Improved Understanding of Changes in Convective Available Potential Energy and Links to the Large-scale Circulation

从分散的爆米花云到高耸的黑色风暴云，雷暴的形成是夏季下午的常见景象。大气的对流有效势能（CAPE）促进了这种云的蓬勃生长。正式的CAPE是假定没有周围空气混入上升的空气羽流中，通过比周围空气轻的大气部分上升的饱和空气羽流的浮力所做的功。天气预报员通常从大气温度探测中计算CAPE，并用它来预测强对流风暴的可能性。温室气体引起的气候变化的计算机模拟通常显示CAPE随全球平均温度的大幅增加，这一结果引起了人们对雷暴可能因气候变化而变得更常见或更强烈的担忧。但是CAPE会随着全球温度升高而增加的原因尚不清楚，而且缺乏CAPE与温度依赖的理论限制了模式结果的可信度。在之前的资助下，PI的小组开发了一个简单的模型来解释CAPE对温度的依赖。但该理论假定大气处于辐射-对流平衡状态，这种状态近似于温暖的热带海洋上空的大气状态。这一理论作为起点是令人信服的，但不能直接应用于理解陆地或高纬度地区的CAPE变化。因此，该奖项下的工作旨在更全面地了解CAPE与全球气候之间的关系，包括大尺度大气环流的影响。这项研究是通过对耦合模式比对项目产生的气候模式模拟进行检验，并结合在有限范围内使用云解析模式来检验假设的实验来进行的。大气环流的作用是通过计算和分析湿润平均有效势能（MAPE）来评估的，MAPE是大气平均状态转化为能量较低的参考状态所能可逆产生的最大动能。MAPE分析试图将中高纬度大气的平均状态与其通过大尺度环流产生CAPE的潜力联系起来。地表条件对CAPE的影响是研究的另一个重点，因为强烈的日循环温度和湿度对陆地对流的发展起着关键作用。进一步的工作考虑了CAPE变化对Walker环流的影响，Walker环流是西太平洋和东太平洋之间的大尺度翻转环流。考虑到对流风暴（包括冰雹、闪电、龙卷风和山洪暴发）的破坏性影响，以及它们的强度或发生频率可能因气候变化而增加的迹象，这项工作具有社会和科学价值。具有实际意义的成果通过研讨会和其他场所与有关各方分享，研究成果纳入课堂教学和其他教育活动。此外，该项目为研究生提供支持和培训，从而为该研究领域的未来劳动力提供支持。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。

The build-up of a thunderstorm, from scattered popcorn clouds to a dark towering storm cloud, is a common sight on a summer afternoon. The vigorous growth of such clouds is fueled by the convective available potential energy (CAPE) of the atmosphere. Formally CAPE is the amount of work that would be done by the buoyancy force on a saturated plume of air rising through the portion of the atmosphere in which it is lighter than the surrounding air, assuming that no ambient air mixes into the rising plume. Weather forecasters routinely calculate CAPE from atmospheric temperature soundings and use it to predict the likelihood of severe convective storms.Computer simulations of greenhouse-gas induced climate change commonly show large increases of CAPE with global mean temperature, a result which has raised concerns that thunderstorms may become more common or intense as a consequence of climate change. But the reasons why CAPE should increase with global temperature are not clear, and the lack of a theory for the dependence of CAPE on temperature limits confidence in model results.Under previous funding the PI's group developed a simple model which explains the dependence of CAPE on temperature. But the theory assumes that the atmosphere is in a state of radiative-convective equilibrium, a state which approximates the condition of the atmosphere over warm tropical oceans. The theory is compelling as a starting point but cannot be directly applied to understand CAPE change over land or at higher latitudes. Work under this award thus seeks a more general understanding of the relationship between CAPE and global climate, including the effects of large-scale atmospheric circulation. The research is conducted through examination of climate model simulations produced for the Coupled Model Intercomparison Project, combined with experiments using a cloud resolving model on a limited domain to test hypotheses. The role of atmospheric circulation is assessed through calculation and analysis of moist mean available potential energy (MAPE), the maximum amount of kinetic energy that can be reversibly produced from the mean state of the atmosphere by transforming to a lower energy reference state. The MAPE analysis seeks to relate the mean state of the atmosphere in middle and high latitudes to its potential to generate CAPE through large-scale circulations. The impact of land surface conditions on CAPE is another focus of the research, as the strong diurnal cycle of temperature and moisture plays a key role in the development of convection over land. Further work considers the impact of changes in CAPE on the Walker circulation, a large-scale overturning circulation between the western and eastern Pacific.The work has societal as well as scientific value given the damaging effects of convective storms, including hail, lightning, tornados, and flash floods, along with indications that their intensity or frequency of occurrence may increase due to climate change. Results with practical implications are shared with interested parties through workshops and other venues, and research results are incorporated into classroom teaching and other educational activities. In addition, the project provides support and training to a graduate student, thereby providing for the future work force 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.