Global climate research

What controls the mean depth of the planetary boundary layer?

The depth of the planetary boundary layer (PBL) is a climatologically important quantity that has received little attention on regional to global scales. In this study a 10-yr climatology of PBL depth from the UCLA atmospheric GCM is analyzed using the PBL mass budget. This GCM is particularly useful for PBL studies because it goes to extraordinary lengths to consistently couple the budgets of mass, heat, and moisture between the PBL and the free troposphere. Using the PBL mass budget allows processes affecting PBL depth to be separated. Based on the dominant physical processes, several PBL regimes are identified. The most common regimes are stratocumulus-topped boundary layers, dry convective boundary layers over land, stable boundary layers, and shallow boundary layers associated with deep convection.

These regimes exhibit large-scale geographic organization. Locally generated buoyancy fluxes and static stability control PBLdepth nearly everywhere, though convective mass flux has a large influence at tropical marine locations. Virtually all geographical variability in PBL depth can be linearly related to these quantities. While dry convective boundary layers dominate over land, stratocumulus-topped boundary layers are most common over ocean. This division of regimes leads to a dramatic land-sea contrast in PBL depth, evident in this figure showing the 10-year mean PBL depth from the simulation. Aside from the difference between land and ocean, there are also regions of deep PBL over eastern ocean basins, which are coincident with stratocumulus regions. High latitudes are dominated by the frigid surface conditions, leading to a relatively persistent shallow stable boundary layer.

Diurnal effects keep mean PBL depth over land shallow despite large daytime surface fluxes. The daily development of a deep convective boundary layer and its subsequent collapse to a nocturnal stable boundary layer leads to significant daily exchange of heat and mass between the PBL and free troposphere for most land locations. This diurnal cycle is absent over the ocean, where mixing is accomplished by turbulent entrainment. Employing a simple model of the convective boundary layer, we show that consistent treatment of remnant air from the deep daytime PBL is necessary for proper representation of the diurnal behavior over land.

To illustrate the differences in PBL regimes, this figure shows a scatterplot of PBL depth and buoyancy flux with points color-coded by (a) cumulus mass flux, (b) stability of the lower troposphere, and (c) incidence of stratus clouds. The figure separates land and ocean points, since diurnal variation makes the distributions very different. Some PBL regimes are clearly evident here. For example, tropical ocean locations are characterized by relatively shallow PBL with low mean buoyancy fluxes and high convective mass flux. Also evident are high-latitude marine locations, dominated by shallow boundary layers and small buoyancy flux (red and orange points in (b)). Stratus-topped PBLs have a range of depths, but generally have substantial buoyancy fluxes, driven mostly by cloud-top cooling. Many locations, both land and ocean, also exhibit seasonal shifts in PBL regime related to changes in PBL clouds. These shifts are controlled by seasonal variations in buoyancy flux and static stability. Seasonal variations in PBL regime contribute significantly to the spread of the mean characteristics shown in the scatterplot.

Using the PBL mass budget separately for ocean and land locations and investigating diurnal and seasonal variations in PBL depth, an understanding of the spatial distribution of PBL depth is reached. This analysis takes a global perspective, anticipating global PBL datasets providing sufficient information for a similar analysis, which could be useful for understanding the PBL and its role in climate as well as comparison with other climate models.

Download the publication (Medeiros et al. 2005) describing these results in more detail.

Brian Medeiros, Alex Hall, and Bjorn Stevens make up the team that performed this research.