Clouds, Circulation, and Climate Sensitivity in a Radiative-Convective Equilibrium Channel Model

Large-scale atmospheric circulation, and its interaction with organized moist convection across many scales, sets the patterns of tropical cloud clover and relative humidity and their sensitivity to climate change. It has been proposed that an increase in the amount of organized convection with warming might strengthen circulations and lead to a reduced climate sensitivity. We explore changes in clouds and circulation in response to uniform SST change in a set of radiative-convective equilibrium simulations with the System for Atmospheric Modeling (SAM) cloud resolving model. We use a non-rotating, highly elongated three-dimensional channel domain of length >10⁴ km, with interactive radiation and surface fluxes and fixed sea-surface temperature varying from 280–310 K. Convection self-aggregates and establishes large-scale circulations across this full range of temperatures, including multiple moist and dry bands in alternating regions of ascent and subsidence. Compared to observed tropical soundings, there is a realistic amount of variability in humidity. Simulations at 300 K also show a surprisingly realistic distribution of large-scale vertical velocities at 500 hPa, despite homogeneous boundary conditions. We find that large-scale overturning circulations decrease in strength with warming, by 6% K^-1, with part of this weakening likely attributable to changing spatial scale of the overturning cells. We find that aggregation of convection makes the total longwave feedback (Cess-type) modestly more negative, increasing its magnitude from -1.6 to -2.1 W m^-2 K^-1. We also discuss how cloud feedbacks in small-domain RCE (without large-scale circulation) compare to cloud feedbacks in the large-domain channel simulations, and find that in both cases, high cloud fraction is reduced with warming.