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

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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.