopause to global, fast, large scales in the thermosphere is independent of geomagnetic conditions. The various characteristics of these large-scale waves may explain observations of LSTIDs at magnetically quiet times. We also find that this body force creates a localized ‘‘mean’’ horizontal wind in the direction of the body force. For the plume at 2120 UT, the wind is southward with an estimated maximum of vmax �� 400 m s � 1 that is dissipated after � 4h . We also find that the induced body force direction varies throughout the day, depending on the winds in the lower thermosphere.

Mechanism for the Generation of Secondary Waves in Wave Breaking Regions

Abstract: The authors propose that the body force that accompanies wave breaking is potentially an important linear mechanism for generating secondary waves that propagate into the mesosphere and lower thermosphere. While the focus of this paper is on 3D forcings, it is shown that this generating mechanism can explain some of the mean wind and secondary wave features generated from wave breaking in a 2D nonlinear model study. Deep 3D body forces, which generate secondary waves very efficiently, create high-frequency waves with large vertical wavelengths that possess large momentum fluxes. The efficiency of this forcing is independent of latitude. However, the spatial and temporal variability/intermittency of a body force is important in determining the properties and associated momentum fluxes of the secondary waves. High spatial and temporal variability accompanying a wave breaking process leads to large secondary wave momentum fluxes. If a body force varies slowly with time, negligible secondary wave fluxes result. Spatial variability is important because distributing ‘‘averaged’’ body forces over larger regions horizontally (as is often necessary in GCM models) results in waves with smaller frequencies, larger horizontal wavelengths, and smaller associated momentum fluxes than would otherwise result. Because some of the secondary waves emitted from localized body force regions have large vertical wavelengths and large intrinsic phase speeds, the authors anticipate that secondary wave radiation from wave breaking in the mesosphere may play a significant role in the momentum budget well into the thermosphere.

Gravity wave dynamics and effects in the middle atmosphere

Abstract: [1] Atmospheric gravity waves have been a subject of intense research activity in recent years because of their myriad effects and their major contributions to atmospheric circulation, structure, and variability. Apart from occasionally strong lower-atmospheric effects, the major wave influences occur in the middle atmosphere, between ∼ 10 and 110 km altitudes because of decreasing density and increasing wave amplitudes with altitude. Theoretical, numerical, and observational studies have advanced our understanding of gravity waves on many fronts since the review by Fritts [1984a]; the present review will focus on these more recent contributions. Progress includes a better appreciation of gravity wave sources and characteristics, the evolution of the gravity wave spectrum with altitude and with variations of wind and stability, the character and implications of observed climatologies, and the wave interaction and instability processes that constrain wave amplitudes and spectral shape. Recent studies have also expanded dramatically our understanding of gravity wave influences on the large-scale circulation and the thermal and constituent structures of the middle atmosphere. These advances have led to a number of parameterizations of gravity wave effects which are enabling ever more realistic descriptions of gravity wave forcing in large-scale models. There remain, nevertheless, a number of areas in which further progress is needed in refining our understanding of and our ability to describe and predict gravity wave influences in the middle atmosphere. Our view of these unknowns and needs is also offered.

Simulation of the seeding of equatorial spread F by circular gravity waves

Abstract: [1] The Naval Research Laboratory three-dimensional simulation code SAMI3/ESF is used to study the response of the postsunset ionosphere to circular gravity waves. We model the coupling of both circular (local) and plane wave (nonlocal) gravity waves to the bottomside F layer as a mechanism for triggering equatorial plasma bubbles. Results support the hypothesis that nonplane gravity waves can more strongly couple to the F layer than plane gravity waves. Results also show that the coupling of the seed wave to the F layer depends on the (nonlocal) growth rate and the local electron density at the position of the seed wave.

Recent developments in gravity-wave effects in climate models and the global distribution of gravity-wave momentum flux from observations and models

Abstract: Recent observational and theoretical studies of the global properties of small-scale atmospheric gravity waves have highlighted the global effects of these waves on the circulation from the surface to the middle atmosphere. The effects of gravity waves on the large-scale circulation have long been treated via parametrizations in both climate and weather-forecasting applications. In these parametrizations, key parameters describe the global distributions of gravity-wave momentum flux, wavelengths and frequencies. Until recently, global observations could not define the required parameters because the waves are small in scale and intermittent in occurrence. Recent satellite and other global datasets with improved resolution, along with innovative analysis methods, are now providing constraints for the parametrizations that can improve the treatment of these waves in climate-prediction models. Research using very-high-resolution global models has also recently demonstrated the capability to resolve gravity waves and their circulation effects, and when tested against observations these models show some very realistic properties. Here we review recent studies on gravity-wave effects in stratosphere-resolving climate models, recent observations and analysis methods that reveal global patterns in gravity-wave momentum fluxes and results of very-high-resolution model studies, and we outline some future research requirements to improve the treatment of these waves in climate simulations. Copyright © 2010 Royal Meteorological Society and Crown in the right of Canada

Large-Scale Dynamics of the Convection Zone and Tachocline

Abstract: The past few decades have seen dramatic progress in our understanding of solar interior dynamics, prompted by the relatively new science of helioseismology and increasingly sophisticated numerical models. As the ultimate driver of solar variability and space weather, global-scale convective motions are of particular interest from a practical as well as a theoretical perspective. Turbulent convection under the influence of rotation and stratification redistributes momentum and energy, generating differential rotation, meridional circulation, and magnetic fields through hydromagnetic dynamo processes. In the solar tachocline near the base of the convection zone, strong angular velocity shear further amplifies fields which subsequently rise to the surface to form active regions. Penetrative convection, instabilities, stratified turbulence, and waves all add to the dynamical richness of the tachocline region and pose particular modeling challenges. In this article we review observational, theoretical, and computational investigations of global-scale dynamics in the solar interior. Particular emphasis is placed on high-resolution global simulations of solar convection, highlighting what we have learned from them and how they may be improved.