Auxiliary Information

Vortex Circulation on Venus:
Dynamical similarities with Terrestrial Hurricanes

A simple two dimensional model used for simulation of the dynamical instability in tropical cyclones has been adapted to investigate the dynamical instability in the hemispheric vortices centered on each pole of Venus. The investigation was triggered by the VIRTIS observations of the inverted S-shape around the south pole of Venus during the insertion orbit of Venus Express [Piccioni et al., 2007]. Confirming not only the hemispheric symmetry of the atmospheric circulation on Venus in terms of features observed, VIRTIS also presented a day-night view of the planet from a unique polar perspective, by capturing the day side in reflected ultraviolet and by recording the emitted radiation from the night hemisphere in near infrared (e.g., Figure 1b). This instantaneous day-night global view of Venus over the pole confirmed the vortex organization determined from the space time composite view generated by Suomi and Limaye [1978].

Figure 1a. S-shaped pattern seen near the south pole of Venus in April 2006 in the 5.05 µ VIRTIS observations [from Piccioni et al., 2007]. The bright features show higher radiance values compared to the darker hues and likely reflect warmer temperatures found in the lower atmosphere. However, it is also likely that subsidence, expected from the converging flow is also responsible for the warming of the lower atmosphere somewhat.

Figure 1b. A composite view of the day and night side of Venus from the VIRTIS instrument on Venus Express (ESA Image SEM49273R8F). This global view of Venus is a mosaic of several images taken by the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) on board ESA's Venus Express on 18 May 2007, at a distance of about 66 000 km from the planet. The images were obtained at 1.7 µ (left) and 3.8 µ (right) wavelength, The wavelength used to obtain the left-hemisphere composite (1.7 µ) provides a dramatic global view of the night-side clouds in the lower atmosphere (approximately 45 km), while the wavelength used to obtain the right-hemisphere composite (3.8 µ) provides a view of the day-side cloud top.

The presence of the inverted S-shape and its appearance in the early VIRTIS data prompted us to look at the barotropic instability from the meridional profiles of the zonal flow which had been extended to higher latitudes [Limaye, 2007]. A profile relative vorticity is needed for the stability analysis and hence some comments about the determination of the vorticity profile are necessary. While Venus is completely covered with clouds, the tracking of (light or dark) ultraviolet features in the global images obtained from orbit is still challenging due to the peculiar cloud morphology. Discrete, isolated cloud features at high latitudes are rare. Cloud tracking is therefore difficult and the resulting flow somewhat uncertain in its precise shape. Nevertheless, different estimates of the profiles are generally consistent in the main details, and some temporal variability has been noted on time scales of weeks to months and perhaps longer. Note that for dynamical studies, we need measurements and a computation of cyclostrophically balanced flow is not sufficient (as it would preclude meridional motions).

The relative vorticity profile for Venus is not very well known and does not cover the critical polar region; therefore fits to the calculated relative vorticity profile extrapolated to the center of the vortex based were used. The relative vorticity was estimated from the results of cloud tracking using Pioneer Venus Orbiter images acquired in 1980.

Vorticity was determined from cloud motions averaged in latitude bins and error in the vorticity was estimated from the measurements. Beyond 75°, the analytical profiles in Fig. 1c were arbitrarily extrapolated to either zero or 2 x 10^-5 s^-1 at the pole. Since we are interested in the instability of large scale motions, the idealized, smoothed data are acceptable. The typical degree of small-scale variability in the meridional vorticity profile is not yet confidently known from observations.

While recent cloud tracking data from low resolution imagery taken aboard Venus Express [Markiewicz et al., 2007] have revealed a similar atmospheric state, the high latitude vorticity is still not reliably known. Near infrared imagery from Venus Express during April 2006 [Piccioni et al., 2007] revealed an S-shaped feature around the southern pole (Fig. 1) that was nearly a mirror image of feature seen in the northern pole from Pioneer Venus in 1979.

Figure 2. Animated output from the model, initialized with the profile from Pioneer Venus, shows the development of an S-shape similar to that seen in the Pioneer Venus observations as well as from Venus Express (flipped due to with mirror symmetry). Evolved time is indicated in hours on the top left of the image.	Figure 3. Animation of Hurricane Howard (thermal infrared images) over about a day showing the development of a similar "S" shaped transient feature in the core. Comparison of the model simulation for the Venus vortex (Figure 2) with the evolution of the core region of Hurricane Howard (Figure 3), which also exhibited a similar transient S-shape is shown here.

