Title：Understanding the Atmospheric Adjustment to CO2 Perturbation at the Process Level
Lecturer：Prof. Yi Huang（Department of Atmospheric and Oceanic Sciences, McGill University）
Time：2:00 PM May 24, 2019
Venue： Lecture Hall D103, School of Atmospheric Sciences, Xianlin Campus
Abstract：Climate model comparisons show that there is considerable uncertainty in the atmospheric temperature response to CO2 perturbation, even when the sea surface temperatures are identically prescribed. This uncertainty results from the lack of process-level understanding of the temperature adjustment. Here, we aim at isolating the effects of different processes, including radiation, convection and large-scale circulation, on the temperature adjustment, through a set of numerical experiments using a hierarchy of climate models. We find that radiative adjustment triggers and largely controls the zonal mean atmospheric temperature response pattern. This pattern is characterized by stratospheric cooling, lower tropospheric warming and a warming center near the tropical tropopause. The dynamical processes largely counteract the destabilizing effect of the radiative process. The effect of local convection is to move atmospheric energy vertically, which cools the lower troposphere and warms the upper troposphere. The adjustment due to large-scale circulation further redistributes energy along the isentropic surfaces across the latitudes, which cools the low-latitude lower troposphere and warms the middle-latitude upper troposphere and stratosphere. Within this process-level dissection of atmospheric response, we develop a method to quantitatively attribute the equilibrium temperature response and its uncertainty to the radiative process. The temperature response uncertainty is found to result from the uncertainties in both CO2 forcing and radiative Jacobians. This calls into attention the importance of Jacobians in the radiation code intercomparison and validation. We find that the errors in the temperature response that may be attributed to radiation code inaccuracy may reach several tenths Kelvin for surface temperature and more than one Kelvin for atmospheric temperatures in the case of quadrupling CO2.
Brief introduction to the speaker：Dr. Huang is an Associate Professor in the Department of Atmospheric and Oceanic Sciences at McGill University. He obtained his Ph.D. from Princeton University in 2008 and was a U.S./NOAA Climate and Global Change Postdoctoral Fellow before joining the McGill faculty in 2011. Dr. Huang’s research aims at understanding atmospheric radiation and applying it in atmospheric remote sensing. His work has advanced radiative transfer theories and especially the understanding of the radiative forcing that drives global warming, e.g., the logarithmic dependency and spatial distribution of CO2 forcing.