Hurricane at Saturn’s South pole observed by Cassini ISS in 2006.
Ulyana A. Dyudina, Andrew P. Ingersoll, Shawn P. Ewald
150-21, Geological and Planetary Sciences, Caltech, Pasadena, CA, 91125, USA
Ashwin R. Vasavada
Jet Propulsion Laboratory, Caltech, Pasadena, CA, 91125, USA
Received
;
accepted
Possibly for submission to Icarus.
Revised as of June 7, 2007
For co-authors only
The author list is preliminary. Please send your suggestions.
Number of manuscript text pages: 16, Figures: 9
Temporary remarks are shown in bold font and will be removed later.
suggestions are marked UD at the end of the note. Ulyana
My
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Proposed running head:
Hurricane at Saturn’s South pole
Send proofs and correspondence to:
Ulyana A. Dyudina
150-21 Caltech, Pasadena, CA 91125, USA
E-mail: [email protected]
Phone: (626)395-6824
Fax: (626)585-1917
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Abstract
Cassini camera has seen something never before seen on another planet – a hurricanelike storm at Saturn’s south pole with a well-developed double-wall eye, ringed by towering
clouds. The inner wall of the eye is 1.1-1.3 degrees planetocentric latitude (or 1000-1200
km) away from the center, the outer wall is 2.2 degrees (2100 km) from the pole. This
double-wall structure existed on the pole for at least two years since previous Cassini
observations in 2004. The eye is cloud-free at higher altitudes and has a few small deep
clouds inside. A movie taken by Cassini’s camera over a three-hour period reveals winds
increasing towards the storm’s eye blowing clockwise at speeds reaching 160 m/s. The eye
of the storm rotates at angular velocity of 25-30 degrees per hour, which is close to the 33.7
degrees per hour Saturn’s rotation rate. As a result Saturn’s South pole rotates almost
twice as fast as the rest of the planet. The cyclonic central vortex is surrounded by multiple
anti-cyclonic smaller clouds. The two cloud walls around the the eye are so steep that they
cast shadows on the lower clouds inside the eye. The shadows follow the Sun’s direction as
Saturn rotates. From the shadow lengths, the inner wall is 50-100 km high. The outer wall
is 30-40 km high.
Keywords: (1) SATURN, ATMOSPHERE (2) ATMOSPHERES, DYNAMICS (3)
METEOROLOGY (4) SPECTROSCOPY ! CHECK ICARUS FOR MORE
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1.
Introduction.
Strong winds around Saturn’s South pole had been seen before by Cassini ISS shortly
before the Saturn orbit insertion in June 2004 (Sánchez-Lavega et al. 2006; Vasavada et al.
2006). Our October 2006 observations show that two years later the winds persist. The
unprecedented 20 km/pixel resolution of the 2006 polar movies allowed detecting two
cloud walls around the polar storm’s eye because the steep walls cast shadows visible in
the high-resolution images. High spatial resolution also allowed to measure rotation of
small-scale vortices outside the storm’s eye. The 2006 movies were taken in multiple filters
probing different atmospheric depths, and we also discuss the vertical structure of the flow.
Because some parts of the polar storm’s structure are remarkably similar to terrestrial
hurricanes, we discuss those similarities and differences throughout the paper.
Section 2 discusses morphology of the clouds and their vertical structure. Section 3
presents our wind speed measurements. Section 4 shows the vorticity of the small clouds
around the polar vortex compared to the vorticity of the zonal flow, which forms the polar
vortex. Section 5 estimates the height of the two cloud walls around the polar storm’s eye.
Section 6 discuss possible polar storm’s origin in comparison with terrestrial hurricanes.
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2.
Cloud structure.
