Cloud transitions and decoupling in the shear-free stratocumulus-topped PBL

Large-eddy simulations are used to investigate basic dynamical processes thought to be responsible for decoupling of stratocumulus as PBL air moves equatorward in the trades. The simulations indicate that shear-free stratocumulus -topped mixed layers are unable to remain well mixed if so doing requires negative buoyancy fluxes anywhere in the flow interior. The tendency toward a two-layer structure becomes particularly strong as the negative area in the buoyancy flux profile necessary to maintain a mixed layer increases to more than 10\% of the positive area. The simulations provide convincing evidence that flux-partitioning closures for entrainment are incompatible with the turbulent dynamics of the shear-free stratocumulus-topped planetary boundary layer. Lastly the simulations demonstrate that velocity scales for free convection depend on the geometry of the buoyancy-flux profile, and not simply its integral.