Global circulation as the main source of cloud activity on Titan
Sébastien Rodriguez, Stéphane Le Mouélic, Pascal Rannou, Gabriel Tobie, Kevin H. Baines, Jason W. Barnes, Caitlin A. Griffith, Mathieu Hirtzig, Karly M. Pitman,… 展开作者
Sébastien Rodriguez, Stéphane Le Mouélic, Pascal Rannou, Gabriel Tobie, Kevin H. Baines, Jason W. Barnes, Caitlin A. Griffith, Mathieu Hirtzig, Karly M. Pitman, Christophe Sotin, Robert H. Brown, Bonnie J. Buratti, Roger N. Clark, Phil D. Nicholson
Clouds on Titan result from the condensation of methane and ethane and, as on other planets, are primarily structured by circulation of the atmosphere. At present, cloud activity mainly occurs in the southern (summer) hemisphere, arising near the pole and at mid-latitudes from cumulus updrafts triggered by surface heating and/or local methane sources, and at the north (winter) pole, resulting from the subsidence and condensation of ethane-rich air into the colder troposphere. General circulation models predict that this distribution should change with the seasons on a 15-year timescale, and that clouds should develop under certain circumstances at temperate latitudes (~40\degree) in the winter hemisphere. The models, however, have hitherto been poorly constrained and their long-term predictions have not yet been observationally verified. Here we report that the global spatial cloud coverage on Titan is in general agreement with the models, confirming that cloud activity is mainly controlled by the global circulation. The non-detection of clouds at latitude ~40\degree N and the persistence of the southern clouds while the southern summer is ending are, however, both contrary to predictions. This suggests that Titan's equator-to-pole thermal contrast is overestimated in the models and that its atmosphere responds to the seasonal forcing with a greater inertia than expected.
Air-Sea Interactions on Titan: Lake Evaporation, Atmospheric Circulation, and Cloud Formation
泰坦丰富的湖泊和海洋通过一种通常被称为空气-海洋相互作用的过程与大气交换甲烷蒸汽和能量。这种湍流交换过程的研究与大气中尺度模式耦合到一个平板模式表示下垫面湖泊。通过几十个二维数值模拟，研究了湖泊尺度、有效湖混合层深度、背景风速、气湖温差和大气湿度对海气相互作用过程的影响。一般的，准稳定的解决方案是一个非常弱的背景羽流环流驱动的浮力蒸发的甲烷与一个强大的相反的热直接（海风）环流驱动的湖泊和温暖的内陆空气之间的热对比的非线性叠加。具体的解决方案取决于所选择的大气和湖泊属性参数的值，但这两个环流的叠加的一般解决方案是持久的。与前人的分析工作相一致，感热通量和潜热通量趋向于相反和相等的值，使得它们的比值Bowen比在大多数（但不是全部）准稳态解中接近-1.0。重要的是，在几乎所有的情况下，通量的绝对星等的趋势非常小的值，这样的平衡解决方案也几乎是一个平凡的解决方案，其中海气能量交换约3 W m-2或less. In所有情况下，一个凉爽，潮湿，静态稳定的浅海层，近平静的风和小湍流通量交换与一个较冷的底层湖泊产生的模型。湖泊的温度、空气的海洋属性和海风的强度取决于初始条件，在较小程度上取决于边界条件。
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Astrochemistry Laboratory, NASA Goddard Space Flight Center（NASA戈达德太空飞行中心天体化学实验室）；Department of Physics, Catholic University of America（天主教大学物理系）；Aeolis Research（埃俄利斯研究）；Department of Earth and Planetary Sciences, Yale University（耶鲁大学地球与行星科学系）；School of Earth Sciences, University of Bristol（布里斯托尔大学地球科学学院）；Space Telescope Science Institute（太空望远镜科学研究所）；Division of Geological and Planetary Sciences, California Institute of Technology（加州理工学院地质与行星科学部）
