Fine Vertical Structures at the Cloud Heights of Venus Revealed by Radio Holographic Analysis of Venus Express and Akatsuki Radio Occultation Data

Abstract

Radio occultation (RO) is one of the most efficient techniques for studying fine vertical structures in planetary atmospheres. However, the geometrical optics (GO) method, which has been used for the analysis of RO data, suffers blurring by the finite width (Fresnel scale) of the radio ray and cannot decipher multipath propagation, which also prevents retrieval of fine structures. Here we apply Full Spectrum Inversion (FSI), which is one of the radio holographic methods, to RO data taken in Venus Express and Akatsuki missions to retrieve fine structures in Venus' cloud-level atmosphere. The temperature profiles obtained by FSI achieve vertical resolutions of~150 m, which is much higher than the typical resolution of 400-700 m in GO, and resolve structures in multipath regions. Thin, near-neutral layers are found to be ubiquitous at cloud heights; we suggest here that those layers are caused by the mixing associated with the breaking of short-wavelength gravity waves. The wavenumber spectra of small-scale structures are consistent with the semiempirical spectrum of saturated gravity waves and show larger amplitudes at higher latitudes. Temperature profiles in the high latitudes frequently show a sharp temperature minimum near the cloud top, below which the vertical temperature gradient is near adiabat, implying that the sharp temperature minimum is created by adiabatic cooling associated with convective plumes that impinge on the overlying stable layer.Using the Venus Express radio occultation data, Tellmann et al. (2012) studied the meridional distribution of the temperature perturbation and revealed an enhanced wave activity at high latitudes. Ando et al. (2015) studied, using the same data set, the vertical wavenumber spectrum of the temperature and showed that the spectra are roughly consistent with the semiempirical spectrum of saturated gravity waves; this feature implies occurrence of wave breaking (Tsuda et al., 1991). A major caveat is that the observed static stabilities are positive throughout the atmosphere except in the middle and lower cloud regions; neutral or nearneutral stability layers, which are indicative of turbulence generation by breaking gravity waves, are hardly IMAMURA ET AL. 2151

HDO and SO2 thermal mapping on Venus

“…-The SO 2 plume distribution as a function of local time seems to show a minimum occurrence around noon, with two possible maxima around the terminator. The depletion around 12:00, if confirmed, could be the signature of convection inhibition (Imamura et al 2018), or photochemical processes associated with the incidence angle, or dynamical transport. Presently, the limited amount of data prevents us from drawing a firm conclusion on this phenomenon.…”

“…A possible explanation for this depletion might be a suppression of the cloud level convection, which could inhibit transport through the cloud layer, as suggested by Imamura et al (2018). Other possible explanations might be a photochemical process, as suggested by the subsolar depletion, and/or a dynamical wave pattern.…”

Stationary Features at the Cloud Top of Venus Observed by Ultraviolet Imager Onboard Akatsuki

“…Characterization of topographic gravity waves in a broad altitude region is crucial for understanding the momentum deposition by those waves and the resultant deceleration of the superrotation above the cloud. Note, however, that waves with vertical wavelengths of tens of kilometers (section ) cannot be detected by radio occultation measurements, which focus on vertical wavelengths shorter than several kilometers (e.g., Imamura et al, ; Tellmann et al, ).…”

Gravity Wave Packets in the Venusian Atmosphere Observed by Radio Occultation Experiments: Comparison With Saturation Theory

“…(2015) used, Imamura et al. (2018) showed that thin near‐neutral layers frequently exist in the Venusian atmosphere, using high‐vertical‐resolution temperature profiles obtained by a radio holographic method from the Venus Express and Akatsuki radio occultation experiments. Examples of the vertical profile of the static stability are shown in Figure 1, where dT / dz is the vertical gradient of the temperature, g is the gravitational acceleration and C p is the specific heat at constant pressure given by Seiff et al.…”