Collaborative Research: Climate Feedbacks in Radiative-Convective Equilibrium--The Role of Self-Aggregation of Convection in A Multi-Model Ensemble of Idealized Simulations

长波辐射与对流垂直输送感热和潜热之间的相互作用是地球天气和气候的重要组成部分。在没有对流的情况下，长波辐射的冷却效应产生的垂直温度分布是不稳定的。不稳定性导致对流，对流向上输送感热和潜热，抵消长波冷却，使大气重新稳定。这种辐射冷却和对流热输送之间的协同相互作用导致了一种称为辐射-对流平衡（RCE）的状态，它为气温与海拔高度的关系提供了一个基本的解释。RCE对于我们理解全球温度对温室气体浓度变化的敏感性（即气候敏感性）、水文循环对全球温度变化的响应以及大尺度大气环流的发展也是至关重要的。早期的RCE模拟使用了高度简化的近似，只捕获了对流的整体效应，但现在的RCE模拟使用了更复杂的模型，这些模型代表了单个对流云及其与周围空气的相互作用。pi和其他机构的模拟结果表明，对流云聚集成团的趋势随着温度的升高而增加。对流聚集的温度依赖性可能对气候敏感性有影响，因为聚集的云集中在相对较小的区域，在地球上留下的晴空比分解的对流大得多。在晴朗的天空下，对太空的冷却更有效，因此，由于聚集增强而增加的晴朗天空面积构成了负反馈，抵消了导致聚集增加的部分变暖。由于对流聚集而产生负反馈的可能性对理解气候变化及其潜在的社会影响具有明确的意义。但对流聚集的机制及其对温度的依赖性，以及对流聚集对气候敏感性的影响程度尚不清楚。进一步的考虑是，聚集程度及其对温度的依赖可能在不同的模式之间不一致，而pi已经努力组织了一个国际RCE模式比较项目（RCEMIP）来解决模式依赖问题。在这里，pi使用RCEMIP的模拟来探索聚合的物理和动力学及其对温度的依赖，使用各种诊断来测试他们自己先前工作和合作者的假设。使用全球云解析模式——大气模拟系统（SAM），在专门的配置下进行了额外的模拟，旨在促进或抑制聚集，从而评估其对气候敏感性的影响。该项目还包括观测比较和设计适合RCE模拟的简化物理包。由于气候变化对社会的影响以及更好地估计温室变暖的可能后果的可取性，这项工作具有更广泛的影响。云、环流和气候敏感性的相互作用已被世界气候研究计划确定为一个巨大的挑战。该项目通过pi参与RCEMIP，以及相关的云反馈模式比对项目和全球大气系统研究工作，建立了国际科学合作。特别是首席项目负责人在她的校园为RCEMIP建立了一个数据门户，提供模型输出、诊断分析软件和文档，从而促进社区参与项目。该项目还为一名学生和一名博士后提供支持和培训，从而为该研究领域提供劳动力发展。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 interplay between longwave radiation and the vertical transport of sensible and latent heat by convection is a key ingredient of the weather and climate of the earth. In the absence of convection the cooling effect of longwave radiation produces a vertical temperature profile which is unstable. The instability leads to convection, which transports sensible and latent heat upward and counteracts the longwave cooling, restabilizing the atmosphere. This cooperative interaction between radiative cooling and convective heat transport leads to a state known as radiative-convective equilibrium (RCE), which provides a basic explanation for the dependence of air temperature on altitude. RCE is also fundamental to our understanding of the sensitivity of global temperature to changes in greenhouse gas concentration (referred to as climate sensitivity), the response of the hydrological cycle to changes in global temperature, and the development of large-scale atmospheric circulation.Early simulations of RCE used highly simplified approximations which only captured the bulk effects of convection, but RCE simulations are now performed with more sophisticated models that represent individual convective clouds and their interactions with the surrounding air. Results of such simulations by the PIs and others suggest that the tendency of convective clouds to aggregate into clusters increases with increasing temperature. The temperature dependence of convective aggregation could matter for climate sensitivity, as aggregated clouds are concentrated in relatively small regions, leaving clear skies over a larger portion of the earth than disaggregated convection. Cooling to space is more effective under clear skies, thus an increase in clear sky area due to enhanced aggregation constitutes a negative feedback, counteracting some of the warming that produced increased aggregation. The possibility of a negative feedback due to convective aggregation has clear implications for understanding climate change and its potential societal impacts. But the mechanisms of convective aggregation and its temperature dependence, and the extent to which aggregation affects climate sensitivity are not clear. A further consideration is that the extent of aggregation and its temperature dependence may not be consistent from one model to another, and the PIs have worked to organize an international RCE model intercomparison project (RCEMIP) to address the issue of model dependence.Here the PIs use simulations from RCEMIP to explore the physics and dynamics of aggregation and its temperature dependence using a variety of diagnostics to test hypotheses taken from their own prior work and from their collaborators. Additional simulations are performed using the System for Atmospheric Modeling (SAM), a global cloud resolving model, in specialized configurations designed to promote or suppress aggregation, thereby providing an assessment of its effect on climate sensitivity. The project also includes observational comparisons and the design of a simplified physics package suitable for RCE simulations.The work has broader impacts due to the societal impacts of climate change and the desirability of better estimates of the likely consequences of greenhouse warming. The interaction of clouds, circulation, and climate sensitivity has been identified as a grand challenge by the World Climate Research Program. The project builds international scientific collaboration through the PIs' participation in RCEMIP, as well as the related Cloud Feedback Model Intercomparison Project and the Global Atmospheric System Studies effort. In particular the lead PI has established a data portal for RCEMIP on her campus to provide model output, diagnostic analysis software, and documentation, thereby facilitating community engagement in the project. The project also provides support and training for a student and a postdoc, thereby providing workforce development 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.