Nature of the Atmospheric Dynamics on Venus from Power Spectrum Analysis of Mariner 10 Images

Abstract

Mariner 10 Venus images are analyzed into power spectra of the brightness field for planetary zonal wavenumbers n≥3 and for latitudes from 55°?25°N. These spectra are examined for significant features and latitudinal variation as well as compared with cloud brightness distribution spectra similarly determined for the Earth. The Venus image spectra are found to have a systematic variation as a function of latitude. In the equatorial region the average power spectrum lm an approximately ?2.7 power law behavior for n>5, while in midlatitudes the average spectrum is characterized by a slope of about ?1.8 for n≥3. Although a flattening of the equatorial region spectrum for wavenumbers less than 5 is noted, no obvious peaks are superimposed on the general power law behavior of the Venus spectra. This is in contrast to the result for the Earth cloud brightness spectra, which exhibit a noticeable peak near wavenumber 5 or 6 for the midlatitude region. The Venus image spectra are further interpreted under the assumption that the UV cloud features serve as markers of large-scale dynamical processes and thus can reveal the characteristics of the eddy kinetic energy spectrum. Support for this assumption is provided by the results of a comparison of the cloud brightness spectra for the Earth with observed eddy kinetic energy spectra. The characteristics of the Venus spectra at low latitudes suggest that the observed clouds are in a region of high static stability in which the large-scale turbulence is essentially two-dimensional, and that the important energy sources are at the largest scales (n<5), with the spectrum exhibiting a slope close to the theoretically expected ?3 value for an inertial subrange at larger wavenumbers. It is concluded that the less steep slope observed for the midlatitude spectrum is quite possibly the manifestation of the importance of barotropic eddy disturbances in that region.