Info

Energy2green Wind And Solar Power System

Wind Energy DIY Guide

Get Instant Access

Figure 5.2. Zonal wind structure of the giant planets. Solid line: Jupiter; dotted line: Saturn; dashed line: Uranus; dot-dashed line: Neptune. The sources of these wind speeds are: Jupiter, Vasavada (2002), Porco et al. (2003); Saturn, Sanchez-Lavega et al. (2000); Uranus, Allison et al., 1990 (unweighted Voyager fit); Hammel et al. (2001); Neptune, Sromovsky et al. (1993).

Figure 5.3. Zonal wind structure of the giant planets plotted separately and with regions of cyclonic vorticity shaded in gray.

associated with visible cloud features, where cyclonic vorticity shear regions (i.e., regions rotating in the same sense as the planet, resulting in anti-clockwise relative rotation in the northern hemisphere (du/dy < 0) and anti-clockwise rotation in the southern hemisphere (du/dy > 0) are seen as visibly dark, relatively cloud-free "belts", while anticyclonic vorticity shear regions (i.e., regions rotating in the opposite sense to the planet, resulting in clockwise relative rotation in the northern hemisphere (du/dy > 0) and anti-clockwise rotation in the southern hemisphere (du/dy < 0) are seen as visibly bright cloudy "zones". The mean cloudiness of the belts/zones is confirmed by observations of the thermal emission at 5 ^m, where cloud-free belts allow radiance to escape from the warm 5 bar to 8 bar pressure levels underneath, while the cloudy zones appear dark. Hence, the visible and near-infrared albedo is closely anti-correlated with the 5 ^m brightness (e.g., Irwin et al., 2001). The belt/zone structure of Saturn appears much blander than that of Jupiter, due to the greater obscuration by tropospheric haze (Chapter 4), but there appears to be a more fundamental difference in that there is less correspondence between wind shear and albedo. In fact, the Saturn belts/zones appear better correlated with mean zonal wind than they do with wind shear. The appearance of Saturn in the 5 ^m has now been recorded from ground-based observations (Yanamandra-Fisher et al., 2001) and by Cassini VIMS (Baines et al., 2005). While there is a greater component of reflected sunlight in Saturn's 5 ^m spectrum, the observed radiance is, like Jupiter, principally

Figure 5.4. Zonal wind structure of the giant planets superimposed onto representations of their visible appearance. The wind speeds have been converted into degrees longitude/rotation of the planet. Note that the figures in the disks refer to degrees latitude and longitude—not to wind speed. Planetary images based on NASA data and processed by James Hastings-Trew and Bjorn Jonsson.

Figure 5.4. Zonal wind structure of the giant planets superimposed onto representations of their visible appearance. The wind speeds have been converted into degrees longitude/rotation of the planet. Note that the figures in the disks refer to degrees latitude and longitude—not to wind speed. Planetary images based on NASA data and processed by James Hastings-Trew and Bjorn Jonsson.

due to thermal emission of the deep atmosphere, modulated by cloud opacity in the 2 bar to 3 bar region, especially towards the longwave end of the 5 ^m window. However, unlike Jupiter, there is observed to be little correlation between the 5.2 ^m brightness and the visible albedo, although the main bright 5 ^m band seen between 38°S and 49°S (planetocentric) does seem to coincide with an eastward jet, although puzzlingly with the anticyclonic side, rather than the cyclonic side, as is usually seen for Jupiter. Since the 5 ^m observations map the total cloud opacity above the 5 bar level on Saturn there again seems to be less, or perhaps a different correlation between cloud opacity above 5 bar and zonal wind flow at the cloud tops.

Measurements of mean thermal emission as a function of latitude by spacecraft such as Voyager and Cassini may be used to calculate the zonal thermal vertical wind shears which, combined with measured cloud top winds (assuming the pressure level of this is known accurately) can be used to generate the variation of zonal wind speed with height. On all four giant planets, the zonal winds are found to decay with height to zero within about 3-4 scale heights of the cloud tops. The nature of the frictional force that this braking implies is not clear, although it is believed that the breaking of gravity waves or eddy motions may play a role. The latitude dependence of temperature in the upper troposphere may also be used to deduce meridional motion. Latitude bands with warmer than average temperatures imply horizontal convergence and thus presumably subsidence, while cooler latitudes imply divergence and thus upwelling from below. On Jupiter it is found that anticyclonic, more cloudy latitudes are cooler in the upper troposphere while cyclonic, less cloudy latitudes are warmer, leading to the canonical view of the flow in the Jovian atmosphere (shown in Figure 5.5) that zones are regions of moist, upwelling air and belts are regions of subsiding, dry air. However, Cassini observations suggest that this canonical view may be incorrect since convective clouds and lightning are found to occur exclusively in cyclonically sheared belts (Ingersoll et al., 2000; Porco et al., 2003). Similarly for Saturn, although convective storm activity is less vigorous than for Jupiter, three major storms have been observed in the cyclonically sheared region at 35°S, and these z

ZONE BELT ZONE BELT ZONE

Was this article helpful?

0 0
Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

Get My Free Ebook


Post a comment