Abstract
The reason for stationary gravity waves at Venus’ cloud top to appear mostly at low latitudes in the afternoon is not understood. Since a neutral layer exists in the lower part of the cloud layer, the waves should be affected by the neutral layer before reaching the cloud top. To what extent gravity waves can propagate vertically through the neutral layer has been unclear. To examine the possibility that the variation of the neutral layer thickness is responsible for the dependence of the gravity wave activity on the latitude and the local time, we investigated the sensitivity of the vertical propagation of gravity waves on the neutral layer thickness using a numerical model. The results showed that stationary gravity waves with zonal wavelengths longer than 1000 km can propagate to the cloud-top level without notable attenuation in the neutral layer with realistic thicknesses of 5–15 km. This suggests that the observed latitudinal and local time variation of the gravity wave activity should be attributed to processes below the cloud. An analytical approach also showed that gravity waves with horizontal wavelengths shorter than tens of kilometers would be strongly attenuated in the neutral layer; such waves should originate in the altitude region above the neutral layer.
Highlights
A surprising discovery on the Venus atmosphere in the 2010s is a bow-shaped temperature structure by Longwave Infrared Camera (LIR) onboard the Venus orbiter Akatsuki (Fukuhara et al 2017b)
This study revealed, using three-dimensional numerical model, that gravity waves with zonal wavelengths longer than 1000 km excited near the surface can propagate to the cloud top and produce temperature disturbances as observed, despite the existence of a neutral layer with the thickness of up to 15 km in the cloud
The results strongly suggest that the dependence of the observed wave amplitude on the local time and the latitude are not attributed to the variation of the neutral layer thickness along the local time and the latitude but is caused by excitation and/or transmission processes in the lower atmosphere
Summary
A surprising discovery on the Venus atmosphere in the 2010s is a bow-shaped temperature structure by Longwave Infrared Camera (LIR) onboard the Venus orbiter Akatsuki (Fukuhara et al 2017b). LIR detects thermal infrared radiation from the cloud top at an altitude of approximately 65 km and provides images of the horizontal distribution of the temperature at this height (Fukuhara et al 2011, 2017a; Taguchi et al 2007). The bow-shaped structure extended from the northern hemisphere to the southern hemisphere with a length of approximately 10,000 km (Fukuhara et al 2017b). The bow-shaped structure remained at the geographical location around the local time of 16:00 for at least 5 days, despite the strong westward wind of about 100 m s−1 blowing at the cloud-top altitude. The location corresponds to Aphrodite Terra, a large highland.
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