CAREER: Understanding Low-cloud Feedbacks Using Large-eddy Simulation of Spatially Developing Cloud Transitions

一个从加利福尼亚飞往澳大利亚的旅行者可能会从一个平坦的、完整的云层中起飞，然后看着云层变得凹凸不平，当飞机经过温暖的水域时，层积云从层积云中伸出来。最终，云层破裂，海洋出现在蓬松的积云之间，这些积云开始很小，但在温暖的赤道太平洋上空成长为高塔。云层的破裂，被称为层积云到积云的转变，对气候很重要，因为云层通过将阳光反射到太空来冷却地球，而海洋吸收了大部分落在上面的阳光。这里的一个问题是，层云到积云的转变与海表温度（SST）有关，因此，温室气体增加导致的海表温度变暖可能会导致在连续的层积云云甲板上有利于分离良好的积云，从而导致额外的变暖。对这种反馈的关注，即变暖导致进一步变暖，引起了人们对层积云向积云转变的强烈兴趣。研究层积云的研究人员在大涡模拟（LES）模型中发现了一种强大的工具，在这种模型中，多云的大气在网格上模拟，其中风速、温度、湿度和云量等流动变量定义在网格点上，网格点之间相距可能几十米。LES模型产生了令人惊讶的真实的云模拟，但它们需要相当大的计算能力，而且它们模拟的云场宽度不超过几十公里。小的区域严重限制了云跃迁的研究，因为云跃迁发生在数百公里之外。进一步的限制是云的行为受到大尺度大气环流的强烈影响，特别是哈德利单体（北半球和南半球各一个）在行星尺度上的翻转。哈德利单体的特点是在最温暖的海温上空高耸积云的上升运动与副热带上空的下沉平衡。在LES模拟中，翻转运动和伴随的大尺度大气特征必须由外部施加，这是不幸的，因为云过渡的变化可能以反馈云过渡的方式影响翻转环流。该奖项下的研究解决了LES模型的可访问域大小与层积云到积云过渡覆盖的更大区域以及更大的哈德利细胞之间的不匹配。该策略涉及三个LES模型配置的层次结构的开发和使用，其中第一个是具有周期性边界的小域LES域（从域的西侧流出的云重新进入东侧）。小区域可以在逐渐变暖的海温上空缓慢移动，从而引起云的转变。第二种配置是一条长通道，北端流入（代表加利福尼亚附近的情况），南端流出（“澳大利亚”端）。该区域很窄，但在纵向上是周期性的，以节省计算费用，而且不像实际的太平洋样带那么长。然而，初步工作表明，如果南北两端施加海温对比，并伴有适当的下沉，该区域能够产生层积云到积云的转变。第三种结构与第二种相似，只是更深，两端封闭，从而产生自己的翻转循环，比哈德利单元小，但与之相似。几个实验正在计划中，比如用不同的二氧化碳浓度来研究气候变暖时云层过渡的变化。模式结果的分析部分是由水平对流理论指导的，水平对流理论指的是沿一个区域的顶边界或底边界的温度差异所产生的对流行为。水平对流在海洋学中得到了广泛的研究，但并不是理解云和大尺度大气翻转的通用框架。这个职业奖的教育和推广部分利用了LES模型模拟的云的现实性。该项目与美国国家大气研究中心（NCAR）的可视化服务和研究实验室（VisLab）合作，创建了一个云图像和动画的集合，将作为一个在线云画廊提供给公众。云画廊的一个版本将在康涅狄格州的公共博物馆威廉本顿艺术博物馆（WBMA）展出。该项目还包括将艺术融入科学教学的努力，承认在艺术作品中对云和湍流的敏锐描绘，如达芬奇的瀑布素描。其中一项努力是通过焦耳研究员项目进行的，这是一个为期六周的K-12教师暑期项目。Joule研究员与项目团队合作，为他们的课堂教学开发视觉材料。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。

A traveler flying from California to Australia might take off through a flat, unbroken cloud layer and watch as the layer gets lumpy, with stratocumulus clouds protruding from the stratus layer as the flight tracks over warmer water. Eventually the cloud deck breaks up and the ocean appears between puffy cumulus clouds, which start out small but grow into towers over the warm equatorial Pacific. The breakup of the cloud deck, referred to as the stratocumulus to cumulus transition, matters for climate as a cloud deck cools the earth by reflecting sunlight to space while the ocean absorbs most of the sunlight that falls on it. One concern here is that the stratus to cumulus transition is tied to sea surface temperature (SST), thus the warming of SSTs by greenhouse gas increases could lead to additional warming by favoring well-separated cumulus clouds over a continuous stratocumulus cloud deck. Concern over this feedback, in which warming begets further warming, has prompted intense interest in the stratocumulus to cumulus transition.Researchers studying stratocumulus clouds have found a powerful tool in Large Eddy Simulation (LES) models, in which the cloudy atmosphere is simulated on a grid mesh with flow variables like wind speed, temperature, moisture, and cloud amount defined on a lattice of gridpoints spaced perhaps a few tens of meters apart. LES models produce surprisingly realistic simulations of clouds, but they require considerable computing power and the cloud fields they simulate are not more than a few tens of kilometers wide. The small domain is a severe limitation in studies of the cloud transition, which takes place over hundreds of kilometers. A further limitation is that cloud behavior is strongly influenced by the large-scale