Meteorology Of Jupiter General circulation and zonal structure

Jupiter emits 1.67 x more radiation than it receives from the Sun indicating a substantial internal heat source and hence, presumably, vigorous convection. The zonal structure of Jupiter appears to be neutrally stable with well-defined belts and zones (summarized in Figure 1.4 and reviewed in great detail by Rogers, 1995). In fact, this canonical stable belt/zone structure is a little misleading since the dark belts occasionally brighten and the bright zones occasionally darken with typical timescales ranging from days to years. However, since the advent of space missions it has become clear that the zonal wind structure associated with the bands is far more invariant and that atmospheric motion may best be referred to the fast-moving "jet-streams'' seen in the zonal wind flow. Belts and zones occur in pairs, or domains, to the north and south of the equatorial zone, with belts bounded by an eastward-flowing jet-stream on the side closest to the equator, and a westward-flowing jetstream on the poleward side. Thus, the belts are regions of cyclonic vorticity and the zones regions of anticyclonic vorticity (as described earlier). Sandwiched within this general zonal flow structure are several short-lived and long-lived ovals, of which the largest and most long-lived are the Great Red Spot (GRS) shown in Figures 5.12 and 5.13 (see color section for both), which lies at 22°S (planetographic) between the South Equatorial Belt (SEB) and South Tropical Zone (STrZ), and the South Temperate Belt-south (STBs) white oval, which lies at 32.6°S (planetographic) between the South Temperate Belt and the South Temperate Zone (STZ). Jupiter's visible cloud features are shown in Figures 5.14 (see color section) and 5.15 and its appearance at visible, UV, and near-infrared wavelengths is shown in Figure 5.16.

Figure 5.15. Southern hemisphere of Jupiter observed by Cassini ISS in December 2000. The Galilean satellite Io is visible in the middle right together with its shadow. The image shows several of the main cloud features of Jupiter. The GRS is clearly visible, together with small SSTB white ovals to the south. The turbulence in the EZ is clear, as are two of the dark plumes on its northern edge. In the NEB, a bright white transitory convective cloud is clearly seen in the process of being torn apart by the horizontal wind shear. Courtesy of NASA.

Figure 5.15. Southern hemisphere of Jupiter observed by Cassini ISS in December 2000. The Galilean satellite Io is visible in the middle right together with its shadow. The image shows several of the main cloud features of Jupiter. The GRS is clearly visible, together with small SSTB white ovals to the south. The turbulence in the EZ is clear, as are two of the dark plumes on its northern edge. In the NEB, a bright white transitory convective cloud is clearly seen in the process of being torn apart by the horizontal wind shear. Courtesy of NASA.

As mentioned previously, there is a clear correlation between cloud opacity and latitudinal wind shear with anticyclonic latitudes appearing bright at visible wavelengths and dark at 5 ^m, and cyclonic latitudes appearing dark at visible wavelengths, but bright at 5 ^m. This correlation becomes less clear at wavelengths where the atmosphere is more opaque and thus where most of the observed reflection comes from the upper troposphere. This is probably because the small aerosols detected near the tropopause are transported horizontally from zone to belt on a timescale short compared with the precipitation time.

Figure 5.16. Three images of Jupiter observed by Cassini ISS on 8 October 2000, as it approached Jupiter during its flyby. The image on the left was taken through the blue filter and appears similar to Jupiter's visible appearance. The middle image is recorded in the ultraviolet. At this wavelength, light is Rayleigh-scattered from the upper atmosphere and the disk appears bright unless there are high abundances of upper-tropospheric and stratospheric hazes. The strong haze absorption near the poles is clearly visible as is the increased haze abundance over the EZ. A wavelike pattern is also clear at the northern edge of this haze, which appears correlated with the plumes/hotspots seen in the blue image. The image to the right is recorded in the near-infrared at 0.89 ^m where methane absorption is strong and so only light scattered by high-altitude hazes is visible. This image is almost the negative of the UV image at equatorial latitudes although the anti-correlation breaks down at polar latitudes indicating the peculiar properties of the aerosols near the poles. The high haze opacity over the GRS is clearly visible. Courtesy of NASA.

