The long road to equilibrium: Temporal scaling of marine boundary layers*

Clouds forming in the atmospheric boundary layer play a crucial role in the Earth's energy balance. Climate projections are sensitive to the amount of low-cloud cover and small variations in stratocumulus area coverage can produce energy-balance changes comparable to those due to greenhouse gases. The response of the marine atmospheric boundary layer to the large-scale circulation is investigated using idealized large-eddy simulations (LES). The effects of the large-scale circulation are modeled by variations in the sea surface temperature and large-scale divergence, which span a typical subtropical-value parameter space. All simulations are initialized with a balanced free troposphere and a stratocumulus-topped mixed layer. The boundary layers are time-integrated until a statistically steady state is attained. The vertically integrated liquid water potential temperature equation is used to investigate the contribution to the energy balance of the various physical processes. In cloud regimes with a high area fraction stratiform cloud the dominant balance is between cloud long wave cooling and subsidence warming. In cloud regimes with little or no stratiform cloud the key processes are cloud long wave cooling, subsidence warming, and the surface heat flux. The column integrated energy budget is used to study the boundary layer approach to equilibrium. Subsidence warming has the longest time scale during the boundary layer approach to steady state. For boundary layers with significant stratiform cloud the time adjustment is exponential. The LES model results support theoretical estimates that the adjustment time scale is proportional to the inverse of the large-scale divergence.