Numerical Simulation of the Stratus-to-Cumulus Transition in the Subtropical Marine Boundary Layer. Part II: Boundary-Layer Circulation

Abstract

The progression from the stratus-topped boundary layer (STBL) to the trade cumulus boundary layer (TCBL) during a simulated stratus-to-cumulus transition (SCT) involves two intermediate stages: the deep stratus-topped boundary layer (DSTBL) and the “cumulus-under-stratocumulus” boundary layer (CUSBL). The DSTBL, like the STBL, has an active circulation that extends from the surface to the cloud top. The CUSBL, like the TCBL, has an active subcloud-layer circulation that is linked to the cloud layer by narrow cumulus updrafts. It is called a “cumulus-coupled” boundary layer. A generally applicable convective updraft/downdraft partitioning scheme based on trajectory analysis was developed and used to analyze the boundary-layer circulation changes during the simulated SCT. The circulation analysis revealed that as the SST increased and the boundary layer changed from an STBL to a TCBL the updraft fraction in the cloud layer decreased, the convective updrafts strengthened, and the convective downdrafts weakened. The convective mass flux in the cloud layer decreased significantly as SST increased, while in the subcloud layer it changed little. The differences between updraft and downdraft properties and cloud-base levels gradually increased as SST increased. An analysis of the vertical acceleration components of the convective updrafts and downdrafts suggests that there are three steps in the transition from an STBL circulation to a TCBL circulation. First, the STBL deepens due to increased surface buoyancy fluxes as it moves over increasing SST but remains well mixed. Next, the DSTBL gradually changes into the two-layer CUSBL. During this step, negative buoyancy in downdrafts originating near cloud top becomes less important, while positive buoyancy in (cumulus) updrafts becomes more important. This indicates that cloud-top entrainment instability does not play a significant role in the SCT. Finally, the overlying stratocumulus deck gradually dissipates and only the underlying cumulus clouds of a typical TCBL remain. This general sequence of events is supported by recent observational evidence.