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Figure 5.5. Stone's (1976) canonical meridional flow diagram of Jupiter where bright "zones" are regions of upwelling and thus divergence at the top of the atmosphere, and "belts" are regions of convergence and subduction. While this model fits well with the Jovian cloud structure and zonal winds, it less successfully models the flows in the other giant planet atmospheres. From Stone (1976). Reprinted by permission of the University of Arizona Press. © Arizona Board of Regents.

outbreaks are correlated with electrostatic discharges thought to be due to lightning. These observations led Ingersoll et al. (2000) to suggest that the meridional flow we see at the cloud tops is not that of the deeper atmosphere below the water clouds and that, in fact, the deep flow is contrary to the upper flow (as can be seen in Figure 5.6). Moist convection in the form of thunderstorms requires the existence of convective available potential energy (CAPE). Positive CAPE exists when the background temperature profile is stable with respect to dry convection, but unstable with respect to moist convection. Essentially this requires that the lapse rate in the atmosphere lies somewhere between the DALR and SALR, such that if a parcel rises without condensation it is denser than the surrounding air and sinks back to its original level whereas if condensation occurs, the parcel is less dense than the surroundings and continues rising, condensing volatiles as it goes. For this scenario to be able to explain the conditions seen in the atmospheres of Jupiter and Saturn requires the existence of a stable layer underlying the condensation region, which inhibits dry convection below the water condensation level, but allows occasional vigorous events to penetrate to the level of positive CAPE leading to sudden, massive convective events (Showman and de Pater, 2005). Without such a stable layer, the CAPE would be quickly dissipated. In such a model the confinement of thunderstorms to cyclonic regions of Jupiter and Saturn can then be explained by the fact that anticyclonic regions are high-pressure areas that will press down on the underlying layers, increasing the thickness of the underlying stable layer, whereas cyclonic regions are low-pressure areas that will have a thin underlying stable layer and thus be more susceptible to convective outbursts. Such a scenario then explains why the abundance of ammonia at altitudes above the 5 bar level is so low on Jupiter (and Saturn): with convection in this region limited to small moist convective thunderstorms, the abundance of ammonia in the 1 bar to 2 bar region is much lower than expected.

Through horizontal mixing, some of this air reaches the zonal regions and is then upwelled to condense at levels considerably higher than expected from the deep abundances.

Jupiter and Saturn have similar zonal wind patterns, with strong eastward, or prograde, jets at the equator and rapidly varying wind direction towards both poles. At the equator, the air is effectively rotating faster than the bulk of the planet and thus is described as "super-rotating". How such a state might be driven is extremely puzzling since it can be proven that no axisymmetric (i.e., zonal mean) process can lead to such a state. Instead, nonlinear wave interactions are required such as diffusive small-scale eddies, or gravity/Kelvin/Rossby waves (discussed in Section 5.3.2). The nature of the super-rotation of the equatorial atmospheres of Jupiter and Saturn, and indeed the observed super-rotation of the slowly rotating worlds of Venus and Titan is an area of very active research.

The zonal wind profiles of Uranus and Neptune are completely different from those of Jupiter and Saturn with a very smooth, slow variation of zonal wind speed with latitude, and retrograde equatorial jets. Uranus had a particularly bland appearance at visible wavelengths during the Voyager 2 flyby in 1986, with a hint of brighter albedos at the equator and poles and darker albedos in midlatitudes where the winds are eastward. Although the appearance of Uranus has become more dynamic as it passed through its Northern Spring Equinox in 2007, the generally darker appearance of the midlatitudes still holds true. Neptune also seems to show the same correspondence. Hence, the wind shear is anticyclonic at midlatitudes and cyclonic at the poles suggesting upwelling at midlatitudes in the upper tropospheres and subsidence at the equator and poles. This view is supported by Voyager, Hubble Space Telescope (HST), and ground-based observations of these planets that show the main convective activity in the upper methane cloud layers to occur at just these darker midlatitudes. In addition, measurements of zonal temperature structure show the upper troposphere to be warm at the equator and poles, but cooler at midlati-tudes, which fits nicely with this picture. Hence, the brighter albedos observed at the equator and poles are believed to be due partly to the concentration of upper tropo-spheric haze in the convergence zones above the poles and equator and also partly due to either the cloud top pressure, or color of the main presumed H2S cloud. Why the atmospheric circulations of Uranus and Neptune should be so different from those of Jupiter and Saturn is not known.

For many years, the jets were assumed to be confined to the upper weather layer of the atmosphere and not extend into the interior. The accepted explanation of the observed belts and zones was as mentioned earlier and shown in Figure 5.5 that, due to unspecified frictional drag processes, in anticyclonic shear regions where the Coriolis force is balanced by high pressures (assuming the geostrophic approximation) the flow is divergent within the clouds and at the cloud tops, but convergent beneath and so anticyclonic regions were preferentially heated from below by latent heat release (Ingersoll, 1990). This heating would act to sustain the anticyclone by heating the core, and from the thermal wind equation increasing the anticyclonicity with altitude, thus causing more divergence in the clouds and so on. This explanation is certainly consistent with the zonal cloud structure on Jupiter, but is far less successful in explaining the belt/zone structure of the other planets and also does not explain why convective thunderstorms on both Jupiter and Saturn occur only in cyclonic regions as we saw earlier. It also suffers from further flaws: (1) it does not actually explain the banded appearance of the planets since it works equally well for isolated anticyclonic spots as it does for extended anticyclonically sheared zones; and (2) it relies on difficult-to-observe (and estimate) processes such as frictional drag and latent heat release. One aspect of this picture that can easily confuse is that on the Earth we are familiar with anticyclones (high-pressure regions) being regions of downwelling, cloud-free air while cyclones (low-pressure regions) are regions of upwelling, cloudy air. This is clearly the opposite of what is found on the giant planets! However, it must be remembered that the Earth has a definite lower boundary and thus it is the frictional forces near the ground that cause convergence at the base of low pressures and divergence at the base of high pressures. On the giant planets there is no such lower boundary and the nature of any deep frictional forces that may be responsible for any meridional flow is unclear. However, the giant planets do appear to have an upper boundary in that the zonal winds are found to decay with height, presumably due to some kind of frictional damping. If this is also the main source of the friction driving the meridional circulations then the "highs" and "lows" on the giant planets have friction at their tops rather than at their bottoms, which would account for their inverted properties relative to their terrestrial cousins.

For all the giant planets, although there is evidence for significant turbulence and wave activity, the zonal winds appear to be extremely stable, and have not altered greatly since observations of these planets began (with the possible exception of Saturn, whose equatorial jet has slowed by some 100 m s_1 since the Voyager observations). In order to investigate the stability, driving, and dissipation of zonal mean circulation, we need to introduce the basic theory of eddy motion in the giant planet atmospheres.

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