Dynamics of the Venus atmosphere from a Fourier-transform analysis .

The radiance escaping from the Venus deep atmosphere into space at 1.74 μm, one of the atmospheric window, is modulated by the opacity of the lower clouds at about 47 km and it can be effectively used to probe the layer at this altitude. We have used the VIRTIS (Visible and InfraRed Thermal Imaging Spectrometer) instrument onboard the Venus-Express spacecraft able to obtain images of the South hemisphere with a typical projected field of view covering 40◦ in latitude and 50◦ in longitude when observed from the apocenter, at a distance of about 65000 km. From this radiance it is possible to characterize different dynamical regimes present in the lower cloud. The observed cloud features have very different morphologies, from bland regions with a few structures to chaotic regions highly contrasted. VIRTIS observations are effectively used to map the dynamical recurrent patterns in the lower clouds layer and these are investigated through a Fast Fourier Transform analysis. The method uses a numeric set of indicators, defined starting by the FFT power spectrum, to differentiate automatically the cloud regions with laminar and turbulent dynamical regimes. The difference between the spectral slopes of the FFT at low frequency is identified as an indicator to detect automatically a laminar or turbulent behavior regime. Another identified parameter related to the cloud turbulent-laminar aspect is the difference between the power spectrum of the first wavenumber and the mean value of the higher power spectrum wavenumbers. This is able to highlight the different dynamical conditions in case of turbulence at the lower scale and thus it removes the ambiguity present in the range of distribution of the spectral slopes when transverse laminar flows respect to the zonal motion or gravity wave on laminar flow are present.