Meteorology Of Uranus General circulation and zonal structure

The amount of energy emitted by Uranus is at most only 1.06 x that received from the Sun indicating that Uranus has a very low internal heat source and, one might

Figure 5.35. Saturn's North Polar Hexagon (NPH) as viewed by Cassini VIMS in March 2007 at 5 ^m, showing thermal emission from the deep atmosphere, modulated by variable cloud opacity in the 2-3 bar region. Courtesy of NASA.

suspect, a sluggish circulation system driven primarily by latitudinal variations of the solar flux, which at Uranus' distance from the Sun is a meager 3.7 Wm~2. Another difference between Uranus and Jupiter/Saturn is Uranus' extremely large obliquity, which means that Uranus receives direct sunlight over both poles as well as the equator during the course of a Uranian year. [NB: Even though both poles experience a night lasting half a Uranian year long, they receive annually 50% more sunlight per unit area than the equator.] The final difference between Uranus and Jupiter/Saturn is that the visible hydrogen-helium atmosphere is only a small fraction of the total planetary mass and thus the Taylor-Proudman column hypothesis is unlikely to apply due to the boundary between the hydrogen-helium outer atmosphere and the denser icy-rock interior occurring at a depth of just 5,000 km below the visible cloud tops. During the Voyager 2 flyby mission, when Uranus' South Pole was facing almost directly towards the Sun, the dominant circulation must have been one that efficiently redistributed relatively warm polar air over the planet. Modeling the planet as a black sphere in radiative equilibrium with the received insolation, the expected equator-to-pole temperature difference would be of the order of 10 K. However, very little thermal variation was found in the 0.5 bar to 1 bar pressure levels by Voyager 2 (Flasar et al., 1987; Hanel et al., 1986). This lack of thermal variation is surprising given that estimates of the para-H2 fraction and the eddy-mixing coefficients suggest that the vertical and meridional circulation of Uranus is very sluggish. How such an apparently sluggish atmosphere can have such small temperature gradients, given its differential radiative forcing, is very unclear.

Deeper in the atmosphere, microwave observations reveal a clear zonal structure, with latitudes polewards of 45°N or 45° S appearing significantly brighter than midlatitudes (Hofstadter and Butler, 2003). While these differences could just conceivably be due to temperature variations at the 50 bar level, it seems more plausible that the variations in brightness are due to a higher abundance of ammonia gas at latitudes less than ±45°, suggesting either meridional circulation with air rising at midlatitudes and descending at the poles, or alternatively that convective overturning polewards of ±45° is somehow inhibited. Long-term monitoring of Uranus' microwave brightness (Klein and Hofstadter, 2006) shows seasonal variations consistent with this broad latitudinal structure. Interestingly, VLA observations in 2006 at microwave wavelengths detect some banded structure at latitudes less than ±45° (Orton et al., 2007c), which may point to a latitudinal variation in convective overturning rather than more general Hadley-like overturning of the atmosphere. The apparent special significance of the ±45° latitude boundary is further emphasized by the fact that long-term monitoring of Uranus' disk-averaged visible albedo is also well matched by a model where latitudes of less than ±45° are darker than polar latitudes by a factor that varies from 0.72 to 0.98 (Hammel and Lockwood, 2007), as was discussed in Chapter 4.

Although very little temperature variation was seen by Voyager 2 in the 0.5 bar to 1 bar pressure range, significant variation was seen in the upper troposphere/lower stratosphere (60-200 mbar), where the equator was found to be actually warmer than the pole and where midlatitudes were found to be colder than the equator by K (Flasar et al., 1987). Ground-based VLT observations in 2006 of the temperature near 100mbar (Orton et al., 2007c), found the same variation in brightness temperature even though almost an entire Uranian season had elapsed since the Voyager 2 observations. This upper-tropospheric temperature structure is also seen in Neptune's atmosphere and is strongly indicative of upwelling and divergence at midlatitudes (causing cooling) followed by meridional circulation to the poles and equator, where the air then converges (causing heating) and descends.

Although Uranus had very little visible belt/zone structure during the Voyager epoch (Smith et al., 1986, and Figure 1.9), with only a very slight north-south asymmetry apparent in broadband visible images (with South Polar latitudes appearing slightly brighter due to thicker methane haze), enough variable features existed to allow cloud tracking and thus determination of cloud top winds by the Voyager 2 cameras, which was discussed earlier in Section 5.2.2. Thermal wind shears calculated from the retrieved temperature fields of Voyager 2 suggested that zonal winds decayed with height with a vertical scale of ~10 scale heights or 300 km.

Convective activity has become significantly more vigorous in the lead-up to Uranus' Northern Spring Equinox in 2007 and further wind speed data have been deduced from more recent HST (Hammel et al., 2001) and Keck observations (Hammel et al., 2005a). However, even though Uranus has become more convectively active than it was during the Voyager 2 flyby, the zonal winds do not appear to have changed significantly. Images of Uranus recorded by the HST at the end of the 1990s (Karkoschka, 1998b, 2001; Rages et al., 2004) (Figure 5.36, see color section) revealed that the brightness of the South Polar latitudes had decreased since the Voyager epoch and a noticeably bright "zone" had appeared at 40°S to 50°S, together with a less bright zone appearing, for a time, at 70°S. Since then, as Uranus approached its Northern Spring Equinox in 2007 (Figure 5.37, see color section), the contrast of the zone at 45°S has been seen to decrease and the beginnings of a zone at 45°N has been detected. Observations of Uranus during the previous equinox revealed a very different zonal appearance (Alexander, 1965; Karkoschka, 2001) and there was much anticipation that rapidly changing solar forcing would lead to dramatic changes in Uranus' atmosphere during the 2007 equinox. Hence, many observations of Uranus have taken place during 2006-2008 and it appears that the atmosphere has indeed changed significantly during this event, but this time the changes have been very well observed and are currently being analyzed.

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