Low cloud reduction in a greenhouse‐warmed climate: Results from Lagrangian LES of a subtropical marine cloudiness transition

Abstract Lagrangian large‐eddy simulations of a composite stratocumulus to cumulus transition case over the subtropical northeast Pacific Ocean are subject to perturbed forcings that isolate the cloud response to CO 2 , to overall tropical warming, and to increased inversion stability over the subtropical subsidence regions. These simulations show that a tropical surface warming of 4 K induces substantial stratocumulus thinning via a thermodynamic mechanism: increased cloud layer humidity flux in a warmer climate induces an entrainment liquid‐flux adjustment that dries the stratocumulus cloud layer, whether well mixed or cumulus coupled. A radiative mechanism amplifies this response: increased emissivity of the free troposphere due to increased CO 2 and water vapor reduces radiative driving of turbulence in a stratocumulus‐capped boundary layer; a thinner stratocumulus layer accompanies less turbulence. In combination, a 4 K warming and CO 2 quadrupling greatly reduce low cloud and weaken the simulated shortwave cloud radiative effect by over 50%. Large increases in inversion stability in the stratocumulus regions could counter much of this cloudiness reduction.