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 cover and relative humidity and their sensitivity to climate change. It has been proposed that an increasing tendency of convection to self-aggregate with warming might lead to a reduced climate sensitivity by drying the atmosphere and allowing more longwave radiation to escape to space. We explore cloud and circulation changes in response to uniform SST change in a set of radiative–convective equilibrium simulations with the System for Atmospheric Modeling (SAM) cloud-system 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 into multiple moist and dry bands across this full range of temperatures. Simulations at 300 K show a surprisingly realistic distribution of large-scale vertical velocities at 500 hPa, despite homogeneous boundary conditions. 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 large-scale overturning circulations and clouds change in response to warming, although we have less confidence in low cloud changes and the shortwave feedbacks they cause.