Large-Eddy Simulations of the Stratocumulus to Shallow Cumulus Cloud Transition

We describe a large-eddy simulation (LES) that provides an Eulerian perspective of the stratocumulus to shallow cumulus cloud (Sc-Cu) transition, one of the key cloud characteristics over the subtropical oceans. The present approach consists of two separate LESs running concurrently. The first is a steady-state LES with horizontally homogeneous conditions. The second is a horizontally inhomogeneous LES of the boundary layer, where the use of downstream and upstream boundary conditions replaces the periodic boundary conditions. The first LES generates a sheet of stratocumulus cloud with statistically steady properties. The variables on a vertical slice of the domain are then used as the upstream boundary condition of the second LES, where the transition is triggered by spatially evolving, but fixed-in-time, climatological factors, such as sea surface temperature (SST) and subsidence. The LES runs until it achieves a statistically steady evolution of the Sc-Cu transition. The results are compared to those of the Lagrangian view of transition cases with the time-varying large-forcing. This provides insights on cloud response to spatial-dependent aspects without the inherent time-lag effects.