Variation of Venusian Gravity Wave Absolute Momentum Fluxes and Drag as Retrieved from the Akatsuki Mission

Abstract

Using temperature retrievals from Akatsuki radio occultation measurements, we characterize gravity wave activity as a function of vertical wavenumber and altitude and, for the first time, estimate the absolute horizontal momentum fluxes and the magnitude of the associated gravity wave drag (i.e., wave acceleration), which quantify the potential effects of these waves in the Venusian middle atmosphere between 40--95 km. Observed temperature perturbations, which are indicative of atmospheric gravity wave activity, reach amplitudes of approximately $\pm$10 K, and significant momentum flux (10--30 m$^2$ s$^{-2}$) and wave drag (0.003--0.03 m s$^{-2}$) are detected across all analyzed profiles. The inferred wave drag represents a lower bound on the total gravity wave-induced drag in the Venusian atmosphere. Momentum flux tends to increase exponentially with altitude below approximately 50--60 km, then peaks and attenuates at higher altitudes. Wave drag becomes prominent where momentum flux begins to decrease, which is a consequence of wave dissipation. Both quantities exhibit multiple altitude-localized maxima, which is consistent with upward wave propagation followed by dissipation at different altitudes for different vertical wavelengths. Damping due to gravity wave nonlinear interactions is likely to play the major role in limiting the growth of wave amplitudes and fluxes with height. These features are observed across a range of latitudes and local times. Overall, the results provide observational constraints on gravity wave momentum transport and dissipation in the Venusian middle atmosphere and could guide numerical models in their effort to quantify wave-mean flow interactions in Venus's atmosphere.