[P-PS04] Advancing the science of Venus in the golden age of exploration

[PPS04-P02] Reproducibility of Cold Collar by Observation System Simulation Experiment on Venus

*Yukiko Fujisawa¹, Norihiko Sugimoto¹, Chi Ao², Asako Hosono³, Hiroki Ando⁴, Masahiro Takagi⁴, Itziar Garate Lopez⁵, Sebastien Lebonnois⁶ (1.Keio University, 2.Jet Propulsion Laboratory, California Institute of Technology, 3.University of Tokyo, 4.Kyoto Sangyo University, 5.Bilboko Ingeniaritza Eskola (UPV/EHU), 6.Sorbonne Universite)

Keywords:Venus atmosphere, Data assimilation, Observing system simulation experiment

Observation System Simulation Experiment (OSSE) is a method to test virtual observation plan using data assimilation method, and can evaluate observation plan by the reproducibility of target phenomena. On the Earth, it is actively used to objectively evaluate weather forecasting capabilities when new observations such as satellite observations are introduced [1]. On the Mars, where observation and numerical model research are most active among planets, the framework of OSSE have been proposed [2]. Our research group has been the first in the world to conduct OSSE on Venus [3-6].

The radio occultation (RO) observations using multiple small satellites are expected not to be interrupted by a thick global cloud layer at about 48–70 km altitudes, and to enable frequent observations of the global vertical temperature distribution at 40–90 km altitudes. The Venus orbiters "Akatsuki" and "Venus Express" have been observing the horizontal temperature distribution near the cloud top by the Longwave Infrared Camera (LIR).

In this study, we focus on a phenomenon peculiar to Venus called “cold collar”, in which the polar region becomes warmer than the surrounding latitudes of 60–80 ° at 65 km altitude, and the OSSEs are carried out assuming RO and LIR observation by satellites to be launched in the future.

For the Venus assimilation system, “ALEDAS-V" (AFES LETKF Data Assimilation System for Venus) [7] based on LETKF (Local Ensemble Transform Kalman Filter) is used. For the ensemble forecast, the general circulation model “AFES-Venus” [8] based on AFES (Atmospheric GCM For the Earth Simulator) is used. For the pseudo-observation data, we use the temperature output of the "IPSL Venus GCM" (now called the Venus “Planetary Climate Model; PCM”) [5] which represent ideal cold collar by radiative forcing. In the RO OSSE, assuming observations by three satellites, the temperature at 40–90 km altitudes corresponding to observation points obtained by orbit calculations is used as pseudo-observation data. The observation points are globally scattered, and the observation frequency is about twice a day in the area north of latitude 75°N. On the other hand, in the LIR OSSE, dayside temperature in the northern hemisphere at 70 km altitude is used, and the observation frequency is twice a day.

Figures a–c are the temperatures at 30–90 °N at 67 km altitude on the 50th Earth day after the start of the experiment. In the experiment without assimilation (free run; FR), the cold collar is not reproduced at 67 km altitude. On the other hand, the experiments assimilated the RO and LIR observations reproduced cold collar. Figures d–f are the latitude-altitude cross sections of the residual mean meridional circulation (vector) and its vertical component (color) for the average of 60–90th Earth day. In the experiments assimilated the RO and LIR observations, downward flow in the polar region is enhanced and reach lower altitudes than in the FR. The study with atmospheric general circulation model in Ando et al. [9] showed that the downward flow of the residual mean meridional circulation warmed the atmosphere in polar region by adiabatic heating, thus forming the cold collar. In this study, the assimilation results consistent with those of Ando et al.

[1] Masutani, M., J. S. Woollen, S. J. Lord, G. D. Emmitt, T. J. Kleespies, S. A. Wood, S. Greco, H. Sun, J. Terry, V. Kapoor, R. Treadon, and K. A. Campana (2010), J. Geophys. Res., 115.
[2] Reale, O., T. Fauchez, S. Teinturier, S. Guzewich, S. Greybush, and J. Wilson (2021), Bulletin of the AAS, 53(4).
[3] Sugimoto, N., M. Abe, Y. Kikuchi, A. Hosono, H. Ando, M. Takagi, I. Garate-Lopez, S. Lebonnois, and C. Ao (2019b), Journal of Japan Society of Civil Engineers A2: Applied Mechanics, 75(2), 477–486.
[4] Sugimoto, N., Y. Fujisawa, M. Shirasaka, A. Hosono, M. Abe, H. Ando, M. Takagi, M. Yamamoto, (2021), Atmosphere, Vol.12, No.1, 14, 16pp.
[5] Sugimoto, N., Y. Fujisawa, M. Shirasaka, M. Abe, S.-Y. Murakami, T. Kouyama, H. Ando, M. Takagi and M. Yamamoto (2022), Atmosphere, 13, 182.
[6] Sugimoto, N., Fujisawa Y., Komori, N., Ando, H., Kouyama T. and Takagi, M (2022), Geosci. Lett., Vol.9, 44.
[7] Sugimoto, N., A. Yamazaki, T. Kouyama, H. Kashimura, T. Enomoto, and M. Takagi (2017), Scientific Reports, 7(1), 9321.
[8] Sugimoto, N., M. Takagi, and Y. Matsuda (2014a), J. Geophys. Res. Planets, 119, 1950–1968.
[9] Ando, H., N. Sugimoto, M. Takagi, H. Kashimura, T. Imamura, and Y. Matsuda (2016), Nat. Commun., 7, 10398.