Vertical structure of the axi-asymmetric temperature disturbance in the Venusian polar atmosphere: Comparison between radio occultation measurements and GCM results

Vertical temperature profiles at 40-75km around 80 degrees N in the Venus polar vortex are retrieved over 13Earth days almost continuously from radio occultation measurements (Venus Express radio occultation) in the Venus Express mission. They show periodical variations with a dominant period of approximate to 3.1Earth days. These fluctuations are confined in an altitude range of 45-65km with a local minimum at approximate to 58km altitude, where the static stability abruptly increases with height. The phase of the temperature fluctuations is almost reversed at the 58km level and varies little above and below this altitude. A numerical simulation of a Venusian atmospheric general circulation model (GCM) shows that the axi-asymmetric temperature disturbance with zonal wave number 1 is predominant at 50-75km levels in the model atmosphere. The vertical structure of the reproduced disturbance agrees quite well with that retrieved by the radio occultation measurement: amplitude of the temperature fluctuation has a local minimum and its phase is reversed at the altitude (65km in the model) where the static stability rapidly changes as in the observations. Above and below this altitude, the phase is almost constant in the vertical direction. The relationship among the temperature, horizontal winds, and geopotential height associated with the simulated disturbance suggests that the axi-asymmetric temperature disturbance observed in the Venus polar region can be interpreted as neutral barotropic Rossby waves related to barotropic instability in the polar region. Plain Language Summary Vertical structure of the axi-asymmetric temperature disturbance, which has been observed by previous infrared measurements, is clarified by Venus Express radio occultation measurements. In addition, Venusian general circulation model (GCM) named Atmospheric GCM for the Earth Simulator for Venus could reproduce both horizontal and vertical structures of the disturbance, and its three-dimensional structure is also proposed for the first time. Moreover, we clarified that the disturbance is a barotropic and neutral Rossby wave related to barotropic instability in the polar region.