Characterization of mesoscale gravity waves in the upper and lower clouds of Venus from VEX‐VIRTIS images

Abstract

Images obtained from the Visible and InfraRed Thermal Imaging Spectrometer (VIRTIS)‐M instrument onboard Venus Express present visible trains of alternating bands of cloud brightness in two different layers: at the upper cloud tops (∼66 km altitude) observed in the dayside hemisphere using reflected ultraviolet light (380 nm) and in the lower cloud (∼47 km altitude) observed in the nightside hemisphere using thermal radiation (1.74 μm). The waves are nearly zonal (with the bands perpendicular to latitude circles), have wavelengths of 60–150 km, propagate westward with low phase velocities relative to the zonal flow, and are confined in wave packets of 400 to 1800 km in length. The waves in the lower cloud observed in the infrared are widely distributed around the planet, and their appearance varies widely throughout the VIRTIS data set. The locations of both types of waves seem not correlated with latitude, local times, surface topography, or the structure of the wind. In both cases the characteristics of the waves correspond to gravity waves propagating in confined stable layers of the atmosphere. We examine the properties of these waves in terms of a linear model and perform a simple analysis to discuss the vertical stability of the atmosphere within Venus clouds.

Overview of Akatsuki data products: definition of data levels, method and accuracy of geometric correction

“…Recent numerical studies suggest that both the thermal tide (e.g., Matsuda 2006, 2007) and meridional circulation mechanisms (e.g., Yamamoto and Takahashi 2004) might be viable in the Venusian atmosphere, in which the vertical and horizontal transport of angular momentum due to thermal tides, eddies, and mean meridional circulation plays an important role. Other kinds of waves, such as the four-and fiveday waves found in UV images of the cloud top (e.g., Del Kouyama et al 2013Kouyama et al , 2015 and small-scale disturbances suspected to be related to gravity waves (e.g., Sagdeev et al 1986;Hinson and Jenkins 1995;Peralta et al 2008;Garcia et al 2009) may also play important roles in the establishment and maintenance of the atmosphere's super-rotation. Schubert et al (1980) suggested that the zonal-mean meridional circulation consists of several vertically stacked cells extending from the equator to poles, based on wind profiles obtained by entry probes.…”

“…We have no observational results as yet for the zonally averaged meridional circulation in the Venusian atmosphere. To elucidate the dynamical effects of meridional circulation, waves, and eddies, their threedimensional structures on both the dayside and nightside must be understood (Peralta et al 2008).…”

Overview of Venus orbiter, Akatsuki

“…The specific disturbance that provides energy to the Kelvin wave has not been identified. Small-scale atmospheric structures indicative of internal gravity waves were observed in temperature profiles (Hinson and Jenkins, 1995) and in cloud images Peralta et al, 2008). Oscillating vertical winds encountered by Vega balloons over highlands are attribute to orographically-excited gravity waves (Sagdeev et al, 1986); orographic waves, however, should transport eastward momentum upward and decelerate the super-rotation.…”

Development and in-flight calibration of IR2: 2-μm camera onboard Japan’s Venus orbiter, Akatsuki

“…VEX/VIRTIS-M images revealed numerous mesoscale waves in the upper and lower atmosphere (Peralta et al 2008), confined to poleward of 40°S. Because of VEX's polar orbit, however, detection of waves might have been biased to higher southern latitudes.…”