[M-GI30] Computational sciences on the universe, galaxies, stars, planets, and their environments

[MGI30-P08] Non-hydrostatic global simulations of the Venus atmosphere: effects of vertical convection

*Hiroki Kashimura¹, Hisashi Yashiro², Seiya Nishizawa³, Hirofumi Tomita³, Masahiro Takagi⁴, Norihiko Sugimoto⁵, Kazunori Ogohara⁴, Takeshi Kuroda⁶, Kensuke Nakajima⁷, Masaki Ishiwatari⁸, Yoshiyuki O. Takahashi¹, Yoshi-Yuki Hayashi¹ (1.Department of Planetology, Graduate School of Science, Kobe University, 2.Center for Global Environmental Research, National Institute for Environmental Studies, 3.RIKEN Center for Computational Science, 4.Faculty of Science, Kyoto Sangyo University, 5.Department of Physics, Keio University, 6.Department of Geophysics Tohoku University, 7.Department of Earth and Planetary Sciences, Flculty of Sciences, Kyushu University, 8.Department of Cosmosciences, Graduate school of Science, Hokkaido University)

Keywords:Venus atmosphere, Non-hydrostatic, Vertical convection, Numerical experiment

Venus is fully covered by thick clouds of sulfuric acid, and its atmospheric circulation and inherent phenomena are not well understood. Recent observations by the Venus Climate Orbiter/Akatsuki have revealed a variety of atmospheric phenomena, from the planetary-scale bow-shaped structure (Fukuhara et al., 2017) and streak-structure (Kashimura et al., 2019) to a front-like structure to small-scale vortices and waves of several hundred kilometers (Limaye et al., 2018). There have also been active attempts to reproduce these phenomena using Venusian atmospheric general circulation models and to understand the mechanisms involved. In particular, AFES-Venus (Sugimoto et al., 2014) has realized high-resolution simulations of the Venus atmosphere, and many structures have been analyzed (e.g., Kashimura et al., 2019; Takagi et al., 2018; Sugimoto et al., 2022; Takagi et al., 2022). However, AFES-Venus is a hydrostatic model, which cannot explicitly express vertical convection. The vertical convection in the cloud layer is not only interesting in itself but is also very important in the Venusian atmosphere because the neutral or low-stability layer resulting from convection is closely related to the formation of the planetary-scale bow-shaped structure and the streak-structure. Though non-hydrostatic regional models have been used to study convective activities and the resulting gravity waves (e.g., Baker et al., 1998; Imamura et al., 2014; Yamamoto 2014, Lefèvre et al., 2017), due to the limitation of the domain size, effects of the convective activities on large-scale phenomena have not been investigated.

We are developing a non-hydrostatic Venusian atmospheric general circulation model to realize a global simulation that explicitly represents convective activities in the cloud layer. We utilized SCALE-GM (http://r-ccs-climate.riken.jp/scale/) for the dynamical core. We imported the solar heating and Newtonian cooling functions used in AFES-Venus (Tomasko et al., 1980; Crisp et al., 1986; Sugimoto et al., 2014) to SCALE-GM. Then, we performed numerical experiments with zonally uniform heating and cooling and obtained features such as the superrotation and streak-structure similar to those obtained by AFES-Venus. Here, note that the used thermal forcing leads the atmosphere to a hydrostatically stable state (though it is close to neutral) and does not directly drive convective motions in the cloud layer.

In this study, we attempted to perform global calculations with vertical convection by providing a thermal forcing that brings a hydrostatically unstable state. Specifically, the forcing stability (i.e., the stability of the reference temperature field for Newtonian cooling) at 55-60 km altitude was changed from 0.1 K/km to negative values (e.g., -1.0 K/km), and a diurnal component of solar heating was added. Calculations were performed at two resolutions with horizontal grid spacings of about 52 km (glevel 7) and 26 km (glevel 8).

Numerical results show that vertical convection occurs in the region of the night side at both resolutions. However, the horizontal scales of the expressed vertical convection are about 500 km at glevel 7 and 200 km at glevel 8, which are larger than the observed convection scales and resolution-dependent. In other words, an accurate representation of convection requires higher resolution. On the other hand, based on our experience with global non-hydrostatic calculations of the Martian atmosphere (the vertical heat and momentum fluxes by convection were underestimated when the horizontal scale of convection was too large due to insufficient resolution), we consider the effect of convective motions to the large-scale structure can be evaluated to some extent by these numerical results. We will show the basic features of the circulation and temperature structures and discuss the role of convection in the Venus atmosphere.