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Venusian cloud structure and variation are strongly linked to atmospheric dynamics. Past near‐infrared measurements have found cloud variation such as zonal‐wavenumber‐1 cloud marking and cloud discontinuity. However, their formation mechanism is still not well understood. To investigate the Venusian cloud structure and its variation, we have developed a Venus GCM incorporating cloud condensation, evaporation, sedimentation, and simple atmospheric chemistry to represent the H2SO4 cycle. The GCM takes into account cloud particles with radii of 0.3, 1.0, 1.26, and 3.13 μm (Modes 1, 2, 2', and 3, respectively) based on past in situ observations. The simulated latitudinal trends of the cloud top and bottom structures are qualitatively consistent with past observations. Zonally averaged cloud mass loading was the largest and smallest in low and middle latitudes, respectively, and maintained by a mechanism similar to that of past 2‐D numerical studies. At the equator, the column integrated optical depth at 1 μm varied between 33 and 50, which is in good agreement with past observations. This variation consists of two types of cloud mass loading changes between 46 and 52 km. One is a rapid small‐scale variation induced by gravity waves. The other is a quasi‐periodic zonal‐wavenumber‐1 variation coupled with an equatorial Kelvin wave, which is similar to the observed cloud marking. Our results showed that the vertical wind associated with the Kelvin wave is essential for maintaining the quasi‐periodic variation, along with the condensation/evaporation by the temperature variation. The vertical‐wind‐induced cloud generation also suggests a relationship to the cloud discontinuity. Plain Language Summary: Venus is completely shrouded in thick sulfuric acid (H2SO4) clouds. The Venusian cloud structure is determined by atmospheric circulation, chemistry, and cloud microphysics, and its morphology is thought to manifest atmospheric activities such as waves and instabilities. Near‐infrared observations have discovered quasi‐periodic features in low latitudes, such as planetary‐scale cloud marking and discontinuity. However, the formation mechanism of the quasi‐periodic features is not well understood because of the complicated cloud process. In this study, we have developed a three‐dimensional global numerical model which simulates cloud condensation/evaporation and transport processes coupled with simple chemistry representing the H2SO4 cycle in the Venusian atmosphere. The simulated latitudinal trends of the cloud top and bottom structures are qualitatively consistent with past observations. In low latitudes, a planetary‐scale cloud marking propagates quasi‐periodically, consistent with previous near‐infrared observations. Our results suggest that vertical wind associated with a planetary‐scale wave is essential for maintaining the cloud marking. This cloud formation mechanism is possibly related to the cloud discontinuity suggested by recent observations. Key Points: Cloud opacity variation in low latitudes is investigated by a Venus general circulation model with simple cloud parameterizationThe simulated opacity variation is consistent with past observations, composed of variations induced by gravity and Kelvin wavesVertical wind associated with the Kelvin wave is essential for maintaining the quasi‐periodic cloud variation [ABSTRACT FROM AUTHOR]

The first year (December 2015 to November 2016) of IR2 after Akatsuki's successful insertion to an elongated elliptical orbit around Venus is reported with performance evaluation and results of data acquisition. The single-stage Stirling-cycle cryo-cooler of IR2 has been operated with various driving voltages to achieve the best possible cooling under the given thermal environment. A total of 3091 images of Venus (1420 dayside images at 2.02 μm and 1671 night-side images at 1.735, 2.26, and 2.32 μm) were acquired in this period. Additionally, 159 images, including images of stars for calibration and dark images for the evaluation of noise levels, were captured. Low-frequency flat images (not available in pre-launch calibration data) have been constructed using the images of Venus acquired from near the pericenter to establish the procedure to correct for the IR2 flat-field response. It was noticed that multiple reflections of infrared light in the PtSi detector caused a weak but extended tail of the point-spread function (PSF), contaminating the night-side disk of Venus with light from the much brighter dayside crescent. This necessitated the construction of an empirical PSF to remove this contamination and also to improve the dayside data by deconvolution, and this work is also discussed. Detailed astrometry is performed on star-field images in the H-band (1.65 μm), hereby confirming that the geometrical distortion of IR2 images is negligible. [ABSTRACT FROM AUTHOR]

Highlights: [•] A new simulation for the Venus mesosphere was compared with direct wind observation. [•] The observations showed much higher variability than that produced by Kelvin wave. [•] Most of the variability can be potentially explained by the gravity wave breaking. [ABSTRACT FROM AUTHOR]

Abstract: The Venus Climate Orbiter mission (PLANET-C), one of the future planetary missions of Japan, aims at understanding the atmospheric circulation of Venus. Meteorological information will be obtained by globally mapping clouds and minor constituents successively with four cameras at ultraviolet and infrared wavelengths, detecting lightning with a high-speed imager, and observing the vertical structure of the atmosphere with radio science technique. The equatorial elongated orbit with westward revolution fits the observation of the movement and temporal variation of the atmosphere which as a whole rotates westward. The systematic, continuous imaging observations will provide us with an unprecedented large data set of the Venusian atmospheric dynamics. Additional targets of the mission are the exploration of the ground surface and the observation of zodiacal light. The mission will complement the ESA''s Venus Express, which also explores the Venusian environment with different approaches. [Copyright &y& Elsevier]