What is the Planetary Boundary Layer?

We know that friction is generated by the earth's surface to the atmosphere near the surface due its rotation to the earth, but that aloft friction is negligible in comparison. At some point in the atmosphere, there is a zone where friction goes from significant to insignificant. The lower layer of air which is subjected to turbulent (frictional) processes is known as the planetary boundary layer (PBL). The remaining air in the troposphere is known as the free atmosphere (because it is free of frictional influences).

But what is a boundary layer? It turns out that there is a well defined, but quite variable height, where the influence of friction stops. Below this point, the earth's surface has direct influences on motion in the atmosphere. These influences include frictional drag, evopotranspiration (the combined processes of evaporation and transpiration)terrain induced flow modification, heat and energy transfer, and pollution emission. The PBL is that layer which is influenced directly by the earth's surface. Much of the weather we feel and see is a result of changes in the PBL. It is the layer that dissipates the energy from the upper atmosphere. Geostrophic winds above the PBL are often greater then 50 kts (58 mph). If those winds were prevalent at the earth's surface, it would be like having a tropical storm everywhere. Through friction and turbulence in the PBL, the energy in the free atmosphere is dissipated in the throughout the PBL.

Property / Planetary Boundary Layer / Free Atmosphere
Friction / Significant drag against earth's surface. High energy dissipation (due to friction) / Insignificant drag. Little energy dissipation (due to lack of friction).
Turbulence / Continuous turbulence throughout layer. / Turbulence only in convective clouds and near jet stream.
Thickness / Between 100 and 3000 m, diurnal variation over land / Between 8 and 18 km, little variation.
Between PBL and tropopause.
Mixing / Rapid turbulent mixing in vertical and horizontal / Rapid horizontal mixing, a little molecular diffusion.

Structure of the Planetary Boundary Layer

The PBL can be subdivided into four separate component layers: the surface layer, the mixed layer, the stable layer, and the residual layer. We will discuss each of these in turn.

Surface Layer

The surface layer is the sub-layer closest to the earth where turbulent stresses are relatively constant (varying by less than 10 percent of their magnitudes). Therefore, the height of the surface layer is 10 percent of the height of the PBL. Between the surface layer and the earth's surface is a very thin layer called the microlayer (few cm thick). In this layer, molecular viscosity dominates over turbulent motion. Above the surface layer is either the mixed layer or the stable layer, depending on the temperature structure of the PBL.

Mixed Layer

During the daytime, surface heating leads to convective motion in the PBL. Heat transfer from the surface forms rising warm air. Radiative cooling from clouds forms sinking cooler air. Convective motion also leads to significant turbulence which mixes the air within this layer. Because of the convective motion and significant mixing of air, this sub-layer is called the convective layer or mixed layer. Above the mixed layer is a stable layer which prevents the continued upward motion of thermals. This stable layer also restricts turbulence, preventing frictional influences from reaching above the PBL. This stable layer is called the entrainment zone, because it is here where air from above the PBL entrains into the mixed layer. During the day, the mixed layer reaches heights over 1 km and make up the entire layer of the PBL above the surface layer. However, the mixed layer vanishes with the sun as the thermally driven convection ceases.

Stable Layer

After sunset, convective motion dramatically decreases. However, the earth's surface still affects the air, and a stable boundary layer forms (also called the nocturnal boundary layer) . This boundary layer is charaterized by light winds and weaker turbulence than in the mixed layer. The height of the PBL, therefore, decreases significantly during the night. Though the height of the nocturnal layer varies, it is usually less than half that of the mixed layer. Unlike the mixed layer, the stable boundary layer does not have a well-defined top. Instead, it slowly merges with the residual layer.

Residual Layer

As turbulence and the mixed layer decay with sunset, the air maintains many of the state variables that the well-mixed air had. This layer is called the residual layer (because its properties are residuals of the mixed layer) and forms above the stable boundary layer. While the nocturnal boundary layer has a very stable profile, the residual layer tends to have more of a neutral profile. The residual layer does not have contact with the earth's surface, and so is not influenced by turbulent stresses like the stable boundary layer below it. The residual layer is bounded above by a capping inversion, which approximates the height of the daytime height of the mixed layer. This inversion simply prevents entrainment from aloft.

Because the residual layer is not influenced directly by the earth's surface (i.e. no turbulent stresses) it is not considered a boundary layer. However, we include it in our discussion for descriptive purposes. Only the mixed layer and stable layer are true boundary layers.

Mixing Height

Because turbulent fluxes vary based on surface heating and other factors, the height of the PBL also varies. At night, the height of the PBL decreases dramatically as the stable layer forms. The height of the PBL is called the mixing height, because it is the height up to which the air is well-mixed. The mixing height is very important to air quality experts when determining air pollution dispersion.