This study investigates how climate sensitivity depends upon the spatial pattern of radiative forcing. Sensitivity experiments using a coupled ocean-atmosphere model were conducted by adding anomalous incoming solar radiation over the entire globe, Northern Hemisphere mid-latitudes, Southern Ocean, and tropics, respectively, with both positive and negative perturbation considered. The varied forcing patterns led to highly divergent climate sensitivities, with extratropical forcing inducing significantly more global-mean temperature change compared to tropical forcing. This dependence is particularly strong over the Southern Hemisphere, where the climate is nearly twice as sensitive to Southern Ocean forcing as tropical forcing. This dependence of climate sensitivity on the location of radiative forcing stems from covariations between lapse rate feedback, cloud feedback and tropospheric stability. These results contrast with the conventional SST-pattern effect in which tropical surface temperature changes regulate the climate sensitivity, and has important implications for geoengineering and understanding the mechanisms of paleoclimate change.

Most global climate models with convective parameterization have trouble in simulating the observed diurnal cycle of convection. Maximum precipitation usually happens too early during summertime, especially over land. Observational analyses indicate that deep convection over land cannot keep pace with rapid variations in convective available potential energy, which is largely controlled by boundary-layer forcing. In this study, a new convective closure in which shallow and deep convection interact strongly, out of equilibrium, is implemented in atmosphere-only and ocean-atmosphere coupled models. The diurnal cycles of convection in both simulations are significantly improved with small changes to their mean states. The new closure shifts maximum precipitation over land later by about three hours. Compared to satellite observations, the diurnal phase biases are reduced by half. Shallow convection to some extent equilibrates rapid changes in the boundary layer at subdiurnal time scales. Relaxed quasi-equilibrium for convective available potential energy holds in significant measure as a result. Future model improvement will focus on the remaining biases in the diurnal cycle, which may be further reduced by including stochastic entrainment and cold pools.

The atmospheric Green’s function method is a technique for modeling the response of the atmosphere to changes in the spatial field of surface temperature. While early studies applied this method to changes in atmospheric circulation, it has also become an important tool to understand changes in radiative feedbacks due to evolving patterns of warming, a phenomenon called the "pattern effect." To better study this method, this paper presents a protocol for creating atmospheric Green’s functions to serve as the basis for a model intercomparison project, GFMIP. The protocol has been developed using a series of sensitivity tests performed with the HadAM3 atmosphere-only general circulation model, along with existing and new simulations from other models. Our preliminary results have uncovered nonlinearities in the response of the atmosphere to surface temperature changes, including an asymmetrical response to warming vs. cooling patch perturbations, and a change in the dependence of the response on the magnitude and size of the patches. These nonlinearities suggest that the pattern effect may depend on the heterogeneity of warming as well as its location. These experiments have also revealed tradeoffs in experimental design between patch size, perturbation strength, and the length of control and patch simulations. The protocol chosen on the basis of these experiments balances scientific utility with the simulation time and setup required by the Green’s function approach. Running these simulations will further our understanding of many aspects of atmospheric response, from the pattern effect and radiative feedbacks to changes in circulation, cloudiness, and precipitation.

Precipitation changes under global warming are widely studied. However, the direct radiative effect of CO2 on precipitation changes, independent from CO2-induced SST changes, has received less attention. Mechanistic understanding of the CO2 direct effect is therefore important and necessary. Utilizing multiple global atmospheric models, we identify robust summer precipitation changes across North America in response to the direct CO2 forcing. We find that spatial distribution of CO2 forcing at land surface is primarily shaped by climatological distribution of water vapor and clouds. This, coupled with changes in convection and moisture supply resulting from CO2-induced circulation changes, largely determines North America hydroclimate changes. In central North America, increasing CO2 decreases summertime precipitation by warming the surface and inducing dry advection into the region to reduce moisture supply. Meanwhile, for the southwest and the east, CO2-induced northward shift of subtropical highs generates wet advections to mitigate the drying effect from surface warming.

A ~ 50 km resolution atmospheric general circulation model (GCM) is used to investigate the impact of radiative interactions on spatial organization of convection, the model’s mean state, and extreme precipitation events in the presence of realistic boundary conditions. Mechanism-denial experiments are performed in which synoptic-scale feedbacks between radiation and dynamics are suppressed by overwriting the model-generated atmospheric radiative cooling rates with its monthly-varying climatological values. When synoptic-scale radiative interactions are disabled, the annual mean circulation and precipitation remain almost unchanged, however tropical convection becomes less aggregated, with an increase in cloud fraction and relative humidity in the free troposphere but a decrease in both variables in the boundary layer. Changes in cloud fraction and relative humidity in the boundary layer exhibit more sensitivity to the presence of radiative interactions than variations in the degree of aggregation. The less aggregated state is associated with a decrease in the frequency of extreme precipitation events, coincident with a decrease in the dynamical contribution to the magnitude of extreme precipitation. At regional scales, the spatial contrast in radiative cooling between dry and moist regions diminishes when radiative interactions are suppressed, reducing the upgradient transport of energy, degree of aggregation and frequency of extreme precipitation events. However, the mean width of the tropical rain belt remains almost unaffected when radiative interactions are disabled. These results offer insights into how radiation-circulation coupling affects the spatial organization of convection, distributions of clouds and humidity, and weather extremes.

A vertically resolved analysis of the budget equation for the spatial variance of moist static energy (MSE) is used to diagnose processes associated with the development of tropical cyclones (TCs) in a high-resolution general circulation model (GCM) under realistic boundary conditions. Previous studies have shown that radiation provides an important feedback which enhances TC development. Here we examine the vertical contributions to this feedback by performing a series of mechanism-denial experiments in which synoptic-scale radiative interactions are suppressed either in the boundary layer or in the free troposphere. Although the boundary layer makes up a much smaller proportion of the atmospheric column than the free troposphere, the two experiments result in similar magnitude of reduction in global TC frequency, indicating that radiative interactions in the boundary layer and those in the free troposphere are of comparable importance in modulating TC frequency. Using instantaneous 6-houly outputs, an explicit computation reveals spatial patterns of the advection term during different TC stages. Instead of damping the spatial variance of MSE as noted in previous idealized studies, the advection term is found to promote the development of TCs. We attribute this result primarily to the explicit calculation of the advection term, however the influence of SST gradients cannot be ruled out. While the vertical component of the advection term is prominent in the middle troposphere, the horizontal component dominates in the boundary layer. These results provide additional insight of how different physical processes contribute to TC development in GCMs under realistic boundary conditions.