论文标题
棕矮星和孤立的年轻巨型行星的喷气流和示踪剂混合
Jet Streams and Tracer Mixing in the Atmospheres of Brown Dwarfs and Isolated Young Giant Planets
论文作者
论文摘要
对棕矮星和相对隔离的年轻巨大巨型行星的观察提供了前所未有的细节，以在新制度中探测大气动力学。有关管理全球循环的机制的问题仍有待解决。先前的研究表明，由于对流和上覆的分层层之间相互作用而导致的小规模，随机变化的热扰动可以驱动区域喷射流，波和湍流。在这里，我们通过使用两流灰色辐射转移方案来改善以前的一般循环模型，以表示更现实的加热和冷却速率。我们检查了区域喷气机的形成及其时间演变，以及包括云和化学物种在内的被动示踪剂的垂直混合。在相对较弱的辐射和摩擦耗散下，速度高达几百美元m \; s^{ - 1} $的稳健Zonal喷气机是典型的结果。当赤道喷气机表现出明显的垂直风剪切时，赤道喷气机往往是无压力的。耗散强大的模型抑制了喷射的形成，并且在赤道区域中具有各向同性湍流。在相对较低的大气温度下，赤道流的准周期性振荡范围从数十到几个月不等。亚微米云颗粒可以传输到高于冷凝水平的几个比例高度，而较大的颗粒形成较薄的层。云甲板在其云层底部附近是不均匀的。具有化学时标的化学示踪剂$> 10^5 $ S可以驱动到平衡中。对于全球均值示踪剂而言，等效的垂直扩散系数，$ k _ {\ mathrm {zz}} $，从我们的模型中诊断出，通常按$ 1 \ sim10^2 \ rm m^2 \ rm m^2 \; s^{ - 1} $。最后，对于相关条件下，不同类型的示踪剂，我们得出了$ k _ {\ mathrm {zz}} $的分析估计。
Observations of brown dwarfs and relatively isolated young extrasolar giant planets have provided unprecedented details to probe atmospheric dynamics in a new regime. Questions about mechanisms governing global circulation remain to be addressed. Previous studies have shown that small-scale, randomly varying thermal perturbations resulting from interactions between convection and the overlying stratified layers can drive zonal jet streams, waves, and turbulence. Here, we improve upon our previous general circulation model by using a two-stream grey radiative transfer scheme to represent more realistic heating and cooling rates. We examine the formation of zonal jets and their time evolution, and vertical mixing of passive tracers including clouds and chemical species. Under relatively weak radiative and frictional dissipation, robust zonal jets with speeds up to a few hundred $\rm m\;s^{-1}$ are typical outcomes. The off-equatorial jets tend to be pressure-independent while the equatorial jets exhibit significant vertical wind shear. Models with strong dissipation inhibit jet formation and have isotropic turbulence in off-equatorial regions. Quasi-periodic oscillations of the equatorial flow with periods ranging from tens of days to months are prevalent at relatively low atmospheric temperatures. Sub-micron cloud particles can be transported to several scale heights above the condensation level, while larger particles form thinner layers. Cloud decks are inhomogeneous near their cloud tops. Chemical tracers with chemical timescales $>10^5$ s can be driven out of equilibrium. The equivalent vertical diffusion coefficients, $K_{\mathrm{zz}}$, for the global-mean tracer, are diagnosed from our models and are typically on the order of $1\sim10^2\rm m^2\;s^{-1}$. Finally, we derive an analytic estimation of $K_{\mathrm{zz}}$ for different types of tracers under relevant conditions.
