Fletcher Leigh N, Kaspi Yohai, Guillot Tristan, Showman Adam P
1School of Physics and Astronomy, University of Leicester, University Road, Leicester, LE1 7RH UK.
2Department of Earth and Planetary Sciences, Weizmann Institute of Science, Rehovot, 76100 Israel.
Space Sci Rev. 2020;216(2):30. doi: 10.1007/s11214-019-0631-9. Epub 2020 Mar 12.
The atmospheres of the four giant planets of our Solar System share a common and well-observed characteristic: they each display patterns of planetary banding, with regions of different temperatures, composition, aerosol properties and dynamics separated by strong meridional and vertical gradients in the zonal (i.e., east-west) winds. Remote sensing observations, from both visiting spacecraft and Earth-based astronomical facilities, have revealed the significant variation in environmental conditions from one band to the next. On Jupiter, the reflective white bands of low temperatures, elevated aerosol opacities, and enhancements of quasi-conserved chemical tracers are referred to as 'zones.' Conversely, the darker bands of warmer temperatures, depleted aerosols, and reductions of chemical tracers are known as 'belts.' On Saturn, we define cyclonic belts and anticyclonic zones via their temperature and wind characteristics, although their relation to Saturn's albedo is not as clear as on Jupiter. On distant Uranus and Neptune, the exact relationships between the banded albedo contrasts and the environmental properties is a topic of active study. This review is an attempt to reconcile the observed properties of belts and zones with (i) the meridional overturning inferred from the convergence of eddy angular momentum into the eastward zonal jets at the cloud level on Jupiter and Saturn and the prevalence of moist convective activity in belts; and (ii) the opposing meridional motions inferred from the upper tropospheric temperature structure, which implies decay and dissipation of the zonal jets with altitude above the clouds. These two scenarios suggest meridional circulations in opposing directions, the former suggesting upwelling in belts, the latter suggesting upwelling in zones. Numerical simulations successfully reproduce the former, whereas there is a wealth of observational evidence in support of the latter. This presents an unresolved paradox for our current understanding of the banded structure of giant planet atmospheres, that could be addressed via a multi-tiered vertical structure of "stacked circulation cells," with a natural transition from zonal jet pumping to dissipation as we move from the convectively-unstable mid-troposphere into the stably-stratified upper troposphere.
它们各自都呈现出行星带状分布模式，不同温度、成分、气溶胶特性和动力学的区域被纬向（即东西向）风的强烈经向和垂直梯度分隔开。来自到访航天器和地面天文设施的遥感观测揭示了不同条带间环境条件的显著差异。在木星上，低温、高气溶胶不透明度以及准守恒化学示踪剂增强的反射性白色条带被称为“区”。相反，温度较高、气溶胶 depleted、化学示踪剂减少的较暗条带被称为“带”。在土星上，我们通过其温度和风的特征来定义气旋带和反气旋区，尽管它们与土星反照率的关系不像在木星上那么清晰。在遥远的天王星和海王星上，带状反照率差异与环境特性之间的确切关系是一个正在积极研究的课题。这篇综述试图将带和区的观测特性与以下两点协调起来：（i）从木星和土星云层水平上涡旋角动量向向东纬向急流的汇聚以及带中湿对流活动的普遍存在推断出的经向翻转；（ii）从对流层上层温度结构推断出的相反经向运动，这意味着纬向急流在云层上方随高度衰减和消散。这两种情况表明经向环流方向相反，前者表明带中存在上升流，后者表明区中存在上升流。数值模拟成功地再现了前者，而有大量观测证据支持后者。这为我们目前对巨行星大气层带状结构的理解提出了一个未解决的悖论，这个悖论可以通过“堆叠环流单元”的多层垂直结构来解决，当我们从对流不稳定的对流层中部进入稳定分层的对流层上层时，会自然地从纬向急流泵送过渡到消散。
