The first assimilation of Akatsuki single-layer winds and its validation with Venusian atmospheric waves excited by solar heating

Abstract

The planetary missions including the Venus Climate Orbiter ‘Akatsuki’ provide new information on various atmospheric phenomena. Nevertheless, it is difficult to elucidate their three-dimensional structures globally and continuously only from observations because satellite observations are considerably limited in time and space. We constructed the first ‘objective analysis’ of Venus’ atmosphere by assimilating cloud-top horizontal winds on the dayside from the equator to mid-latitudes, which is frequently obtained from Akatsuki's Ultraviolet Imager (UVI). The three-dimensional structures of thermal tides, found recently to play a crucial role in maintaining the super rotation, are greatly improved by the data assimilation. This result is confirmed by comparison with Akatsuki's temperature observations. The momentum transport caused by the thermal tides and other disturbances are also modified by the wind assimilation and agrees well with those estimated from the UVI observations. The assimilated dataset is reliable and will be open to the public along with the Akatsuki observations for further investigation of Venus’ atmospheric phenomena.

Cold Collar Reproduced by a Venus GCM With the Akatsuki Horizontal Wind Assimilation

“…Fujisawa et al. (2022) found that the energy density of the thermal tides at the equator had a maximum at 70–75 km and decreased above 75 km in analysis, while it was almost constant above 70 km in FRF. This can be also explained in terms of the difference of the vertical group velocities of the thermal tides between analysis and FRF.…”

“…As mentioned in Section 1, we use the first objective analysis data (hereinafter referred to as “analysis”) of Fujisawa et al. (2022), which was produced by assimilating the horizontal winds obtained from long‐term continuous measurements by UVI onboard Akatsuki conducted from September to December in 2018. Because the orbit of Akatsuki is equatorial, the UVI measurements can observe the dayside region in low‐ and mid‐latitudes, so that the horizontal winds can be derived at the cloud top level (∼70 km altitude) in latitudes equatorward of ∼50°.…”

“…In the work of Fujisawa et al (2022), the horizontal winds in low-and mid-latitudes derived from ultraviolet images taken by the UltraViolet Imager (UVI) onboard Akatsuki were assimilated for AFES-Venus by ALEDAS-V. The UVI measurements have been conducted for ∼3 Earth months continuously, and Fujisawa et al (2022) have produced the objective analysis data, which is the numerical data created by assimilating the observation results of the Venus atmosphere, for the first time.…”

“…They also found that the phases of the thermal tides are shifted toward the earlier local times by ∼2 hr than the case without the data assimilation and well consistent with UVI (Horinouchi 10.1029/2022JE007689 3 of 14 et al, 2018) and radio occultation (Ando et al, 2018) measurements. In this study, by using the objective analysis data produced by Fujisawa et al (2022), we reproduced the unique thermal structure (cold collar and warm polar region) of the upper atmosphere in the Venus polar region more realistically to investigate how the structure is formed and the impact of the thermal tides on the structure in detail.…”

“…In the work of Fujisawa et al. (2022), the horizontal winds in low‐ and mid‐latitudes derived from ultraviolet images taken by the UltraViolet Imager (UVI) onboard Akatsuki were assimilated for AFES‐Venus by ALEDAS‐V. The UVI measurements have been conducted for ∼3 Earth months continuously, and Fujisawa et al.…”