10:00 AM - 10:15 AM
[PCG21-11] Study of the Venusian cloud formation and distribution in low- and mid-latitudes using a GCM: Effects of atmospheric chemistry and circulation
Keywords:Venus, Sulfuric acid clouds, Atmospheric chemistry, General circulation model, Akatsuki
We used the VGCM modified from the CCSR/NIES/FRCGC AGCM [Ikeda, 2011], with the horizontal resolution of T21 (longitude and latitude grids of about 5.6 degrees) and 52 levels in vertical with the sigma (equivalent to the pressure) coordinate (the top altitude of ~95 km). For the cloud condensation and evaporation processes, the effects of supersaturation are not considered and the radius of cloud aerosol is artificially distributed into 4 modes by ratios based on the vertical profiles shown by Haus and Arnold . Note that our model at the moment does not include the growth or reduce of the particle size, and only traces the advection of produced clouds. Also note that the cloud distributions should modulate the thermal distributions through radiative effects, but the current model still assumes constant heat input profile (as well as the former code by Ikeda ).
Our model includes the chemical processes related to the production and loss of H2SO4 vapor (SO3, SO2 and H2O) in order to reproduce the realistic maintenance processes of cloud distributions. In the model with those chemical processes (hereafter WC model), the simulated latitudinal distributions of the optical thickness of clouds agree with the observational results by the Visible and InfraRed Thermal Imaging Spectrometer (VIRTIS) onboard Venus Express (VEX), and also the simulated vertical profiles of H2SO4 vapor agree with the Magellan radio occultation data, in low- and mid-latitudes (0-70o N). On the other hand, in the model without the chemical processes (hereafter NC model), the cloud opacity distributions become less than half of the observed ones, and also the abundance of H2SO4 vapor around 48 km becomes less than half in comparison with the WC model. Clouds are formed above ~50 km in both models, but the abundances in the WC model are larger than those in the NC model. The difference of the WC model is caused by the production of H2SO4 vapor due to the chemical processes and the condensation of it into the clouds in the upper cloud region (60-80 km). We conclude that our VGCM reproduces the observed features with a good reliability in low- and mid-latitudes with chemical processes.
With this VGCM, we investigated the maintenance and circulation processes of the sulfuric acid clouds and vapor in low- and mid-latitudes. Our model indicated that, in the upper cloud region, the clouds are produced centered at ~65 km altitude, and flow upward and poleward by the meridional circulation and vertical diffusion. Meanwhile, in the lower cloud region (50-60 km), H2SO4 vapor transported by the upward advection and vertical diffusion condenses into the cloud in the equatorial region at the altitude of 50-54 km, and the formed clouds are transported poleward along the meridional circulation. These cycles are consistent with those simulated in a 2-D latitude-altitude cloud formation model by Imamura and Hashimoto  with a given meridional circulation, and we first reproduced the cycle in a VGCM. Using this new VGCM, we showed that the diurnal tide mainly contributed to drive the meridional circulation through the wave-zonal mean flow interaction in low- and mid- latitudes.