On October 11 2006 Cassini observed South pole of Saturn with unprecedented spatial
resolution of 20 km/pixel. Figure 1 shows a mosaic from 14 images of South pole taken
within a 3-hour period. The two walls of the cyclone are the circular structure at around
-87.8◦ latitude and the slightly elliptical structure at around -88.7◦ to -88.9◦ latitude. The
same double-wall structure apparently existed during the ∼50 km/pixel Cassini polar
observations taken in September-October 2004, two years before these ones (Vasavada
et al. 2006), and in ∼30 km/pixel observations taken in July 2004 (Sánchez-Lavega et al.
2006). However with lower spatial resolution, slant viewing, and higher Sun producing
shorter shadows, the 2004 observations were uncapable of detecting the steep eyewalls. The
elliptical shape of the inner eyewall looks remarkably unchanged between the 2004 and 2006
observations. On Earth, more than half of the hurricanes develop double eyewalls sometime
during their lifetime. Hurricane Katrina in 2005 had double eye while hitting New Orleans,
the strong winds extending to the larger radius of the outer wall, which was the reason for
the large affected area. (Taken from the web, get appropriate reference. UD) In
terrestrial hurricanes the outer wall replaces the inner wall with time (Houze et al. 2007).
There are few small clouds inside the inner eyewall in both 2004 and 2006 images. In
mature terrestrial hurricanes the low-level cloud deck inside the eye often shows “swirling”
features (Kossin et al. 2002), which may be analogs for the small clouds inside Saturnian
cyclone’s eye.
Here should probably be some comparison with terrestrial hurricanes: size
(3 to 370 km in diameter, according to wikipedia, the smallest is the strongest),
convective clouds around etc.
Figure 2 provides some insight into vertical structure of the clouds in the cyclone by
using images at different wavelengths. The color planes of the figure are composed from
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the images taken with filters sensitive to different altitudes in the atmosphere (see Porco
et al. (2004) for Sassini filter details). The continuum narrow band filter CB2 (750 nm, red
color plane) senses clouds and aerosols from the top of the atmosphere down to the deepest
levels of the troposphere. The filter at weak methane absorption band MT2 (727 nm, green
color plane) senses down to the intermediate depths. The exact values of the atmospheric
pressure where the methane absorption reaches optical depth one for each wavelengths is
usually derived from radiative transfer models and is somewhat model-dependent. Here we
assume the penetration depths described in Tomasko et al. (1984). According to that model,
at 727 nm the gaseous absorption optical depth one occurs at 1.2 bars at nadir viewing for
a cloud-free atmosphere . At slant viewing (the images in Fig.2 are observed at around 40◦
from horizon) the optical depth one occurs near 0.8 bars. The effect of slant illumination
is even more drastic than the effect of slant viewing. For the images composing Fig. 2
the Sun is 15.5◦ above the horizon at the pole. This translates into optical depth one for
cloud-free atmosphere at ∼ 0.3 bars, which would be even strongly reduced by haze or cloud
shadowing. The illumination angle changes considerably across the image, and so does the
depth of the solar light penetration. The filter at the strong methane absorption band MT3
(889nm, blue color plane) senses the highest part of the atmosphere. At this wavelength the
absorption optical depth one occurs at 0.33 bars for nadir viewing, which corresponds to 0.2
bar for slant viewing, or to 0.08 bars for solar illumination. For such a complex system the
exact cloud heights can only be derived from a thorough inverse radiative transfer model.
Such a retrieval would be uncertain with the ISS data only and would greatly benefit from
combining the ISS data with existing observations in near-infrared and infrared wavelengths
by Cassini VIMS and CIRS. check if CIRS is relevant to that cloud levels UD The
accurate cloud heigh retrieval is out of scope of this paper. However we can give qualitative
estimate of the relative clouds heights by combining CB2, MT2 and MT3 images as red,
green, and blue. The map combined from these three color planes shows the deepest clouds
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as red, intermediate clouds as green, and the holes or absorbers in the lower clouds covered
by high haze as blue. The maps are high-pass filtered at spatial scale of around 24000 km
to correct for uneven solar illumination.The cyclone’s eye looks dark and red.This color
inside the eye indicates cloud-free atmosphere or atmosphere with dark absorbing haze in
the eye with some very deep clouds at the bottom. The strong brightness drop from outside
to inside each of the two eyewalls accompanied with the change from greener to redder
shade indicates that the tall thick clouds outside the eye drop to much lower elevations and
form a steep eyewall. Here should probably be some comparison with terrestrial
hurricanes: cloud-free eye, tall walls etc.
