The temperature dependence of CAPE, cloud-buoyancy, and vertical velocities in radiative-convective equilibrium

The implications of climate warming for tropical convective-cloud updraft velocities and precipitation rates are not well understood. Recent idealized and comprehensive modeling studies have suggested that convective available potential energy (CAPE) in the tropics may increase as the climate warms. But the reasons for this increase and the implications for updraft strength are not known. Here we use simulations of radiative-convective equilibrium over a wide range of CO2 levels to investigate how the behavior of convection changes as the temperature is varied.

CAPE, cloud buoyancy, and updraft velocities all increase as the atmosphere warms in the simulations. The increase in CAPE can be understood through a plume-based model in which the thermal stratification is assumed to remain close to neutral with respect to an entraining plume. The increase in CAPE then follows from increases in the saturation deficit of the mid-troposphere and the vertical extent of convection. Observational support for the importance of the saturation deficit in determining the lapse rate in convecting regions is also presented.

Cloud buoyancy increases less rapidly with warming compared with CAPE in the simulations, but a generalization of the plume model to allow for a spectrum of plumes accounts for the changes with warming in both cloud buoyancy and updraft velocities.