Thermospheric Dissipation of Upward Propagating Gravity Wave Packets

Studies of gravity waves in the MLT suggest that the lower thermosphere can act as a barrier to upward energy propagation; waves with higher phase speeds (and longer vertical wavelengths) are affected less by viscosity and are able to propagate to higher altitudes [e.g., Pitteway and Hines, Geophys. Monograph Series, 18, 1974]. Due to conservation of energy, a wave's amplitude increases as atmospheric density decreases. However, effects of viscosity simultaneously increase with decreasing density. At some altitude, depending on the phase speed, the viscosity becomes significant such that dissipation overtakes the growth of the wave, and its amplitude will decline as energy and momentum are deposited. Typically this occurs in the lower thermosphere for phase speeds less than 100m/s. [Fritts and Alexander, Rev. Geophys, 41, 1003,2003; Vadas and Fritts, JGR, 110, D15103, 2005]. We investigate the propagation of gravity wave packets into the lower thermosphere using a nonlinear compressible model, which incorporates realistic viscosity and thermal conduction. Cases are presented to investigate the effects of viscosity upon wave propagation and dissipation under differing background atmospheric and initial conditions. Model results are obtained using both an empirical MSIS background atmosphere and an ideal isothermal atmosphere, in order to differentiate the effects of varying temperature structure and viscosity. These are also compared with inviscid runs to help elucidate the competing effects of dissipation and vertical growth with decreasing density. We track the altitude of the peak momentum flux and the peak horizontally averaged horizontal and vertical wind velocities with time, to assess the packets evolution as dissipation overtakes growth. For the horizontal and vertical wind fields associated with the gravity waves, wavenumber spectra are analyzed with time to assess the spectral evolution of the packet [e.g. Zhang and Yi JGR, 107, D14, 2002; Vadas and Fritts, JGR, 110, D15103, 2005]. Results suggest that dissipation by viscosity and simultaneous refraction by thermospheric structure strongly influence the upward propagation and evolution of gravity wave packets at amplitudes insufficient for breaking.