Study of Venus atmosphere dynamics
using cloud tracking technique

Toru Kouyama

Space and Planetary Science Group
Department of Earth & Planetary Science
Graduate School of Science

The University of Tokyo

December 2011

Abstract
The cloud tracking method with cross correlations is the most practical mean ofmeasuring the wind velocity with extensive spatial and temporal coverage for a cloudy
planet like Venus. To study the atmospheric dynamics of Venus, an improved cloudtracking method for deriving wind velocities from successive planetary images wasdeveloped. In this method, to reduce measurement errors in wind velocities, we
combined several techniques: redetermining the camera pointing information by fitting
an ellipse to the Venus limb in each image; calibrating optical distortions using Venus
images taken after the orbital insersion of the spacecraft; and correcting erroneous cloud
motion vectors by re-determining the most plausible correlation peak among all of thelocal maxima on the correlation surface by comparing each vector with its neighboring
vectors.
The newly-developed method was applied to the Venusian violet images obtainedby Solid State Imaging system (SSI) onboard the Galileo spacecraft during its Venus
flyby and Venus Monitoring Camera (VMC) onboard Venus Express. Two-dimensional
distributions of the horizontal wind vector were successfully obtained.
From the SSI data, it was found that the solar-fixed component of the windvelocity field in 1990 was similar to that in 1982 obtained by the Pioneer Venus orbiter.
This suggests that the structures of the mean circulation and the thermal tides werelargely unchanged between these epochs. The deviation of the instantaneous zonal windfield from the solar-fixed component shows a distinct wavenumber-1 structure in theequatorial region. On the assumption that this structure is a manifestation of an
equatorial Kelvin wave, the momentum deposition by this Kelvin wave, which is2

subject to radiative damping, would induce a westward mean-wind acceleration of ~0.3
m s-1 per Earth day.
From VMC data over four years, an indication of a periodical, long-term
oscillation of the super-rotation was suggested for the first time. The estimated period of
about 260 d is longer than one Venus year, and thus the oscillation will not be a seasonal
cycle. When the mean zonal wind is relatively fast, its latitudinal profile is nearly flatfrom low to mid-latitudes, while distinct mid-latitude jets emerge when the mean wind
is relatively slow. Since the amplitude of the long-term variation is larger at lower
latitudes, the mid-latitude jets are emphasized during the slow wind period.
Kelvin wave-like perturbations are observed when the background wind isrelatively slow, while Rossby wave-like perturbations are observed when thebackground wind is relatively fast. We investigated the influence of the variation of thebackground zonal wind speed on the upward propagation of Kelvin and Rossby waves
at altitudes 60-100 km assuming that radiative damping is the principal dissipation
process. Results from a linearized primitive equation model suggests that Kelvin waves
can reach the cloud top height in the slow background wind period and that Rossby
waves can reach the cloud top in the fast wind period, being consistent with theobservations. The horizontal wave structures of the modeled Kelvin and Rossby waves
are also in agreement with VMC observations.
Since the momentum deposition by these waves can accelerate and decelerate
the mean flow, these waves will contribute, at least partly, to the periodical oscillation of
the super-rotation. The Kelvin wave in our model showed significant acceleration in low
latitudes and it can be a candidate of the increasing of zonal wind speed. However,
because the Rossby wave decelerates mainly in mid-latitudes, other mechanisms are3

required to decelerate to the zonal wind speed in low latitudes, and the advection flow
may play a key role of the deceleration.

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