NUMERICAL-SIMULATION OF THE STRATUS-TO-CUMULUS TRANSITION IN THE SUBTROPICAL MARINE BOUNDARY-LAYER .1. BOUNDARY-LAYER STRUCTURE

A stratus-to-cumulus transition (SCT) that resembles observations occurred in Lagrangian numerical simulations of the subtropical marine boundary layer over the northeastern Pacific Ocean southwest of California. The Lagrangian approach involves translating the domain along the climatological boundary-layer trajectory at a rate equal to the observed surface wind speed. The SST is increased at a corresponding rate. The simulations did not include drizzle, the diurnal cycle, divergence changes, or mesoscale circulations and thus demonstrate that these processes are not essential for an SCT. A 2D numerical cloud model that can explicitly represent large convective eddies is used. Turbulence at scales smaller than the large eddies is parameterized using a third-moment turbulence closure. This type of model requires no cloud-regime-specific input and is computationally economical for multiday simulations. The results suggest that there are four stages in the transition from the stratus-topped boundary layer (STBL) to the trade cumulus boundary layer (TCBL). The simulated transition involves two intermediate stages: the deep stratus-topped boundary layer (DSTBL) and the ''cumulus-under-stratocumulus'' boundary layer (CUSBL). The DSTBL, like the STBL, is well mixed. The CUSBL has a two-layer structure, like the TCBL, with a well-mixed subcloud layer and a stratified (partly mixed) cloud layer. The transition to a typical TCBL structure preceded the transition to a typical TCBL cloud fraction by about two days. Sensitivity tests indicate that by using diurnally averaged solar radiation with the daytime-averaged solar zenith angle, the model is able to reproduce the diurnally averaged cloud-top height. Tests also suggest that the boundary-layer structure is sensitive to the above-inversion thermodynamic structure.