Planetary‐scale waves are thought to play a role in powering the yet unexplained atmospheric superrotation of Venus. Puzzlingly, while Kelvin, Rossby, and stationary waves manifest at the upper clouds (65–70 km), no planetary‐scale waves or stationary patterns have been reported in the intervening level of the lower clouds (48–55 km), although the latter are probably Lee waves. Using observations by the Akatsuki orbiter and ground‐based telescopes, we show that the lower clouds follow a regular cycle punctuated between 30°N and 40°S by a sharp discontinuity or disruption with potential implications to Venus's general circulation and thermal structure. This disruption exhibits a westward rotation period of ∼4.9 days faster than winds at this level (∼6‐day period), alters clouds' properties and aerosols, and remains coherent during weeks. Past observations reveal its recurrent nature since at least 1983, and numerical simulations show that a nonlinear Kelvin wave reproduces many of its properties.
<p>The spatial and temporal structures of "Enormous Cloud Cover" (ECC), seen in 2.26- and 1.735-&#181;m Venus' night-side images acquired by Akatsuki/IR2, are investigated. The data were acquired on 18th and 27th August 2016 and have been processed newly-developed "Restoration by Deconvolution" (RD) method that effectively removes contaminating light spread from the intense day crescent. Spectral radiances are compared between ECC and "seemingly normal" area (BC = Background Clouds). Attenuation by ECC is stronger at 2.26 &#181;m (~70 to 80 %) than at 1.735 &#181;m (~50 %) due primarily to lower single-scattering albedo of cloud particles at 2.26 &#181;m. Detailed radiative-transfer analyses suggest the followings:</p>
<ol>
<li>ECC consists of mostly mode 3 H<sub>2</sub>SO<sub>4</sub> particles (mean radius 3.65 &#181;m) with optical thickness ~8 atop BC.</li>
<li>The altitude of ECC varies from higher (~60 km near the western end) to lower (~54 km in the middle).</li>
</ol>
<p>A possible scenario to explain these observational characteristics, strong upwelling region near the western end (front of propagating feature), pushing H<sub>2</sub>SO<sub>4</sub> vapor to condensate in high altitudes. After the region of strongest upwelling propagates away, the cloud particles gradually sediment or are pulled back by downwelling motion of atmosphere.</p>
<p>Details of data analysis, interpretation of phenomena with comarison to numerical simulations will be presented.</p>
We have developed a new method ‘Restoration by Deconvolution’ (RD) to restore nightside photometry in the Akatsuki/IR2 images contaminated by spread light from intense dayside crescent. With updated point spread function model for IR2 and incorporation of radiative transfer computations, our RD-method is able to improve the photometric accuracy of nightside emission data in both 2.26µm and 1.735µm filters. Exploiting the enhanced photometric quality, the ‘Enormous Cloud Cover’ (ECC) features observed in both 2016-08-18 and 2016-08-27 data have been investigated. Possible altitude variations in the ECC’s spatial variation measurements from z=52km up to z=60km were found. The observations were interpreted that ECC constituting of large sulfuric acid droplets (mean radius of 3.65µm and the optical thickness ~7-9) experience strong upwelling near the ECC front. This elevates the aerosols up to z=58-60km, then subjected upon by strong downwelling forces the particles to sink by a velocity ~-0.3m/s. The analyses of physical properties and evolutionary behavior of the ECC on both dates (08-18 and 08-27) suggest them to be recurring phenomena in the lower part of Venus’ clouds.
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