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

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 CO2, 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 CO2 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 CO2 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.