Clouds immediately outside the outer eyewall up to latitude of ∼-82◦ are greenish-blue
in color, which indicates thicker than average high haze on top of substantial low cloud.
Multiple bright clouds around the cyclone and inside the eye are red and therefore deep.
The two larger dark spots at the upper left are blue. Blue indicates that the is cloud-free or
contains some absorbing clouds down to the deep levels and is covered by the bright high
haze.
Figure 3 shows the same combination of the color planes as Fig. 2, but the color planes
are high-pass-filtered at small length scale of ∼ 300 km to bring up the color contrast of
the small clouds. The red color of the small bright clouds is more clearly seen than in Fig.
2. Also small-scale features seen at high elevations (green and blue) often correspond to a
dipper low cloud (red).
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3.
Windspeeds.
Cassini took repeated images of the pole during the 3-hour observation. High
spatial resolution and good temporal sampling of 14 images in continuum band filter
(the wavelength of the best contrast for small features) allowed tracking clouds to obtain
accurate wind velocities. A movie combined from these images 1 shows the winds increasing
towards the pole. The cyclone’s eye rotates by ∼ 60◦ within the 3 hours. Figure 4 shows
zonal windspeeds measured by cloud tracking. Two tracking techniques were used. Outside
the -84.5◦ latitude circle the windspeeds were obtained by automatic feature tracker. Inside
the -84.5◦ the automatic tracker failed to obtain accurate wind speeds because of high
speeds, curved trajectories, and multiple linear features. For the circle inside the -84.5◦
latitude we manually tracked individual features 2 . The manual and automatic tracking
agree outside the -84.5◦ circle. The wind increases towards the pole until the outer eyewall
at -87.8◦ latitude. There are no trackable features between the outer wall and latitude of
∼-88.5◦ just outside the inner eye wall. Between latitudes -86.5◦ and -89
◦
the winds reach
thair maximum linear speed of ∼ 150 ± 20 m/s. The features inside the ∼-88.5◦ are drifting
at 20-30 degrees per hour with various speeds not strongly correlated with latitude.
The winds measured in July 2004 Cassini images at - 87◦ were 160±10 m/s
(Sánchez-Lavega et al. 2006). This is consistent with our measurements at this latitude. In
1
http://www.nasa.gov/mpg/162357main pia08332.mpg
2
To obtain accurate zonal windspeed manually we played modified movies combined from
the 14 maps. In each modified movie the maps were rotated back relative to the clouds’
windspeed at some angular velocity around the pole. At particular angular velocity the the
cloud feature rotation and the movie’s back rotation cancels out and stops the motion of
the feature. For each of the test angular velocities we picked the features that stopped their
motion in the modified movies, which produced the data points in Fig. 4
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September-October 2004 the linear speed associated with the rotation of the inner eyewall
at -88.5◦ latitude was 80 to 135 m/s. This is smaller than our 130-160 m/s winspeeds
for this latitude. Although this difference may be real, the uncertinty in the windspeed
measurements is rather hight to assert this windspeed change with confidence. Here
should probably be some comparison with terrestrial hurricanes: eyewall speeds
etc.
We also measured the meridional component of the clouds’ motion which turned to be
zero within the errors for all the clouds except the ones in the inner eye. The features in the
inner eye display a non-systematic drift North or South which is small compared to their
zonal motion. The main mechanism forming terrestrial hurricanes involves inward motion
of the air towards the eye. It is possible that on Saturn similar inflow exists, but is slow
and thus undetectable.
