A few factors are likely involved, including entrainment of air from the free troposphere into the PBL driven by local buoyancy fluxes, the cumulus mass flux out of the PBL, and large-scale vertical motions. To investigate the relative importance of these mechanisms, we analyze a 10-year run of the UCLA atmospheric general circulation model in which the diurnal cycle in solar insolation is eliminated. First, we examine the buoyancy fluxes at the surface and at the PBL top, essentially the entrainment contribution to the PBL mass budget. In mid and high latitudes of the both hemispheres during winter, the surface buoyancy flux is the primary control on boundary layer depth: Over continents, where the frigid land surface extracts heat from the boundary layer, a shallow boundary layer predominates, while over the ocean, where cold air extracts heat from the warmer ocean surface, a much deeper boundary layer occurs. In the subtropical regions where stratus clouds predominate, the distribution of the longwave cooling at the PBL top can account for much of the geographical variation in PBL depth. In the deep tropics, cumulus mass flux and large-scale vertical motions are the main factors controling PBL depth.
The diurnal variation in sunshine also affects the mean depth of the PBL. When we re-run the model with a diurnal cycle, PBL depth is dramatically reduced in continental regions where the land surface is significantly warmer than the PBL during the daytime, and significantly colder at night (mainly mid-latitudes during summertime and desert regions). In these regions, the PBL grows slowly during daytime as it eats into the overlying stratified atmosphere, then collapses when the sun sets and a cool dense layer of air forms at the surface. Because PBL depth comes into equilibrium with its environment more slowly when conditions favor growth of the PBL than when they favor decay, the PBL does not reach as high on average when a strong daily cycle of surface buoyancy forcing occurs. Outside of deserts and mid-latitude continents during summertime, there is hardly any difference in mean PBL depth between the simulations with and without the diurnal cycle, and the factors noted above remain the most important in controling mean PBL depth.