Study on the variation of Venusian polar tropopause using Venus Express radio occultation

In the Venusian atmosphere at 50-70km altitudes, there is a thick cloud layer composed of H₂SO₄ and H₂O liquid. Around the cloud base, the clouds absorb infrared radiation from the lower atmosphere, driving convection in the lower and middle clouds (50–55 km altitude) to form the troposphere. Above this convective layer (around 60 km and higher), atmospheric gravity waves propagate.

Previous studies have revealed that the tropopause height increases up to around 60° latitude and then decreases toward the poles (Ando et al. 2020). However, the mechanism determining the tropopause height remains unclear. On the other hand, as described below, we found that the tropopause height varies by several kilometers with time scales of a few days. The variations seem to be correlated with the quasi-periodic variations in the temperature structure, one of which was reported by Ando et al. (2017). The temperature variations were suspected to be driven by Rossby waves.

This study aims to advance our understanding of the dynamics of the tropopause, focusing on the day-to-day variations in the tropopause height in the polar region. We examine two main issues: first, how planetary-scale waves influence the tropopause height; and second, whether the tropopause height is determined by the strength of convection.

By analyzing the radio occultation data from the Venus Express mission in the polar region, we found that, in addition to the previously known temperature variations caused by planetary-scale waves, the tropopause height varies in sync with these temperature changes (Fig. 1). The variation is attributed to the latitudinal displacement of the tropopause caused by planetary-scale waves.
We also focused on the amplitude of gravity waves as an indicator of the convection strength. The relationship between the gravity wave amplitude and the tropopause height was studied, and no significant correlation was found between them. This implies that the tropopause height may not directly reflect the strength of convection, or that other sources of gravity wave excitation may play a more dominant role.

Fig.1 Time evolution of the vertical distribution of temperature deviation (contours) and the tropopause height (black dots). The temperature deviation is calculated by subtracting the period-averaged temperature profile from each day's profile. The tropopause height is defined as the transition altitude where the static stability is smaller than 8 K/km below and larger than 8 K/km above. The data were obtained from radio occultation observations by ESA’s Venus Express. The top panel uses data observed approximately every day from January 19 to 25, 2008, and the bottom panel from February 9 to March 4, 2008.