The multiple-filter observations shown in Fig. 3 are repeated four times during the
three hours. The four frame color movie3 combined from the images similar to Fig. 2
shows that the clouds at different heights (appearing in different colors) move at the same
velocities. It should be noted though that inside -86◦ latitude only one color plane has small
trackable features. Thus it is impossible to tell if the windspeeds near the eye are the same
at all heights. The oval shape of the inner eye rotates coherently in all colors. It is unclear
though if the eye wall shape tracks the wind motion or acts as a wave.
3
http://www.gps.caltech.edu/∼ulyana/iss/polar movie/movies/color 4frm compressed.av
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4.
Vorticity.
The zonal wind profile from Fig. 4 can be used to calculate the vorticity of the zonal
flow. Also, multiple smaller clouds around the cyclone’s eye show detectable individual
rotation. To compare the vorticity in the small clouds with the background zonal flow we
measured the rotation of these smaller clouds. Figure 5 shows the locations and vorticity
values for the individual clouds that have detectable rotation. The vorticities of the
clouds are calculated as their angular velocity multiplied by two. The angular velocity was
obtained by procedure similar to the manual feature tracking used for the zonal windspeeds,
which involves picking an appropriate stopped motion movie from the set of the test movies
4
. Often it is uncertain which of the movies matches the rotation of the cloud. To test
the uncertainty and to increase the precision of the vorticity measurements we measured
vorticity of each cloud 3-4 times picking the range of reasonably good matches. The
vorticity values (colors of the asterisks in Fig. 5) are the averages of those 3-4 measurements.
Nearly all small clouds rotate counterclockwise (positive vorticity). Remarkably, the largest
features (the two dark spots at the upper left corner of the map) rotate the fastest. For the
smaller clouds the relation between the size and rotation is not systematic.
Figure 6 shows the vorticity of the small clouds compared to the vorticity of the
4
To obtain each cloud’s angular velocity we first determined its drift around the pole
(see the technique description at the footnote in Section 3). Then we made an individual
cloud’s movie from the set of 14 maps such that the center of the movie tracks the cloud
in its motion around the pole. Then we made a set of modified back-rotated movies for
a set of possible angular velocities for the cloud. While simultaneously playing the set of
back-rotated movies we picked the one which stopped the apparent rotation of the cloud.
Because particular clouds are not always covered by all 14 images, some modified movies
have less then 14 time steps, down to as few as 2 time steps.
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background zonal flow. The solid line shows negative zonal flow vorticity at the pole.
Thus, the polar vortex is a cyclone. All the small clouds (asterisks), with the exception
of a few that have near-zero vorticity, have positive vorticity. Thus, the small clouds are
anti-cyclones. The absolute value of both the polar cyclone and the small clouds approaches
the vorticity associated with Saturn’s rotation, which is ∼-3.4×10−4 rad/s. The anticyclonic
vorticity of this order or smaller, as we observe in the small clouds, is expected in the
spreading tops of convective plumes.
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5.
Eyewall heights.
The cyclone’s eyewalls are steep and cast shadows on the lower clouds inside the eye.
Figure 7 shows that the dark crescent-shaped areas inside the walls follow the Sun as
Saturn rotates. This demonstrates that the dark areas are the indeed shadows and not a
dark coloration of the underlying clouds.
Figure 8 shows how we derived the height of the eyewalls from the length of the
shadows. The two maps in the figure are examples of the 9 maps from Fig. 7, which we used
for the eyewall height calculation. We manually picked the points of the apparent end of
the shadow and projected these points along the Sun’s azimuth to the edge of the eyewall.
The reader may judge that the shadow end points are quite uncertain by comparing the left
panels of Fig 8 with the right panels showing the same maps overlaid with our estimated
shadow locations. To obtain the eyewall height we multiplied each shadow length by sine of
the solar elevation angle above the horizon which is around 16◦ for all the images.
