Thermal structure of the Venusian atmosphere from the sub-cloud region to the mesosphere as observed by radio occultation

Abstract

We present distributions of the zonal-mean temperature and static stability in the Venusian atmosphere obtained from Venus Express and Akatsuki radio occultation profiles penetrating down to an altitude of 40 km. At latitudes equatorward of 75°, static stability derived from the observed temperature profiles is consistent with previous in-situ measurements in that there is a low-stability layer at altitudes of 50-58 km and highly and moderately stratified layers above 58 km and below 50 km, respectively. Meanwhile, at latitudes poleward of 75°, a low-stability layer extends down to 42 km, which has been unreported in analyses of previous measurements. The deep low-stability layer in the polar region cannot be explained by vertical convection in the middle/lower cloud layer, and the present result thus introduces new constraints on the dynamics of the sub-cloud atmosphere. The Venusian atmosphere is in striking contrast to the earth's troposphere, which generally has a deeper low-stability layer at low latitudes than at mid-and high latitudes. The dynamics of the Venusian atmosphere remain unclear because Venus is completely globally covered by thick clouds at altitudes of 48-70 km. The thermal structure of the Venusian atmosphere across this cloud layer is important in terms of understanding aspects of the general circulation, such as the mean meridional circulation and baroclinic instability waves, which contribute to meridional heat transport and atmospheric super-rotation. In-situ measurements were made by entry probes around the equator and 60°N as part of the Venera and Pioneer Venus missions, revealing a low-stability (weakly or almost neutrally stratified) layer at an altitude of approximately 50-55 km, a highly stratified layer above 55 km and a moderately stratified layer below 50 km 1. Radio occultation measurements, one of the most useful methods of obtaining vertical temperature profiles, were made as part of the National Aeronautics and Space Administration's (NASA's) Pioneer Venus mission and the European Space Agency's (ESA's) Venus Express mission 2-4 to obtain latitude-height distributions of temperature. These observations revealed that a cold latitudinal band called a "cold collar", which is thought to be formed by dynamics 5 and/or the latitudinal cloud structure 6 , is located at roughly 65° latitude near the cloud top (at nearly 65 km altitude) and surrounds a warm polar region and that temperature increases with latitude above 65 km and decreases with latitude below 60 km. Furthermore, static stability profiles obtained from the Venus Express radio occultation measurements showed that the low-stability layer in the cloud layer is deeper at high latitudes than at low and mid-latitudes 4. However, no in-situ measurements have been made at latitudes poleward of 60°, and the thermal structure below the cloud layer at high latitudes thus remains unknown.

Atmospheric response to high-resolution topographical and radiative forcings in a general circulation model of Venus: Time-mean structures of waves and variances

“… Thermal tides, stationary waves, and general circulation are investigated using a T63 Venus general circulation model (GCM) with solar and thermal radiative transfer in the presence of high-resolution surface topography, based on time average analysis. The simulated wind and static stability are very similar to the observed ones (e.g., Horinouchi et al, 2018 ; Ando et al, 2020 ). The simulated thermal tides accelerate an equatorial superrotational flow with a speed of ~90 m s −1 around the cloud-heating maximum (~65 km).…”

“…Although the zonal jets in the T63 model are ~10 m s −1 weaker than those in the T21 model ( Yamamoto et al, 2019 ), the zonal-flow and temperature structures are similar in the two models. Our T63 simulation (the present work) reproduces the multi-layered stable layers at low latitudes ( Young et al, 1987 ), the polar strongly stable regions above the 10 4 -Pa altitude (>65 km), and the deep low-stability region below the polar stable regions ( Tellmann et al, 2009 ; Ando et al, 2020 ). These structures are formed by the radiative forcing ( Fig.…”

“…2 b and d). As in our previous low-resolution model, our simulation reproduces the UV-tracked horizontal flow around the subsolar point and the equatorial multi-layered and polar structures of static stability ( Horinouchi et al, 2018 ; Ando et al, 2020 ). The simulated thermal tides are vertical and equatorward transporters of zonal momentum ( Fig.…”

Venusian Cloud Distribution Simulated by a General Circulation Model

“…Journal of Geophysical Research: Planets occultation measurements (e.g., Ando et al, 2015Ando et al, , 2020Tellmann et al, 2009Tellmann et al, , 2012. It is inferred from these results that the H 2 O vapor distribution is affected by the vertical motions enhanced in the low static stability layer in high latitudes at 45-62 km levels in the present study.…”

Calibration Techniques for Studying Venus and Mars Atmospheres

“…The first VEX scientific results have been presented by Pätzold [6], showing vertical profiles of Venus ionosphere between 100 and 500 km altitude and a neutral atmosphere between 40 and 90 km altitude. Several studies have been published by Tellmann, Piccialli, Lee, Ando [7][8][9][10][11] regarding also the dynamics of Venus atmosphere as well as its thermal structure. Nowadays Akatsuki spacecraft from the Japanese Space Agency (JAXA) is continuing Venus' exploration and more missions will be sent there in the near future.…”