What controls the climatological depth of the PBL?

The depth of the planetary boundary layer (PBL) is a climatologically important quantity which has received little attention on regional to global scales. This study utilizes the UCLA atmospheric GCM to investigate the large-scale structure of the climatological PBL depth. Working within the framework of a PBL mass budget, the mechanisms controlling the climatological PBL depth are introduced, and several distinct PBL regimes are identified. A hierarchy of models is employed to determine the relative importance of the physical processes involved as well as the significance of diurnal and seasonal variation in solar forcing. We show that the locally generated buoyancy flux is a good proxy for the entrainment term from the mass budget, and that this is the primary control on climatological PBL depth for most locations. Other factors that affect the mean PBL depth include the stability of the lower troposphere and the mass flux due to cumulus convection. Using the GCM and a simple model for the dry convective boundary layer, the diurnal variation of solar insolation is shown to significantly modify the mean PBL depth over dry land regions. The analysis also shows that for many locations there is a marked seasonal variation in PBL depth associated with seasonal changes of PBL regime. This study suggests that knowledge of a few key parameters can be used to estimate the PBL depth quite well, which may prove useful in the development of simplified climate models and analysis of space-borne observations.