Figure 9 shows the resulting heights for outer and inner eyewalls calculated from the
9 maps. The data are plotted versus longitude at which each shadow point projects to
the eyewall. The longitudes are then adjusted to account for the zonal wind at respective
latitude. With such an adjustment the shadows of particular eyewall features (e.g., the
”bulge” of the inner wall oval) appear at the same longitude for all images. We assumed
zero adjustment for the first plot in Fig. 7, which is also shown in the upper panel of Fig.
8. The longitudes in Fig. 9 refer to that frame.
The height of the outer wall is about 30-40 km and does not significantly change with
longitude. The height of the inner wall depends on longitude significantly. At longitude
∼180◦ the shadows are shorter and the corresponding wall height is ∼30-60 km. Shorter
shadows can be seen in the upper image in Fig.8, which is taken at the time when the Sun
illuminated the image from longitude ∼180◦ (up on the map). The Sun proceeds to larger
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longitudes of 350◦ -50◦ with time. The last frame in the time sequence is shown in the lower
panels of Fig. 8. The shadows are longer at those longitudes and the corresponding wall
heights are ∼70-120 km.
Here should probably be some comparison with terrestrial hurricanes: wall
heights relative to the scale height and relative to the eye size etc.
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6.
Discussion.
The structure of the Saturn’s polar storm’s eye is similar to the eye of terrestrial
cyclones. However, terrestrial hurricanes form as a result of a “warm core” - anomalously
warm surface of the ocean. Hurricanes get their energy from the latent heat of water,
condensing in the convective clouds surrounding the hurricane. It is unclear if there
are convective clouds in the Saturn’s polar storm’s area, though the small deep bright
anticyclonec vortices may be such clouds.
The ocean also provides a steady lower boundary for the air circulation in the cyclone.
The air in the cyclone circulates between the Earth’s surface and another stable boundary tropopause. On Saturn no lower boundaries similar to the terrestrial ocean are expected.
The stable upper layer is located at ??? 0.1 or 0.4 bars. The upper boundary likely plays
an important role in the polar vortex.
Terrestrial cyclones are drifting following the global winds while Saturn’s storm is
locked to the South Pole. Thus the polar area must have continuous energy supply to
support the polar storm.
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Acknowledgments.
We thank Ashwin Vasavada for map navigation. This research was supported by the
NASA Cassini Project.
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REFERENCES
Houze, R. A., Chen, S. S., Smull, B. F., Lee, W.-C., Bell, M. M., 2007. Hurricane Intensity
and Eyewall Replacement. Science 315, 1235–1239.
Kossin, J. P., McNoldy, B. D., Schubert, W. H., 2002. Vortical swirls in hurricane eye
clouds. Monthly Weather Review 130, 3144–3149.
Porco, C. C., West, R. A., Squyres, S., McEwen, A., Thomas, P., Murray, C. D., Delgenio,
A., Ingersoll, A. P., Johnson, T. V., Neukum, G., Veverka, J., Dones, L., Brahic,
A., Burns, J. A., Haemmerle, V., Knowles, B., Dawson, D., Roatsch, T., Beurle,
K., Owen, W., 2004. Cassini Imaging Science: Instrument Characteristics And
Anticipated Scientific Investigations At Saturn. Space Science Reviews 115, 363–497.
Sánchez-Lavega, A., Hueso, R., Pérez-Hoyos, S., Rojas, J. F., 2006. A strong vortex in
Saturn’s South Pole. Icarus 184, 524–531.
Tomasko, M. G., West, R. A., Orton, G. S., Teifel, V. G., 1984. Clouds and aerosols in
Saturn’s atmosphere, pp. 150–194. in: Saturn. University of Arizona Press,Tucson,
AZ.
Vasavada, A. R., Hörst, S. M., Kennedy, M. R., Ingersoll, A. P., Porco, C. C., Del Genio,
A. D., West, R. A., 2006. Cassini imaging of Saturn: Southern hemisphere winds
and vortices. Journal of Geophysical Research (Planets) 111, 5004+.
