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

从加利福尼亚州飞往澳大利亚的旅客可能会穿过平坦、完整的云层起飞，看着云层变得凹凸不平，层积云从层状层伸出，航班在较温暖的水域上空运行。最终，云层破裂，海洋出现在蓬松的积云之间，开始时积云很小，但在温暖的赤道太平洋上成长为塔楼。云层的破裂被称为层积云到积云的过渡，这对气候很重要，因为云层通过将阳光反射到太空来冷却地球，而海洋则吸收掉落在云层上的大部分阳光。这里的一个担忧是，层云到积云的转变与海洋表面温度(SST)有关，因此，温室气体增加导致的SST变暖可能会导致额外的变暖，因为它有利于分离良好的积云，而不是连续的层积云。对这种反馈的担忧--变暖导致进一步变暖--引发了人们对层积云向积云转变的浓厚兴趣。研究层积云的研究人员在大涡模拟(LES)模式中发现了一个强大的工具，在网格上模拟多云大气，流动变量如风速、温度、湿度和云量定义在间隔可能几十米的网格点上。LES模型产生了令人惊讶的逼真的云层模拟，但它们需要相当大的计算能力，并且它们模拟的云场不超过几十公里宽。小区域是研究云过渡的一个严重限制，云过渡发生在数百公里以上。另一个限制是，云的行为受到大尺度大气环流的强烈影响，特别是哈德利环流在全球范围内的翻转(南北半球各一个)。哈德利环流的特点是，高耸的积云在最温暖的海温区上升，亚热带地区的下沉平衡了这一点。倾覆运动和伴随的大尺度大气特征必须在大涡模拟中从外部施加，这是不幸的，因为云转换的变化可能会以反馈云转换的方式影响颠覆环流。本奖项下的研究解决了大涡模式可访问的区域大小与层积到积云转换和更大的Hadley单体覆盖的更大区域之间的不匹配问题。该战略包括开发和使用三种LES模式配置的层次结构，第一种是具有周期性边界的小域LES区域(从区域西侧流出的云在东侧重新进入)。较小的区域可以慢慢移动到逐渐变暖的SST上，以诱导云的过渡。第二种形式是一条长长的水道，北端流入(代表加利福尼亚州附近的情况)，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，并且没有实际的太平洋横断面那么长。然而，初步工作表明，如果在北端和南端之间施加SST对比，并伴随适当的下沉，该区域能够产生层积云到积云的转换。第三种结构与第二种结构相似，只是更深，末端封闭，因此它会产生自己的翻转环流，比哈德利环流小，但类似于哈德利环流。计划进行几个实验，例如，用不同的二氧化碳浓度来研究随着气候变暖而发生的云层转变的变化。模型结果的分析在一定程度上是由水平对流理论指导的，水平对流是指沿区域顶部或底部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范围大气翻转的通用框架。这一职业奖的教育和推广部分利用了LES模型模拟的云的真实感。该项目与国家大气研究中心(NCAR)的可视化服务和研究实验室(VisLab)合作，创建了一组云图像和动画，将作为在线云画廊向公众提供。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，承认在达芬奇的瀑布素描等艺术作品中对云和湍流的感性描绘。其中一项努力是通过焦耳研究员计划进行的，这是一个为K-12教师提供的为期六周的暑期计划。焦耳研究员与项目团队合作，为他们的课堂教学开发视觉材料。这一奖项反映了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。

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.