Correlation of Venusian Mesoscale Cloud Morphology Between Images Acquired at Various Wavelengths

Self Cite

Images of Venusian clouds taken at ultraviolet (UV) wavelengths frequently exhibit mesoscale cellular structures at low latitudes. To obtain clues regarding the origin of these structures, we studied the temporal variation of the mesoscale UV contrast using 283‐ and 365‐nm images taken by the ultraviolet imager (UVI) onboard the Akatsuki spacecraft. The 283‐nm channel is mostly affected by SO2 at the cloud top and the 365‐nm channel is mostly affected by an unknown absorber, though both absorbers are thought to influence both wavelengths. We defined the mesoscale as a spatial scale roughly below 1,000 km, where the effect of large‐scale streaky patterns is not significant according to the spatial spectra of 283‐nm images taken by UVI and 2.02‐μm images taken by the near‐infrared camera (IR2) onboard Akatsuki. We found that the temporal variation of the standard deviation of the mesoscale UV contrast is dominated by quasi‐periodic oscillations with amplitudes of 30%–50% relative to the mean value and periods of around 4–5 Earth days. Oscillations of the planetary‐scale UV contrast with similar periods were also observed, suggesting that the mesoscale structures are influenced by planetary‐scale waves. The mesoscale contrast tends to be enhanced when the background albedo decreases in the course of the propagation of planetary‐scale waves. This correlation suggests that the solar heating near the cloud top level feeds the mesoscale dynamics. An enhancement of the mesoscale contrast in the afternoon was seen at a wavelength of 365 nm.

Section: Spatial Structure Of Mesoscale Morphologymentioning

confidence: 98%

Section: Spatial Structure Of Mesoscale Morphologymentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Periodic Variation of Mesoscale Ultraviolet Contrast at the Cloud Top of Venus

Suda,

Imamura,

Lee

et al. 2023

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“…Generally, the cloud top decreases from low and middle latitudes to high latitudes (Haus et al., 2014). In the equatorial region, clouds are more patchy and spatially variable, possibly related to convective activities and waves (Narita et al., 2022; Titov et al., 2008). At middle and high latitudes, cloud spatial patterns are smoother and characterized by streaks and bands, suggesting convectively stable layers there (Titov et al., 2008).…”

Section: Motivationmentioning

confidence: 99%

Three‐Dimensional Venus Cloud Structure Simulated by a General Circulation Model

Shao,

Mendonça,

Dai

2024

The clouds have a great impact on Venus's energy budget and climate evolution, but its three‐dimensional structure is still not well understood. Here we incorporate a simple Venus cloud physics scheme into a flexible GCM to investigate the three‐dimensional cloud spatial variability. Our simulations show good agreement with observations in terms of the vertical profiles of clouds and H2SO4 vapor. H2O vapor is overestimated above the clouds due to efficient transport in the cloud region. The cloud top decreases as latitude increases, qualitatively consistent with Venus Express observations. The underlying mechanism is the combination of H2SO4 chemical production and meridional circulation. The mixing ratios of H2SO4 at 50–60 km and H2O vapors in the main cloud deck basically exhibit maxima around the equator, due to the effect of temperature's control on the saturation vapor mixing ratios of the two species. The cloud mass distribution is subject to both H2SO4 chemical production and dynamical transport and shows a pattern that peaks around the equator in the upper cloud while peaks at mid‐high latitudes in the middle cloud. At low latitudes, H2SO4 and H2O vapors, cloud mass loading and acidity show semidiurnal variations at different altitude ranges, which can be validated against future missions. Our model emphasizes the complexity of the Venus climate system and the great need for more observations and simulations to unravel its spatial variability and underlying atmospheric and/or geological processes.

“…The absorption at 283 nm is due not only to the unknown absorber but also to SO 2 . Thus, the small‐scale features at both wavelengths are positively correlated (Narita et al., 2022), but the correspondence of small‐scale features is not perfect. The winds obtained at the two wavelengths agreed well when the cloud features captured were nearly identical, but when they differed, the zonal winds obtained at 283 nm were almost always faster than those obtained at 365 nm.…”

Section: Introductionmentioning

confidence: 99%

Long‐Term Variability of Mean Winds and Planetary‐Scale Waves Around Venusian Cloud Top Observed With Akatsuki/UVI

Horinouchi,

Kouyama,

Imai

et al. 2024

Self Cite

Since December 2015, Ultraviolet Imager (UVI) onboard Akatsuki has been observing Venus clouds at the wavelengths of 283 and 365 nm. Horizontal winds near the cloud top derived from the UVI images over ∼7 earth years are analyzed to elucidate spatial and temporal variability of the superrotation and planetary‐scale waves. Zonal winds averaged over the analysis period are asymmetric with respect to the equator, being faster in the southern hemisphere. This asymmetry varied temporarily and was occasionally reverted. Comparison of the winds from the two wavelengths suggests that it is uncertain whether the asymmetry is in the wind distribution or in the sensing altitude for winds. Mean zonal winds representing the superrotation exhibited broad low‐frequency variability with spectra resembling the red noise spectra. This is indicative of the presence of internal variability rather than responses to periodical external forcing. Planetary‐scale waves with zonal‐wavenumber 1 at periods around 4 and 5 days, which have been interpreted as equatorial Kelvin and Rossby waves, respectively, are quantified. While the ∼5‐day waves have nearly constant frequencies, the ∼4‐day waves have variable phase speeds that follow the superrotation speed. This result indicates that the ∼5‐day waves are likely to extend over a large depth below the cloud top and that the ∼4‐day waves are likely to be confined near the cloud top. Their possible generation mechanism through the coupling of Kelvin and Rossby waves is discussed. This study further reports wind variability of 10 to 15‐day periodicity, thermal‐tide structure, and comparison with minor species observations.