Sensitivity of low cloud transitions to climate perturbations

The sensitivity of marine boundary layer cloud transitions to climate perturbations is explored in Lagrangian simulations of the stratocumulus to trade cumulus transition over the Northeast Pacific. The setup of the simulations is based loosely on those of Sandu & Stevens (2011, doi:10.1175/2011JAS3614.1), which follows a composite trajectory from stratocumulus regions overlying colder SSTs near the coast towards regions dominated by trade cumulus clouds over warmer waters as the trajectory is advected offshore. In addition to a control simulation with conditions characteristic of the current climate, a number of perturbed climate simulations are performed that include warming (increases in SST with moist adiabatic warming aloft) and also changes in CO2, inversion strength and subsidence. These perturbations are considered both separately and in combination. These simulations are performed with both a large eddy simulation (LES) model (the System for Atmospheric Modeling, SAM) and the single column model from the Community Earth System Model (CESM). The simulations are compared to understand how well the climate model parameterizations within CESM capture the cloud responses to climate perturbations seen in the high-resolution LES.