Hurricane Howard's S-Shape feature lasts less than two hours, whereas the S shaped feature on Venus is seen to evolve more slowly in the model. Venus Express observations of the S-shape are critical in this respect and show a similar slowly changing appearance.

It can be seen from the two different profiles we tried that the growth rate for the instability depends on the exact shape of the vorticity profile from the peak value vorticity radius to the center of the vortex. In this region of Venus however, the cloud motions are very difficult to measure due to lack of good cloud targets and hence the vorticity profile is more uncertain, and hence we can gain some insight in the dynamics of the core region near the pole through the evolution rates of the observed features through the use of the model simulations.

A vorticity profile that does not decrease to the pole impacts the growth rate of the instability as can be seen in the animation shown to the right. Beyond about 75 deg south, the confidence in the cloud tracked motions is low and the relative vorticity estimates are less reliable. We approximated the profile here between the peak vorticity and the pole as shown here.

Since the initial sighting of the Southern "S" shape, continued VIRTIS observations have shown that the "S" shape feature is not permanent, but quite transient. This is also expected from our simulation results, supporting the dynamical similarity between the tropical cyclone circulation and the Venus vortex, except for a ~ 10 times difference in physical scale.

The VIRTIS observations of the Southern polar region of Venus show other shapes in the core region of the vortex (Figure 5) which appear similar to the forms seen in the evolving instability features in the animation . It is therefore highly suggestive that their origins are also due to barotropic instability, indicating that the flow at the cloud level has more shear than the cloud motion measurements might suggest. This is possible due to lack of reliable tracers and adequate combination of image spatial resolution and time interval over which the tracer motions need to be measured.

Besides the morphological similarity between a mature tropical cyclone and the Venus hemispheric vortex, another similarity between the observed features in the vortex circulations of Venus and in terrestrial hurricanes is the presence of transverse waves on the spiral bands, away from the vortex centers (Fig. 6). The lack of such transverse waves in Earth’s polar vortices is suggestive that the dynamics of the Venus polar vortices may have more in common with hurricanes than their more direct terrestrial polar counterparts, perhaps due to the large difference in the rotation rates of Earth and Venus.

There are still many questions about the vortex circulation on Venus. How deep is it? Does it reach down to the surface in the Polar Regions? Night side near infrared images in the Venus atmospheric spectral windows reveal atmospheric circulation near the base of the cloud, or about 48-53 km above the surface of Venus. A key question is whether the subsidence in the core region reaches all the way down to the surface, and if it leads to "westerly" flow near the surface that might counteract the momentum exchange with the surface by the westward (easterly) flow in low latitudes? This is crucial for the long term balance of the angular momentum budget of the atmosphere and the solid planet. Are there secondary circulations associated with the spiral bands? More precise knowledge of the profiles of average zonal and meridional flow at deeper levels below the ultraviolet cloud top level reaching down to the surface on both the day and night side of the planet is needed. Some of these questions will be answered from the data to be collected from Venus Express, but others must wait until more data are obtained from future missions. Even then, capable global circulation models as are being now developed by many groups will be crucial along with the limited dynamical observations that can be made of the global deep circulation of the Venus atmosphere.

References

• Limaye, S. S. (2007), Venus atmospheric circulation: Known and unknown, J. Geophys. Res., 112, E04S09, doi:10.1029/2006JE002814.
• Markiewicz, W. J., et al. (2007), Morphology and dynamics of the upper cloud layer of Venus, Nature, 450, 633-636, doi:10.1038/nature06320.
• Piccioni, G., et al. (2007), South-polar features on Venus similar to those near the north, Nature, 450, 637-640, doi:10.1038/nature06209.
• Taylor, F. W., D. J. Diner, L. S. Elson, D. J. McCleese, J. V. Martonchik, J. Delderfield, S. P. Bradley, J. T. Schofield, J. C. Gille, and M. T. Coffey (1979), Temperature, cloud structure, and dynamics of Venus middle atmosphere by infrared remote sensing from Pioneer Orbiter, Science, 205, 65-67.
• Schofield, J. T., and D. J. Diner (1983), Rotation of Venus's polar dipole, Nature, 305, 116-119.
• Suomi, V.E., and S.S. Limaye. Venus: Further evidence of vortex circulation, Science, 201, 1009-1011 (1978).