Terrestrial Planetary Atmospheres and Climate Extremes: From Earth to Titan

乍一看，土星的卫星土卫六表面温度为零下290华氏度，看起来一点也不像地球。但仔细观察发现了一些奇怪的熟悉场景：卡西尼号任务的照片似乎显示了海洋、湖泊、干涸的湖床和排水沟。就像在地球上一样，这些地表特征是通过降水的作用形成的，但泰坦的雨滴是由液态甲烷而不是液态水组成的。和在地球上一样，土卫六上的雨滴也是以云的形式形成的，尽管云中包含浓缩的甲烷云滴，这些云滴又来自甲烷蒸汽的汇聚，甲烷蒸汽由风从表面蒸发液态甲烷的区域携带。简而言之，土卫六有一个以甲烷为基础的“水循环”，包括甲烷冰和雪、表面的液态甲烷、甲烷云和雨以及甲烷蒸汽，温度足够低，甲烷可以像地球上的水一样改变相态。认识到地球水循环和土卫六甲烷循环的共性，该项目将泰坦视为地球的类比，完全独立地表达了水循环物理和动力学，可以用来加深对我们世界的水循环和气候的理解。虽然水循环的许多特征在土卫六上都有甲烷类似物，但类似物的行为不同，表明了相同的潜在动力，但几个关键参数的值不同。例如，地球有一个热带辐合带(ITCZ)，这是一个与赤道大致平行的狭窄的低纬云带，泰坦也有类似的特征。在这两种情况下，ITCZ都有一个季节性周期，在一年中它会南北移动。但在地球上，这种变化是温和的，ITCZ仍然在热带地区，而在泰坦上，ITCZ一直从北极迁移到南极。这种巨大的差异可以用地球和土卫六之间行星自转速度的差异来解释，因为地球的自转速度是土卫六的16倍。PI和其他人在低得多的自转速度下对地球进行的理想化模拟显示，ITCZ南北迁移的范围相当大。因此，对于ITCZ的迁移而言，自转速度可以看作是一个从类地状态向类泰坦状态转变的动力学参数。这项工作还考虑了另外两个参数：大气中的水蒸气数量和表面的液体数量。在地球上，如果一个典型的大气层柱中的水蒸气被浓缩成液体形式，那么它将达到1到10厘米，但在土卫六上，大气中的甲烷蒸汽量要高出大约100倍(尽管甲烷仍然只占泰坦大气的2%左右，后者大约是98%的氮气)。地球大气中的水蒸气含量随着温度的升高而增加，因此，有些违反直觉的是，当谈到柱状水蒸气含量时，土卫六就像是一个温暖得多的地球。关于地表液体，土卫六就像一个非常干燥的地球，因为地球表面主要是深海，而土卫六只有湖泊和小海。该项目考察了这三个参数的变化在多大程度上会导致类似地球的大气循环和水循环呈现出类似泰坦的行为。其中一种现象是土卫六相对稀薄的云层，尽管有大量的甲烷蒸汽量和降雨量，但仍会出现这种情况。这项工作主要是通过计算机模拟进行的。使用的模式包括马克斯·普朗克气象研究所(MPI)的全球云解析模式ICON、埃克塞特大学的ISCA模式，以及可以在简化设置下模拟对流的大气柱模式。作为该项目的一部分，为ISCA建立了一个包括地下水流的地下水文模型。建模策略的一个关键要素是对饱和比湿度公式(Clausius-Clayperon公式)的修改，该公式允许在不增加平均温度的情况下增加柱状水蒸气。这项工作通过与该项目相关的一些教育活动产生了更广泛的影响。这位PI在他的本科教学中通过一套实际操作的流体力学设备来整合这项研究成果。此外，该项目还支持开发径向流入装置，该装置展示了角动量守恒对大气和海洋中流体流动的重要性，由学生设计作为课堂项目。课程还包括乘坐一艘小型考察船在圣莫尼卡湾进行实地考察，该项目的资金将用于支付旅行费用。该项目为两名研究生提供支持和培训，并在预算中提供资金，允许学生访问埃克塞特和MPI的外国合作者。最后，该项目支持开发陆地水文学组成部分模型，以便与ISCA一起使用，ISCA是一个开源模型。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

At first glance Titan, a moon of Saturn with a surface temperature of -290F, looks nothing like Earth. But closer inspection reveals some oddly familiar scenes: photos from the Cassini mission appear to show seas, lakes, dry lake beds, and drainage channels. As on earth these surface features come about through the action of precipitation, but Titan's raindrops are made of liquid methane instead of liquid water. Also as on Earth, raindrops on Titan form in clouds, albeit clouds consisting of condensed methane cloud droplets, which in turn come from the convergence of methane vapor, carried by the winds from regions in which liquid methane evaporates from the surface. In short, Titan has a methane-based "hydrological cycle", complete with methane ice and snow, liquid methane on the surface, methane clouds and rain, and methane vapor, at temperatures low enough for methane to change phase just as water does on Earth. Recognizing the commonalities of Earth's water cycle and Titan's methane cycle, this project treats Titan as an Earth analog, a completely independent expression of hydrological cycle physics and dynamics which can be used to develop a deeper understanding of the water cycle and climate of our world. While many features of the water cycle have methane analogs on Titan, the behavior of the analogs is different in ways that suggest the same underlying dynamics but with different values of a few key parameters. For example, Earth has an intertropical convergence zone (ITCZ), a narrow low-latitude band of clouds roughly parallel to the equator, and Titan has a similar feature. In both cases the ITCZ has a seasonal cycle in which it shifts north and south over the course of the year. But on Earth the shifts are modest