This manuscript was prepared with the AAS LATEX macros v5.2.
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Fig. 1.— Map of the Southern polar vortex combined as a mosaic from 14 maps produced
from individual ISS images. Each image was taken in continuum band filter CB2 with
the central wavelength 750 nm (Porco et al. 2004). To reduce the effect of varying solar
illumination across the image each color plane is high-pass filtered at the spatial scale of 100
pixels (which is 200 km at the pole and slightly smaller away from the pole according to the
polar stereographic projection of the map). The map is labeled by West longitude.
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Fig. 2.— The false-color map of the polar vortex. Color planes are combined from maps in
filter combinations CL1(central wavelength 750 nm, red color plane), MT2 (727 nm, green
color plane), and MT3 (889 nm, blue color plane). To reduce the effect of varying solar
illumination across the image each color plane is high-pass filtered at the spatial scale of
1200 pixels (which is ∼24000 km.).
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Fig. 3.— The false-color map of the polar vortex. Color planes are combined from maps in
filter combinations CL1(central wavelength 750 nm, red color plane), MT2 (727 nm, green
color plane), and MT3 (889 nm, blue color plane). To reduce the effect of varying solar
illumination across the image each color plane is high-pass filtered at the spatial scale of 150
pixels (which is ∼300 km.).
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Fig. 4.— Zonal wind speeds around South Pole. The asterisks indicate data points taken
from automatic tracking at latitudes equatorward from 84.5◦ South or from manual feature
tracking at latitudes poleward from 84.5 ◦ South. The solid line is a spline interpolation of
the data.
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Fig. 5.— Map from Fig. 1 overlayed by the vorticity data. Asterisks show locations of
the individual features for which vorticity had been measured. The value of the vorticity is
indicated by the asterisks’ color. The vorticity data are also plotted in Fig. 6
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Fig. 6.— Vorticity around South Pole. The solid line shows the vorticity of the zonal flow.
This vorticity is calculated from the spline interpolation of the zonal velocity in m/s (the
lower panel of Fig. 4). For each pair of consecutive points in the solid curve from Fig. 4
the vorticity is calculated as follows. vorticity = (u0 x0 − u1 x1 )/((x0 + x1 )/2)/|dl|, where
u and x are zonal wind and distance to the Saturn’s rotation axis for the first and second
point in the pair, subscribed respectively. |dl| is the distance on Saturn’s surface between
these points along the meridian. The asterisks show the vorticity of individual features (see
the features on the map in Fig. 5). The error bars at the asterisks are calculated from 3-4
measurements for each feature. In the case when all the measurements give the same value,
the error bar equals to the step in the tested vorticities, or 1. × 10−5 rad/sec.
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Fig. 7.— A set of the maps showing how the shadow of the cyclone’s eyewall follows the
Sun. The first map is taken on October 11 (DOY 284), 2006 at 19h. 42 min 31 s. The time
on the maps increases from top to bottom panel and then from left to right, as labeled by
the time in hours from the start of the sequence. The white arrow on each panel shows the
direction at which the Sun illuminates the planet. The arrow points from the Sun to the
illuminated scene.
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Fig. 8.— Two maps demonstrating how the length of the shadows had been estimated. The
left panels show the map, the right planes show the same map with the cyclone eyewalls
outlined in black and shadow lengths measured on this image shown in white.
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Fig. 9.— Height of the outer (upper panel) and inner (lower panel) walls of the vortex. The
height is plotted versus longitude of the wall features in the first image of the sequence (first
panel in Fig. 7, or upper panel of Fig. 8). The shadow lengths are taken from all images
from Fig. 7. Then the longitude of the points on the wall casting the measured shadows is
adjusted to account for the zonal velocity of the wall. The zonal velocities are 17 degrees/hr
and 20 degrees/hr for the outer and inner walls respectively.