Figure 5.16. Three images of Jupiter observed by Cassini ISS on 8 October 2000, as it approached Jupiter during its flyby. The image on the left was taken through the blue filter and appears similar to Jupiter's visible appearance. The middle image is recorded in the ultraviolet. At this wavelength, light is Rayleigh-scattered from the upper atmosphere and the disk appears bright unless there are high abundances of upper-tropospheric and stratospheric hazes. The strong haze absorption near the poles is clearly visible as is the increased haze abundance over the EZ. A wavelike pattern is also clear at the northern edge of this haze, which appears correlated with the plumes/hotspots seen in the blue image. The image to the right is recorded in the near-infrared at 0.89 ^m where methane absorption is strong and so only light scattered by high-altitude hazes is visible. This image is almost the negative of the UV image at equatorial latitudes although the anti-correlation breaks down at polar latitudes indicating the peculiar properties of the aerosols near the poles. The high haze opacity over the GRS is clearly visible. Courtesy of NASA.

The zonal structure of Jupiter was observed by the Voyager spacecraft up to latitudes of ±60° (Limaye, 1986; Smith et al., 1979a, b), and the organized, symmetric zonal structure appeared to diminish towards the pole and be replaced by more chaotic motion. Such an observation was consistent with the model of Ingersoll and Pollard (1982) that the zonal flow of Jupiter arises from the flow of the interior organizing itself into a series of differentially rotating concentric cylinders, which would cease at a critical latitude where a cylinder would be tangential to the molecular-hydrogen/metallic-hydrogen boundary. Observations of the zonal winds by Cassini (Porco et al., 2003; Vasavada, 2002) revealed little change to the winds between ±60°, although the speed of the eastward jet at 24°N had slowed by 40m s-1 to 50 ms-1. However, Cassini showed organized zonal flow to extend to at least to ±70°, and still possess north/south symmetry, which presents a considerable problem for the cylinder model. Although the planet takes on a mottled disorganized appearance polewards of 70°, cloud tracking by Cassini revealed that the features at these latitudes are still organized into regular eastward and westward jets that extend all the way to the poles. Thermal measurements by both Voyager (Hanel et al., 1979a, b) and Cassini indicate that these winds decay with height and tend to zero within 3-4 scale heights of the cloud tops. An unusual feature of the zonal wind structure of Jupiter is that the equatorial jet appears to have "horns" in that the wind speed initially increases away from the equator before rapidly diminishing. Such a structure is consistent with a small Hadley cell centered at the equator, where air rises at the equator (thus forming the zone's bright clouds), then moves polewards at the cloud tops, and picks up zonal speed due to the conservation of angular momentum, or equivalently the conservation of vorticity, before descending at the edges of the zone.

Long-term imaging of Jupiter at 18 ^m (Orton et al., 1994) and 7.4 ^m (Orton et al., 1991), sounding the 250mbar and 20mbar levels, respectively, have yielded unique information on seasonal variability, albedo correlation, and wave motion in the Jovian atmosphere. Although Jupiter has very small obliquity, clear seasonal variations are seen at both pressure levels, especially at high latitudes. At the 250 mbar level the seasonal maxima/minima occur roughly 2 years after the solstices. Such a time lag is expected since the atmosphere has a finite heat capacity and solar heating is balanced by increased radiation to space at a later date. The radiative time constant (Chapter 6) at 250mbar is estimated (Orton et al., 1994) to be 6 x 107 s or 1.9 years, which is consistent with observations. A seasonal cycle is also seen at 20mbar, but at this altitude there appears to be no lag between the solstice and maximum temperature, which is inconsistent with radiative equilibrium models and suggests that additional factors affect the stratospheric temperatures. Periods of upper-tropospheric equatorial cooling in 1980 and 1992 coincided with a visible whitening of the Equatorial Zone (EZ), consistent with an episode of increased upwelling and condensation of cloud particles. However, the equatorial cooling observed in 1988 did not correspond to any albedo change. Interesting variability in the strong prograde jet at 20°N was observed between 1984 and 1990 where, from the thermal wind equation, it appeared that the jet went from a condition where it decayed with height, to one where it remained almost constant with height! During this period the North Temperate Belt (NTB) brightened in the middle of 1987 and then darkened in 1990 during a major outbreak of white and dark spots in the STB. In addition, the North Equatorial Belt (NEB) broadened to the north in 1988 and then receded in 1999 leaving an array of brown barges (Orton et al., 1994). Whether or not these changes were caused or influenced by the apparent change of vertical structure of the 20°N jet is not known.

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