Vertical Propagation of Venusian Thermal Tides Above and Below Clouds Revealed by Akatsuki Radio Occultation

Abstract

The vertical structure of the thermal tide in the Venusian low-latitude region (0$°$–30$°$ latitudes) was investigated by the Akatsuki radio occultation measurements. The result shows that the phase of the diurnal tide little varies in the vertical direction, while that of the semidiurnal tide tilts toward earlier local times with increasing altitude above 65 km and tilts in the same direction with decreasing altitude below 50 km. This indicates that the semidiurnal tide is excited between 50 and 65 km altitudes and propagates upward above these heights and downward below. The vertical momentum flux associated with the semidiurnal tide in low latitudes was calculated above 58 km, and the associated acceleration rate was estimated to be $\sim$0.30 m $s^{-1}$ ${\text{day}}^{-1}$ near the cloud top, which is comparable to those expected in the previous numerical studies. Our results observationally confirmed the simultaneous upward and downward propagations of the thermal tide, which can account for the vertical shear of the Venusian atmospheric superrotation in low latitudes, supporting the previous theoretical predictions.