The Dynamics of the Atmospheric Planetary Boundary Layer

Abstract

The boundary-layer concept is well known in fluid dynamics. It is always related to the presence of a rigid boundary in the flow which can be either laminar or turbulent. The most characteristic feature of the boundary layer is the existence of a mean-flow shear in a direction perpendicular to the boundary. In this sense, the surface layer (h ~ 30–50 m) of the atmosphere, studied in Chapter 9, is a boundary layer. However, the dynamic influence of the Earth’s surface can be found much more higher — up to h _g = 1–2 km in the middle latitudes. To this height the wind velocity increases (Figure 10.1) and approaches the geostrophic wind u _g , i.e., z → h _g u → u _g . In the free atmosphere (z > h _g ), the wind is nearly geostrophic. This layer with a thickness h _g is called the planetary boundary layer. It is clear that the transition between the PBL and the free atmosphere is smooth and that the surface layer, as defined in Chapter 9, is a sublayer of the PBL — its lowest part. On the Earth’s surface u = v = w = 0.