A Sensitivity Study of the Thermal Tides in the Venusian Atmosphere: Structures and Dynamical Effects on the Superrotation.

In order to resolve discrepancy between recent observational and numerical studies on the thermal tides in the Venusian atmosphere, we investigated by means of a general circulation model how the thermal tides are affected by the static stability in and above the upper cloud layer by using three different distributions of the static stability. The results show that the vertical structure of the semidiurnal tide, which propagates vertically, is strongly affected by the static stability. The diurnal tide, which has an equivalent barotropic structure in 62–73 km altitudes, becomes weaker with the higher static stability although its vertical structure is almost unchanged. The horizontal distribution of the thermal tides with the realistic static stability distribution, which is consistent with radio occultation measurements, agrees with the observations at the cloud top. The meridional angular momentum flux associated with the thermal tides is equatorward in low latitudes near the altitude where the equatorial zonal‐mean wind takes its maximum. This result is consistent with the recent Akatsuki UVI observations, suggesting that the thermal tides could contribute to the maintenance of the superrotation in the equatorial region near the cloud top. In the most realistic case, the zonal‐mean zonal wind is effectively accelerated at rates of 0.2–0.5 m s−1 day−1 in low latitudes at altitudes of 52–76 km by both the meridional and vertical angular momentum transports. The thermal tides also induce significant meridional heat flux, which cannot be ignored in the dynamical effect on the zonal‐mean zonal wind. Plain Language Summary: Thermal tides are planetary‐scale atmospheric waves excited by the solar heating and fixed to the solar motion. These waves with the zonal wavenumbers of 1 and 2 are called "diurnal tide" and "semidiurnal tide," respectively. In the Venusian atmosphere, the most part of solar energy is absorbed in the cloud layer in 45–70 km altitudes, so that the thermal tides are strongly excited in this layer. There is a discrepancy between results obtained by recent observations and those by numerical simulations of the thermal tides. In this study, we resolve this discrepancy by investigating how the thermal tides depend on the temperature distribution in and above the cloud layer. The result shows that the temperature distribution plays important roles on the vertical structure of the semidiurnal tide. Therefore, it is crucially important to adopt a correct temperature distribution in the numerical model in order to reproduce realistic thermal tides consistent with the recent observations. Further, it is found that the thermal tides transport angular momentum meridionally and vertically and heat meridionally. Through this angular momentum transport, the thermal tides could contribute to the maintenance of the superrotation in the equatorial region near the cloud top. Key Points: A realistic structure of the Venusian thermal tide consistent with recent observations is reproduced by improving the static stabilityThe thermal tide induces meridional angular momentum and heat transports as well as vertical angular momentum transport in the cloud layerThe thermal tide could contribute to the superrotation through not only the thermal tide mechanism but also the meridional circulation one [ABSTRACT FROM AUTHOR]