Vertical wavenumber spectra of gravity waves in the Venus atmosphere using Akatsuki radio occultation profiles and comparison with model spectra

This study utilizes Akatsuki radio occultation profiles, recorded from 2016 March to 2022 December, to investigate gravity waves (GWs) in Venus's atmosphere. The latitudinal temperature distribution reveals an increase in temperature above 65 km with latitude, while temperatures decrease below 60 km with latitude. Temperature perturbations, derived by subtracting a polynomial fit from the temperature profiles, were bandpass filtered to isolate vertical wavelengths between 0.5 and 20 km, attributing the resulting perturbations to GW activity. The perturbations were analysed to determine the spectral slope, and amplitudes corresponding to characteristic wavenumbers, which were then compared with linear instability theory (LIT) predictions to assess their applicability to Venus's atmosphere. The vertical wavenumber spectra reveal a general trend of decreasing spectral density with increasing wavenumber, with the slope flattening at lower wavenumbers. Normalizing the estimated temperature perturbations by the background temperature and performing spectral analysis shows higher amplitudes in the mid-latitude and polar regions, ranging from 0.003 +/- 0.003 to 0.007 +/- 0.008 normalized/(cycles km(-1)), with the highest values observed near the South Pole. The spectral amplitudes across the latitude range (90 degrees S to 90 degrees N) were compared with LIT predicted values to evaluate the theory's consistency with observed data. The LIT fails to adequately explain the observed spectral values in Venus's atmosphere at an altitude range of 45-65 km, but LIT predicts well at an altitude range of 65-85 km. A notable GW 'hotspot' is identified in the polar region, within the altitude range of 63-67 km.