Terrestrial Planetary Atmospheres and Climate Extremes: From Earth to Titan

类地行星大气和极端气候：从地球到泰坦

• 批准号: 1912673
• 负责人: Jonathan Mitchell
• 金额: $ 60.15万
• 依托单位: University of California-Los Angeles
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2019
• 资助国家: 美国
• 起止时间: 2019-08-01 至 2023-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1912673&HistoricalAwards=false
• 关键词: Terrestrial; Planetary; Atmospheres; Climate; Extremes

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.

乍一看，土卫六，土星的卫星，表面温度为-290华氏度，看起来一点也不像地球。 但仔细观察会发现一些奇怪的熟悉场景：卡西尼号使命的照片似乎显示了海洋、湖泊、干涸的湖床和排水沟。 和地球上一样，这些表面特征是通过降水作用形成的，但土卫六的雨滴是由液态甲烷而不是液态水组成的。 和地球上一样，土卫六上的雨滴也形成了云，尽管云是由凝结的甲烷云滴组成的，而甲烷云滴又来自甲烷蒸汽的汇聚，这些甲烷蒸汽是由来自液态甲烷从表面蒸发的区域的风携带的。 简而言之，土卫六有一个以甲烷为基础的“水文循环”，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，温度低到足以让甲烷像地球上的水一样改变相态。该项目认识到地球水循环和土卫六甲烷循环的共同点，将土卫六视为地球模拟物，是水文循环物理学和动力学的一种完全独立的表现形式，可用于加深对我们世界水循环和气候的了解。 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，表明相同的潜在动力学，但几个关键参数的值不同。 例如，地球有一个热带辐合带（ITCZ），一个狭窄的低纬度云带，大致平行于赤道，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，在一年中它会向北和向南移动。 但在地球上，这种变化是适度的，ITCZ仍然在热带地区，而在泰坦上，ITCZ从北极一直迁移到南极。 这种巨大的差异可以用地球和泰坦之间行星自转速度的差异来解释，因为地球的自转速度是泰坦的16倍。 PI和其他人对地球自转速度慢得多的理想化模拟显示，ITCZ的南北迁移范围也相应扩大。 因此，就ITCZ迁移而言，旋转速率可以被看作是一个动力学参数，它可以变化，从类地过渡到类钛状态。在这项工作中还考虑了另外两个参数：大气中的蒸汽量和表面的液体量。 在地球上，典型大气柱中的水蒸气如果冷凝成液体形式，将达到一到十厘米之间，但在泰坦上，大气柱中的甲烷蒸气可能要高出一百倍（尽管甲烷仍然只有大约百分之二的泰坦大气，其中大约98%是氮）。 地球大气层中的水蒸气含量随着温度的升高而增加，因此，当涉及到柱蒸气含量时，泰坦就像一个更温暖的地球。关于表面液体，土卫六类似于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。 该项目研究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。 其中一个现象是土卫六的云层覆盖相对较少，尽管甲烷蒸汽量很高，降水量也很大，这在很大程度上是通过计算机模拟进行的。使用的模型套件包括ICON，一个全球云解析模型从马克斯普朗克气象研究所（MPI），Isca模型从埃克塞特大学，和大气柱模型，可以模拟在一个简化的设置对流。作为该项目的一部分，为Isca建立了一个包括地下水流的地面水文模型。 建模策略的一个关键要素是对饱和比湿方程（Clausius-Clayperon方程）的修改，该方程允许在不增加平均温度的情况下增加柱水蒸气。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。 PI通过一套动手流体动力学设备的“坦克天气”将研究结果纳入他的本科教学。 此外，该项目还支持开发一种径向流入装置，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。课程还包括在一艘小型研究船上对圣莫尼卡湾进行实地考察，该项目的资金用于支付旅行费用。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，使学生能够访问埃克塞特和MPI的外国合作者。 最后，该项目支持开发一个土地水文组件模型，用于Isca，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知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

项目成果

Axisymmetric Hadley Cell Theory with a Fixed Tropopause Temperature Rather than Height

固定对流层顶温度而不是高度的轴对称哈德来环流圈理论

• DOI: 10.1175/jas-d-19-0169.1
• 发表时间: 2020
• 期刊: Journal of the Atmospheric Sciences
• 影响因子: 3.1
• 作者: Hill, Spencer A.;Bordoni, Simona;Mitchell, Jonathan L.
• 通讯作者: Mitchell, Jonathan L.

Solsticial Hadley Cell ascending edge theory from supercriticality

来自超临界的夏至哈德来环流上升边缘理论

• DOI: 10.1175/jas-d-20-0341.1
• 发表时间: 2021
• 期刊: Journal of the Atmospheric Sciences
• 影响因子: 3.1
• 作者: Hill, Spencer A.;Bordoni, Simona;Mitchell, Jonathan L.
• 通讯作者: Mitchell, Jonathan L.

Axisymmetric Constraints on Cross-Equatorial Hadley Cell Extent

• DOI: 10.1175/jas-d-18-0306.1
• 发表时间: 2018-10
• 期刊: Journal of the Atmospheric Sciences
• 影响因子: 3.1
• 作者: S. Hill;S. Bordoni;Jonathan L. Mitchell

Effects of Varying Saturation Vapor Pressure on Climate, Clouds, and Convection

变化的饱和蒸气压对气候、云和对流的影响

• DOI: 10.1175/jas-d-22-0063.1
• 发表时间: 2023
• 期刊: Journal of the Atmospheric Sciences
• 影响因子: 3.1
• 作者: Spaulding-Astudillo, Francisco E.;Mitchell, Jonathan L.
• 通讯作者: Mitchell, Jonathan L.

Constraints from Invariant Subtropical Vertical Velocities on the Scalings of Hadley Cell Strength and Downdraft Width with Rotation Rate

恒定副热带垂直速度对哈德来环流强度和下沉气流宽度随旋转速率比例的约束

• DOI: 10.1175/jas-d-20-0191.1
• 发表时间: 2021
• 期刊: Journal of the Atmospheric Sciences
• 影响因子: 3.1
• 作者: Mitchell, Jonathan L.;Hill, Spencer A.
• 通讯作者: Hill, Spencer A.