and the ITCZ remains in the tropics, while on Titan the ITCZ migrates all the way from the north pole to the south pole. This dramatic difference can be explained by the difference in planetary rotation rate between Earth and Titan, as Earth rotates 16 times faster than Titan. Idealized simulations of Earth at much slower rotation rates by the PI and others show a comparable widening of the north-south migration of the ITCZ. Thus, as far as ITCZ migration is concerned, rotation rate can be regarded as a dynamical parameter which can be varied to transition from an Earth-like to a Titan-like state. Two other parameters are considered in the work: the amount of vapor in the atmosphere and the amount of liquid at the surface. On earth the water vapor in a typical atmospheric column would amount to between one and ten centimeters if it were condensed to liquid form, but on Titan the methane vapor in a column of the atmosphere is higher by a factor of perhaps a hundred (although methane is still only about two percent of Titan's atmosphere, which is about 98% nitrogen). The water vapor content of Earth's atmosphere increases with temperature so that, somewhat counterintuitively, when it comes to column vapor content Titan resembles a much warmer earth. Regarding surface liquid, Titan resembles a very dry earth, as Earth's surface is dominated by deep oceans while Titan has only lakes and small seas. The project examines the extent to which variations in the three parameters can cause an Earth-like atmospheric circulation and water cycle to take on Titan-like behaviors. One such behavior is Titan's relatively meager cloud cover, which occurs despite the high methane vapor amount and precipitation which can come in heavy downpours.The work is conducted largely through computer simulations. The suite of models used includes ICON, a global cloud resolving model from the Max Planck Institute for Meteorology (MPI), the Isca model from the University of Exeter, and atmospheric column models which can simulate convection in a simplified setting. A ground hydrology model including subsurface flow is constructed for Isca as part of the project. A key element of the modeling strategy is a modification to the equation for saturation specific humidity (the Clausius-Clayperon equation) which allows column water vapor to be increased without increasing mean temperature.The work has broader impacts through a number of educational activities connected to the project. The PI incorporates results of the research in his undergraduate teaching through a "weather in a tank" suite of hands-on fluid dynamics devices. In addition, the project supports development of a radial inflow device that demonstrates the importance of angular momentum conservation on fluid flow in the atmosphere and ocean, designed by students as a class project. Classes also include a field trip on Santa Monica bay in a small research vessel, and funds from this project are used to cover costs of the trip. The project provides support and training to two graduate students, and funds are provided in the budget to allow the students to visit the foreign collaborators at Exeter and MPI. Finally, the project supports development of a land hydrology component model for use with Isca, an open-source model.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.