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

长波辐射与对流引起的感热和潜热的垂直输送之间的相互作用是地球天气和气候的一个关键因素。在没有对流的情况下，长波辐射的冷却作用会产生一种不稳定的垂直温度廓线。这种不稳定导致对流，对流向上输送感热和潜热，抵消长波冷却，重新稳定大气。辐射冷却和对流热量输送之间的这种协同作用导致了一种被称为辐射对流平衡(RCE)的状态，这为气温与海拔高度的关系提供了基本的解释。RCE对我们理解全球温度对温室气体浓度变化的敏感性(称为气候敏感性)、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。早期对RCE的模拟使用高度简化的近似，仅捕捉到对流的整体效应，但现在RCE模拟使用更复杂的模式来表示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，对流云聚集成星团的趋势随着温度的升高而增加。对流聚集对温度的依赖可能会对气候敏感性产生影响，因为聚集的云集中在相对较小的区域，比分解对流在地球上更大的地方留下了晴朗的天空。在晴朗的天空下，对太空的冷却更有效，因此，由于增强的聚集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一些变暖。对流聚集产生负反馈的可能性对于理解气候变化及其潜在的社会影响具有明显的影响。但对流聚集的机制及其对温度的依赖性，以及聚集对气候敏感性的影响程度尚不清楚。另一个考虑是聚集的程度和它的温度依赖关系从一个模型到另一个模型可能不一致，PI已经努力组织了一个国际RCE模型比较项目(RCEMIP)来解决模型依赖的问题。在这里，PI使用来自RCEMIP的模拟来探索聚集的物理和动力学及其温度依赖，使用各种诊断来检验来自他们自己和他们的合作者的假设。使用大气建模系统(SAM)执行额外的模拟，SAM是一种全球云解析模型，在专门的配置中设计用于促进或抑制聚集，从而提供对其对气候敏感性的影响的评估。该项目还包括观测比较和设计适用于RCE模拟的简化物理方案。由于气候变化的社会影响以及对温室气体变暖可能后果的更好估计，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项重大挑战。该项目通过PIS参与RCEMIP以及相关的云反馈模式比较项目和全球大气系统研究工作来建立国际科学合作。特别是，首席PI在她的园区为RCEMIP建立了一个数据门户，以提供模型输出、诊断分析软件和文档，从而促进社区参与该项目。该项目还为一名学生和博士后提供支持和培训，从而在这一研究领域提供劳动力发展。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。

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.