Atmospheric response to high-resolution topographical and radiative forcings in a general circulation model of Venus: Time-mean structures of waves and variances

Abstract

Thermal tides, stationary waves, and general circulation are investigated using a T63 Venus general circulation model (GCM) with solar and thermal radiative transfer in the presence of high-resolution surface topography, based on time average analysis. The simulated wind and static stability are very similar to the observed ones (e.g., Horinouchi et al., 2018 ; Ando et al., 2020 ). The simulated thermal tides accelerate an equatorial superrotational flow with a speed of ~90 m s −1 around the cloud-heating maximum (~65 km). The zonal-flow acceleration rates of 0.2–0.5 m s −1 Earth day −1 are produced by both horizontal and vertical momentum fluxes at low latitudes. In the GCM simulation, strong solar heating above the cloud top (>69 km) and infrared heating around the cloud bottom (~50 km) modify the vertical structures of thermal tides and their vertical momentum fluxes, which accelerate zonal flow at 10 3 Pa (~75 km) and 10 4 Pa (~65 km) at the equator and around 10 3 Pa at high latitudes. Below and in the cloud layer, surface topography weakens the zonal-mean zonal flow over the Aphrodite Terra and Maxwell Montes, whereas it enhances the zonal flow in the southern polar region. The high-resolution topography produces stationary fine-scale bow structures at the cloud top and locally modifies the variances in the geographical coordinates (i.e., the activity of unsteady wave components). Over the high mountains, vertical spikes of the vertical wind variance are found, indicating penetrative plumes and gravity waves. Negative momentum flux is also locally enhanced at the cloud top over the equatorial high mountains. In the solar-fixed coordinate system, the variances (i.e., the activity of waves other than thermal tides) of flow are relatively higher on the nightside than on the dayside at the cloud top. Strong dependences of the eddy heat and momentum fluxes on local time are predominant. The local-time variation of the vertical eddy momentum flux is produced by both thermal tides and solar-related, small-scale gravity waves on the nightside.

Generation of gravity waves from thermal tides in the Venus atmosphere

“…Baroclinic waves accelerate at ~0.05 m/s/day in the mid-latitudes 24 . The thermal tides also accelerate at 1.0 m/s/day or less in the low-latitudes by Akatsuki observations 44 and ~0.2–0.5 m/s/day by another Venus GCM study 45 . Considering that about half of the acceleration due to thermal tides could be cancelled out by gravity waves, the contribution of gravity waves can be comparable to that of planetary-scale waves and more than that of baroclinic waves, although our estimate was rough and not zonally averaged.…”

Rossby Number Dependence of Venus/Titan‐Type Superrotation and Its Related Intermittency

“…The zonal-mean heating due to the equator-pole temperature contrast is of primary importance as a thermal driver of the general circulation. Although thermal tides also contribute to the equatorial superrotation of Venus in GCMs (Lebonnois et al, 2010;Takagi & Matsuda, 2007;Yamamoto et al, 2019Yamamoto et al, , 2021Yamamoto & Takahashi, 2006), the zonal-flow acceleration by thermal tides is limited to the cloud top region. Furthermore, GCM simulations with more realistic radiative forcing result in more complex meridional circulations with separated overturning cells in the cloud layers from those at deeper levels (e.g., Lebonnois et al, 2010).…”

Winds in the Lower Cloud Level on the Nightside of Venus from VIRTIS-M (Venus Express) 1.74 μm Images

“…The local time dependency for the meridional speed also shows similarities, as the poleward motion is found before midnight, and equatorward-after midnight. Jet-like increases of zonal speed at the lower cloud level near equator and in middle latitudes has been shown by the T63 GCM [34]. Longitudinal variations have also been simulated by the GCM at ~54 km [35].…”