Other
Scientific paper
Aug 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007phdt........13m&link_type=abstract
Proquest Dissertations And Theses 2007. Section 0051, Part 0606 182 pages; [Ph.D. dissertation].United States - - Colorado: Uni
Other
Brightness, Venus, Clouds, Gravity Waves
Scientific paper
The goal of this thesis is to understand the causes of the holes that form in the Venus condensational clouds; and to determine the mechanisms that are responsible for their subsequent evolution. Near Infrared observations of the nightside of Venus reveal regions of high brightness temperatures. These regions of high brightness temperatures are caused by the localized evaporation of the middle and lower cloud decks, which are about 50 to 60 km above the surface of the planet. We simulate the Venus condensational middle and lower cloud deck with the University of Colorado/NASA Ames Community Aerosol and Radiation Model for Atmospheres (CARMA). Our simulated clouds have similar characteristics to the observed Venus clouds. Our radiative transfer model reproduces the observed temperature structure and atmospheric stability structure within the middle cloud region. A radiative-dynamical feedback occurs that generates mixing due to increased absorption of upwelling infrared radiation within the lower cloud region, as previously suggested by others. Using a one-dimensional model, we find that localized variations in temperature structure or in sub-grid scale mixing cannot directly explain the longevity and the optical depth of the clouds. However, vertical motions are capable of altering the cloud optical depth by a sufficient magnitude in a short enough timescale to be responsible for the observed clearings.
The lifetimes of these holes are observed to be moderately short, on the order of ten days. We have also developed a two-dimensional (zonal, vertical) model with simplified dynamics to study the effects of zonal flow on the lifetimes of the holes in the clouds. We find that although wind shear may be negligible within the middle cloud itself, the shear that is present near the top and the bottom of the statically unstable middle cloud region can lead to changes in the radiative-dynamical feedback which ultimately lead to the dissipation of the holes.
Gravity waves have been observed in the Venus atmosphere in the form of temperature scintillations in the Magellan and Pioneer Venus occultation data. Multiple in situ probes and long-duration remote observations indicate the presence of convective motions in the Venus clouds. Dynamical studies by others have suggested that gravity waves can exist in the stable regions of the Venus atmosphere both above and beneath the middle clouds, and likely are triggered by flow past sub-cloud plumes caused by convective overshooting. We present results of a simple two-dimensional model investigating the observable effects that convective motions and gravity waves can have on the condensational Venus cloud. We find that a simplified treatment of convective dynamics generates variation in the Venus condensational cloud consistent with observed variability. We also find that gravity waves launched by obstacles (such as overshooting convective plumes) near cloud base are capable of generating variations in 1.74mm brightness temperature that should be observable by instruments such as VIRTIS on Venus Express . However, a more robust treatment of the atmospheric dynamics is needed to address adequately these interactions with the mesoscale dynamics.
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