[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, l z (z diss ), increases exponentially with altitude, although with a smaller slope for z > 200 km. We also show how the horizontal wavelength, l H , and wave period spectra change with altitude for dissipating GWs. We find that a new dissipation condition can predict our results for l z (z diss ) very well up to altitudes of $500 km. We also find that a GW spectrum excited from convection shifts to increasingly larger l z and l H 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 l z , consistent with observations. Finally, we show that our results agree well with observations.

[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 w Ir /N ( 1 an upward propagating gravity wave dissipates at an altitude given by ' z 1 + H ln(w Ir /2Hjmj 3 n 1 ), where H is the density scale height, N is the buoyancy frequency, n 1 is the viscosity at z = z 1 , and w Ir 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 mn/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 = 0.7 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.Citation: Vadas, S. L., and D. C. Fritts (2005), Thermospheric responses to gravity waves: Influences of increasing viscosity and thermal diffusivity,

We study the response of the thermosphere and ionosphere to the dissipation of gravity waves (GWs) excited by a deep convective plume on 1 October 2005 at 52.5°W, 15.0°S, and 2120 UT in Brazil. Those small‐ and medium‐scale GWs which reach the thermosphere dissipate at z ∼ 120–250 km in a direction opposite to the background wind ∼(1–2) density scale heights below. This localized momentum deposition creates horizontal thermospheric body forces that have large sizes and amplitudes and generates large‐scale secondary GWs and large‐scale traveling ionospheric disturbances (LSTIDs) that propagate globally away from the body force in all directions except that perpendicular to the force direction. For the convective plume at 2120 UT, the secondary GWs have horizontal wavelengths of λH ∼ 2100–2200 km, periods of τr ∼ 80 min, horizontal phase speeds of cH ∼ 480–510 m/s, density perturbations as large as ∣ρ′/∣ ∼ 3.6–5% at z = 400 km, relative [O] perturbations as large as ∼2–2.5% at z = 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 ∼4 h. We also find that the induced body force direction varies throughout the day, depending on the winds in the lower thermosphere.

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.

[1] Gravity waves in the mesopause region (80-105 km) may induce perturbations in OH Meinal Band emissions at $87 km. These perturbations can be observed by ground-based OH airglow imagers. In this paper, we present observations of concentric gravity waves (CGW) by the all-sky OH imager at Yucca Ridge Field Station (40.7°N, 104.9°W) near Fort Collins, Colorado. We find that expanding rings of concentric gravity waves were observed on 9 out of 723 clear nights from 2003 to 2008. In particular, on 11 May 2004, concentric rings were observed for $1.5 h, with nearly perfect circular rings entirely in the field of view during the first 30 min. The centers of the concentric rings occurred at the geographic locations of two strong convective plumes which were active in the troposphere $1 h earlier. We measured the horizontal wavelengths and periods of these gravity waves as functions of both radius and time. These results agreed reasonably well with the internal Boussinesq gravity wave dispersion relation with an assumed zero background wind. Similarly, for the other 8 cases, strong convective plumes occurred prior to the CGW observations near the apparent center of each of the arcs or rings. For the 7 out of the 9 cases, radiosonde data were available up to z = 30-35 km. These data showed that the wind speeds from the tropopause to $30-35 km were smaller than $20-30 m/s. Because 8 of the 9 cases occurred when the total horizontal mean winds were weak and because the horizontal winds below $87 km were less than $20 m/s on 11 May 2004 (according to radiosonde and TIME-GCM model data), we postulate that weak background horizontal winds are likely a necessary condition for gravity waves excited from convective overshooting to be observed as concentric arcs or rings in the OH layer.