UC San Diego

The transition from stratocumulus (Sc) cloud to cumulus (Cu) cloud occurs when air parcels advect from subtropical ocean to tropical ocean. Due to complex physics involved during the transition, it is hard to represent it in large/meso-scale models and the transition itself is crucial to the energy budget for the subtropic environment. The Atlantic Stratocumulus Transition Experiment (ASTEX) was conducted specifically to resolve this problem. In this research, ASTEX case was simulated with Dutch Atmosphere Large Eddy Simulation (DALES) model with two decaying scalars designed to track the origination of air parcels. During 40 hours of simulation for ASTEX, the decoupling stage accounted for 30 hours, showing a clear three-layer-structure: i) a well-mixed surface layer driven by surface heating; ii) a Cu layer with heating and moisture feeding from the surface and cooling and drying from the Sc layer but without sufficient turbulent kinetic energy (TKE) to mixed them together; iii) a well-mixed Sc layer driven by radiative cooling. During the transition, Sc layer becomes thinner and the downward non-local mixing becomes weaker, which further promotes the decoupling.

To help build a clear representation of the Sc-Cu transition in the Numerical Weather Prediction model (NWP), a quadrant analysis based on a decaying scalar and vertical velocity perturbation with vertical velocity probability density function (wPDF) filter was used separating the turbulent shell and coherent structures. A clear dynamic cycle among turbulent shells and interactions between the turbulent shells and coherent structures was observed. To show the gradual decrease of the vertical mixing within Sc layer during transition, a comparable downdraft initiated from cloud base was added to the existing state-of-art stochastic Eddy-diffusivity Mass-flux (EDMF) planetary boundary layer (PBL) parameterization.