Oslo joint seminar in atmospheric, ocean and climate science, Nov. 7
Title: The contribution of radiative transfer error and base state diversity to inter-model spread in climate responses
Speaker: Ryan Kramer, NASA Goddard Space Flight Center
Speaker: Ryan Kramer, NASA
Changes in the composition of the atmosphere induce radiative perturbations that instigate all other climate changes. The forcing-feedback framework is widely used for diagnosing these perturbations and for understanding why climate models respond differently to identical emission scenarios. In this talk we first focus on uncertainty in effective radiative forcing, which consists of two parts - the instantaneous radiative forcing, measuring the perturbation in radiative fluxes due solely to a change in the forcing agent, and rapid adjustments, which measures the radiative perturbations induced by the atmosphere’s response to the instantaneous radiative forcing, before surface temperature responds. Recent efforts to diagnose uncertainty in effective radiative forcing have largely focused on rapid adjustments, in part because diagnostics of instantaneous radiative forcing are not routinely provided in climate model inter-comparison studies. We apply the radiative kernel technique to a multi-model ensemble of climate model simulations to highlight that instantaneous radiative forcing and not rapid adjustments are the dominant contributor to inter-model spread in effective radiative forcing, defined at the top-of-atmosphere and at the surface.
We next quantify the contribution of differences in model base state and the parameterization of radiative transfer to instantaneous radiative forcing spread. While the former source represents actual uncertainty, the latter exists despite radiative transfer being extremely well constrained by theory and observations. We highlight that addressing radiative transfer error therefore serves as a low hanging fruit for reducing inter-model spread in climate and hydrological sensitivity. Radiative transfer error and base state diversity also impact the radiative kernels used to diagnose radiative forcing and feedbacks. Despite a quickly rising number of radiative kernels, little work has been conducted to understand whether these sources of spread limit the effectiveness of the method as a diagnostic tool. We highlight notable differences between GCM-based radiative kernels, especially when applied to the surface energy budget, and discuss what this implies for how we evaluate radiative responses in models and observations. Finally, in relation to this work, we introduce radiative kernels based on satellite observations, which we use to estimate radiative feedbacks in the A-train satellite observational record.
What is the Joint Oslo Seminar (JOS):
Atmospheric and climate sciences have a stronghold in Oslo among the four institutions University of Oslo, the Meteorological Institute, CICERO and NILU. This joint seminar invites renowned international experts to contribute to an informal series of lectures, meant to create interaction with the Oslo atmospheric and climate science community on recent highlights and analysis in the field. All seminars will be held on Thursdays (Noon -1pm) and lunch (sandwiches) will be served on a “first-come-first-served”-basis.