NSFGEO-NERC: Analyses of the pattern effect on radiative feedbacks

Overview The "pattern effect" refers to the contribution of the spatially-varying pattern of temperature change to the global-mean climate feedback. The role of the pattern effect in climate sensitivity is generally probed and quantified in targeted numerical experiments forced with prescribed sea-surface temperatures. The current approaches have two shortcomings: 1) The current numerical set-up is focused on climate models forced with fixed surface temperatures. Whether the set-up provides a robust estimate of the time-varying climate feedback in coupled climate simulations and observations remains unclear. 2) The current numerical experiments do not prescribe land-surface temperatures. Thus the contribution of the spatially-varying pattern of land-surface temperature change to the climate feedback is unknown. The proposed research will address the two shortcomings through two research Streams. The first Stream is guided by the hypothesis that statistical methods provide a practical and physically-meaningful approach for investigating the pattern effect, and can be applied to both observations and coupled climate model output. The research will include developing and testing a hierarchy of statistical methods that quantify the pattern effect in observations and existing coupled climate simulations. The results will provide new insights into the regional physical processes that drive variations in the global feedback due to both internal variability and external forcing in both models and observations. The second Stream is guided by the hypothesis that land-surface temperatures have a substantial influence on local and remote clear-sky feedbacks and cloud-radiative effects, and thus on the global feedback parameter. The research in Stream 2 will quantify the role of land-surface temperatures in the global radiative flux and climate feedbacks by performing the first numerical surface temperature patch experiments that apply the Green's function approach to both ocean and land areas. The proposal is a joint NSF/NERC project. The research will be led by Drs David WJ Thompson and Maria Rugenstein in the Department of Atmospheric Science at Colorado State University, and Dr Paulo Ceppi in the Department of Physics at Imperial College London. It will include collaboration with Drs Timothy Andrews and Duncan Ackerley at the UK Met Office, who will serve as scientific advisors on the project and facilitate the development of the numerical patch experiments. Intellectual Merit The research will lead to new insights into our understanding of the physical processes that determine the global climate feedback parameter and thus climate sensitivity. Together, the research in Streams 1–2 will provide tools that can be used to estimate the global climate feedback parameter with statistical methods alone and thus from observations – potentially opening new opportunities to observationally constrain climate feedback and sensitivity. The research will provide insights into the importance of land-surface temperatures in the global radiative response, and thus potentially a refined estimate of the global climate feedback parameter and its time variation. It will provide a numerical template for exploring the role of land-surface temperatures on radiative feedbacks and climate sensitivity. Broader Impacts The impacts for society include improved understanding of the global climate feedback and thus the amplitude of the climate response to increasing greenhouse gases. In the process, the work will foster international collaboration between experts on climate dynamics and feedbacks at Colorado State University in the US, Imperial College and the Met Office in the UK. It will provide a framework for prescribing land-surface temperatures in climate models and thus a potential new template for surface temperature patch (Green's function) experiments. The proposal will provide mentoring and funding support for two graduate students at CSU and for a postdoctoral researcher at Imperial College. It will foster international collaboration between students and early career researchers working in the UK and USA, and provide funds for the students to attend meetings and seminars relevant to their graduate education. It will support a series of outreach activities in the local school district in Fort Collins, CO. The research will additionally contribute to ongoing model development efforts for the upcoming phase 7 of CMIP. In particular, the prescribed land-temperature template developed for Stream 2 will be employed by the UK Met Office for new experiments run for the Radiative Forcing Model Intercomparison Project (RFMIP). The model source code modifications and new Green’s functions will be made publicly available to the international research community.