Dayside cloud top structure of Venus retrieved from Akatsuki IR2 observations

We describe the dayside cloud top structure of Venus as retrieved from 93 images acquired at a wide variety of solar phase angles (0-120 degrees) using the 2.02-mu m channel of the 2-mu m camera (IR2) onboard the Venus orbiter, Akatsuki, from April 4 to May 25, 2016. Since the 2.02-mu m channel is located in a CO2 absorption band, the sunlight reflected from Venus allowed us to determine the cloud top altitude corresponding to unit aerosol optical depth at 2.02 mu m. First, the observed solar phase angle dependence and the center-to-limb variation of the reflected sunlight in the region equatorward of 30 degrees were used to construct a spatially averaged cloud top structure characterized by cloud top altitude z(c), Mode 2 modal radius r(g,2), and cloud scale height H, which were 70.4 km, 1.06 mu m, and 5.3 km, respectively. Second, cloud top altitudes at individual locations were retrieved on a pixel-by-pixel basis with an assumption that r(g,2) and H were uniform for the entire planet. The latitudinal structure of the cloud top altitude was symmetric with respect to the equator. The average cloud top altitude was 70.5 km in the equatorial region and showed a gradual decrease of similar to 2 km by the 45 degrees latitude. It rapidly dropped at latitudes of 50-60 degrees and reached 61 km in latitudes of 70-75 degrees. The average cloud top altitude in the region equatorward of 30 degrees showed negligible local time dependence, with changes up to 1 km at most. Local variations in cloud top altitude, including stationary gravity wave features, occurred within several hundreds of meters. Although long zonal or tilted streaky features poleward of similar to 45 degrees were clearly identifiable, features in the low and middle latitudes were usually subtle. These did not necessarily appear as local variations at the cloud top level, where mottled and patchy UV patterns were observed, suggestive of convection and turbulence at the cloud top level.