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太阳光的量和地球向太空发射的红外辐射量。但云的气候效应是各种令人眼花缭乱的云类型、结构和大小的综合效应。原则上，这种聚合效果可以在逐云的基础上计算，详细模拟所有云，然后将它们各自的贡献相加。但这样的模拟是不切实际的，而且无论如何都不能为云的行为提供令人满意的解释。另一种方法是寻找至少近似地由大尺度大气的整体特性来确定云的细微尺度特性的方法。这种方法的优点是集中在可能受气候变化影响的云特性上，并可能起到放大或减缓温室气体增加引起的变暖的作用。在最近的工作中，PI发展了一种理论，其中大尺度大气的层化对热带对流云的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，地面太阳加热产生的对流势能通过云中上升的空气和周围势能较低的晴朗空气之间的水平湍流混合来耗散。由于云的两侧构成了一个边界，通过该边界发生混合跨越势能降，该理论意味着大气势能(特别是饱和静能)表面的云周长度与大气在该水平上的翻转运动的稳定性之间的关系。在准平衡状态下，这个关系进一步表明，具有给定周长的云的数量与该周长成反比。此外，穿过云层两侧的质量通量与云层与其环境之间的势能差有关。这一关系可以用来表明，高对流势能云比低对流势能云(波尔兹曼分布的一种形式)要罕见得多。在该奖项下的工作利用卫星观测和模型模拟来测试该理论关于云发生的周长和能量相关性的预测。进一步的工作涉及云数和周长之间的关系可以在多大程度上被用来推断具有给定水平面积的云数(可能遵循科尔卡克定律)，因为云面积与云的气候影响有直接联系。最后，我们将探讨该理论对热带对流云变化应对气候变化的影响。气候变暖时，大气稳定度有望增加，稳定度与周长的关系表明，变暖将增加云量和总周长。但需要做一些工作来协调这一结果与其他基于热力学的热带云层变化与变暖的论点。鉴于人们对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所产生的变暖程度的努力中，云层可能是最大的不确定性来源，这里研究的细尺度云特性与大尺度大气条件之间的关系可能有助于减少这种不确定性。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，从而在这一研究领域建立未来的劳动力队伍。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。

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.