Variability of Venusian Polar Tropopause Studied by Radio Occultation and a Venusian GCM

In the Venusian atmosphere, a cloud layer composed mainly of sulfuric acid exists at altitudes of 50–70 km. Near the cloud base, infrared radiation emitted from the lower atmosphere is absorbed, driving convection in the lower and middle clouds (approximately 50–55 km) and forming the troposphere. In addition, a large-scale meridional circulation is thought to exist, in which air rises at low latitudes, flows poleward above the cloud top, and descends at high latitudes to return to the troposphere, producing adiabatic heating above the polar troposphere. Poleward heat transport by baroclinic disturbances, suggested by Venus GCM studies, might also affect the tropopause structure. As a result, the high-latitude tropopause will be formed by the combined effect of multiple dynamical processes, including convection, large-scale circulation, and wave activity. Therefore, investigating temporal variations in tropopause structure can provide insight into how the vertical structure of the Venusian atmosphere is maintained.

Based on temperature data from radio occultation observations by the Venus Express spacecraft, Ando et al. (2017) identified temperature variations on timescales of several days in the polar region and attributed them to planetary-scale waves. By analyzing a larger data set, we further showed that the tropopause height also varies on similar timescales and that there is a strong positive correlation between the temperature below the tropopause and the tropopause height. These features suggest meridional advection of the background atmospheric structure associated with planetary-scale waves, but direct verification has been difficult because meridional wind cannot be obtained from radio occultation observations.

In this study, we investigate this mechanism using numerical simulations with the global non-hydrostatic Venus atmospheric model, Venus SCALE-GM, to complement the analysis of observational data and investigate the dynamical mechanism of the observed variations. The model reproduces a positive correlation between temperature anomalies and tropopause height, and also shows a positive correlation between convective intensity and tropopause height. To clarify the role of waves, we investigated the meridional wind and temperature fields around 55 km altitude, which exhibit a prominent wavenumber-1 structure at high latitudes. Wavenumber–frequency analysis indicates a period of about 6 Earth days and a zonal propagation slower than the background flow, consistent with a Rossby wave. The meridional displacement associated with the wave is estimated to be approximately 5° in latitude, and we are investigating whether this displacement can explain the tropopause height variations.