New Tools for Quantifying Cloud Response to Varying Climate States

云在地球气候中扮演着核心角色，因为它们调节着到达地球的阳光数量和地球向太空发射的红外辐射量。但是，云的气候效应是一种综合效应，它涵盖了令人眼花缭乱的各种云的类型、结构和大小。原则上，这种累积效应可以在逐云的基础上计算，详细模拟所有的云，然后将它们各自的贡献加起来。但是这样的模拟是不切实际的，而且无论如何也不能对云的行为提供令人满意的解释。另一种方法是寻找云的精细尺度特性至少近似地由大尺度大气的整体特性决定的方法。这种方法的优点是侧重于可能受气候变化影响的云特性，这些云特性可能会放大或缓和温室气体增加造成的变暖程度。在最近的工作中，pi发展了一种理论，其中大尺度大气的分层对热带对流云的精细尺度复杂性施加了热力学约束。该理论假设了一种准平衡状态，在这种状态下，太阳在地面加热产生的对流位能被云层中上升空气与周围低位能的晴空之间的水平湍流混合耗散。由于云的侧面构成了一个边界，通过这个边界混合发生在势能下降处，该理论暗示了大气势能（特别是饱和静态能）表面的云周长与大气在该水平面上对倾覆运动的稳定性之间的关系。在准平衡状态下，这种关系进一步表明具有给定周长的云的数量与该周长成反比。此外，通过云侧面的质量通量与云与其环境之间的势能差有关。这种关系可以用来表明，高对流势能的云在指数上比低势能的云更罕见（玻尔兹曼分布的一种形式）。该奖项下的工作利用卫星观测和模型模拟，测试了该理论关于云发生的周长和能量依赖的预测。进一步的工作解决了云数和周长之间的关系在多大程度上可以用来推断给定水平面积的云数（可能遵循Korcak定律），因为云面积与云的气候效应有直接的联系。最后，将探讨热带对流云随气候变化而变化的理论含义。在气候变暖的情况下，大气稳定性预计会增加，稳定性与周长之间的关系表明，变暖将增加云数和总周长。但是需要做一些工作来使这一结果与其他基于热力学的关于热带云量随变暖而变化的论点相一致。考虑到气候变化的严重性及其对人类和环境的影响，这项工作具有社会意义。在确定温室气体浓度增加造成的变暖程度的努力中，云可能是最大的不确定性来源，本文研究的细尺度云特性与大尺度大气条件之间的关系可以帮助减少这种不确定性。该项目包括里尔大学的参与者，从而促进了国际研究合作。它还为两名研究生和一名本科生提供支持和培训，从而在该研究领域建立未来的劳动力。该奖项反映了美国国家科学基金会的法定使命，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。

Clouds play a central role in Earth's climate as they regulate both the amount of sunlight reaching the Earth and the amount of infrared radation the Earth emits to space. But the climatic effect of clouds is an aggregate effect over a bewildering variety of cloud types, structures, and sizes. In principle this aggregate effect can be calculated on a cloud-by-cloud basis, simulating all clouds in detail and then adding up their individual contributions. But such simulations are impractical and in any case do not provide a satisfactory explanation for cloud behaviors. An alternative approach is to look for ways in which the fine-scale properties of clouds are determined, at least approximately, by the bulk properties of the large-scale atmosphere. This approach has the advantage of focusing on cloud properties which are likely to be influenced by climate change and may act to amplify or moderate the amount of warming caused by greenhouse gas increases.In recent work the PIs have developed a theory in which the stratification of the large-scale atmosphere exerts a thermodynamic constraint on the fine-scale complexity of tropical convective clouds. The theory assumes a quasi-equilibrium state in which convective potential energy generated by solar heating at ground level is dissipated by horizontal turbulent mixing between rising air in clouds and surrounding clear air, which has lower potential energy. Since the sides of a cloud constitute a boundary through which mixing occurs across a potential energy drop, the theory implies a relationship between the length of cloud perimeter on a surface of atmospheric potential energy (specifically saturated static energy) and the stability of the atmosphere to overturning motions at that level. In quasi-equilibrium this relationship further implies that the number of clouds with a given perimeter length is inversely proportional to that perimeter length. Also, the mass flux through the sides of a cloud is related to the potential energy difference between the cloud and its environment. This relationship can be used to show that clouds of high convective potential energy are exponentially more rare than clouds of lower potential energy (a form of the Boltzmann distribution).Work under this award tests the theory's predictions regarding the perimeter and energy dependence of cloud occurrence using satellite observations and model simulations. Further work addresses the extent to which the relationship between cloud number and perimeter length can be used to infer the number of clouds with a given horizontal area (possibly following Korcak's law), as cloud area has a direct connection to the climatic effects of clouds. Finally, the implications of the theory for changes in tropical convective clouds in response to climate change will be explored. Atmospheric stability is expected to increase in a warmer climate, and the relationship between stability and perimeter length suggests that warming will increase cloud number and total perimeter. But some work is required to reconcile this result with other thermodynamically-based arguments for changes in tropical cloud cover with warming.The work has societal relevance given concerns regarding the severity of climate change and its impacts on people and the environment. Clouds are perhaps the greatest source of uncertainty in efforts to determine the amount of warming produced by increases in greenhouse gas concentrations, and the relationships between fine-scale cloud properties and large-scale atmospheric conditions examined here could help to reduce this uncertainty. The project includes participants at the University of Lille and thus promotes international research collaboration. It also provides support and training to two graduate students and one undergraduate, thereby building the future workforce in this research area.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.