atmospheric circulation, particularly the planetary-scale overturning of the Hadley cells (one each for the Northern and Southern Hemispheres). The Hadley cells feature rising motion in the towering cumulus clouds over the warmest SSTs balanced by subsidence over the subtropics. The overturning motion and accompanying large-scale atmopsheric characteristics have to be externally imposed in LES simulations, which is unfortunate as changes in the cloud transition may influence the overturning circulation in ways that could feed back on the cloud transition.Reseach under this award addresses the mismatch between the accessible domain sizes for LES models and the much larger areas covered by the stratocumulus to cumulus transition and the even larger Hadley cells. The strategy involves the development and use of a hierarchy of three LES model configurations, the first of which is a small-domain LES domain with periodic boundaries (clouds flowing out of the western side of the domain reenter on the eastern side). The small domain can be slowly moved over progressively warmer SSTs to induce the cloud transition. The second configuration is a long channel with inflow at the northern end (representing conditions near California) and outflow at the southern end (the "Australian" end). The domain is narrow but periodic in the longitudinal direction to save on computational expense, and not as long as an actual Pacific transect. Nevertheless preliminary work shows that the domain is capable of producing the stratocumulus to cumulus transition if an SST contrast is imposed between the northern and southern ends, along with appropriate subsidence. The third configuration is similar to the second only much deeper and closed at the ends so that it generates its own overturning circulation, smaller than but analogous to a Hadley cell.Several experiments are planned, for instance with varying carbon dioxide concentration to study changes in the cloud transition as climate warms. Analysis of model results is guided in part by the theory of horizontal convection, meaning the behavior of convection generated by temperature contrasts along the top or bottom boundary of a domain. Horizontal convection has been extensively studied in oceanography but is not a common framework for understanding clouds and large-scale atmospheric overturning.The education and outreach component of this CAREER award takes advantage of the realism of clouds simulated by LES models. The project works with the Visualization Services and Research Lab (VisLab) at the National Center for Atmospheric Research (NCAR) to create a collection of cloud images and animations that will be offered to the public as an online Cloud Gallery. A version of the Cloud Gallery will be exhibited at the William Benton Museum of Art (WBMA), a public museum in Connecticut. The project also involves efforts to incorporate art into science teaching, acknowledging the perceptive portrayal of clouds and turbulence found in artworks such as Da Vinci's sketches of waterfalls. One such effort is conducted through the Joule Fellows program, a six week summer program for K-12 teachers. Joule Fellows work with the project team to develop visual materials for their classroom teaching.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.