The temperature dependence of rainfall intensity, CAPE, and vertical velocities in radiative-convective equilibrium

Radiative-convective equilibrium is simulated over a wide range of CO2 levels using a cloud-system resolving model. As the atmosphere warms, the model simulates increases in convective available potential energy (CAPE), cloud buoyancy, and updraft velocities. The increase in CAPE can be understood through an entraining plume model of convection in which cloud buoyancy is assumed to be small. The plume model shows that the CAPE increase is due to an increase in the saturation deficit of the mid-troposphere as the surface is warmed, as well as an increase in the height of the level of neutral buoyancy.

The increase in cloud updraft velocities simulated by the model, while large, occurs primarily in the upper troposphere, and hence affects the precipitation rate only weakly. The rate of increase of precipitation intensity with warming is roughly consistent with the increase in moisture content at the surface for sufficiently long accumulation periods. Higher rates of increase of precipitation intensity are found for shorter accumulation periods at low temperatures, and the reasons for this behavior are discussed.