| Summary: | The term planetary boundary layer (PBL) is often used as a synonym for the Ekman layer (i.e., for the region in which the convergence of the vertical flux of momentum is of the same order as the Coriolis force and the pressure gradient). The definition favored in this chapter is somewhat broader and includes the regions on both sides of the interface in which the vertical fluxes, not only of momentum but also of heat, moisture, and salinity, determine the vertical distribution of these properties. Such a definition may suggest as many different boundary layers as there are transported properties. This may be the case, but the various fluxes are coupled with each other to such an extent that it is usually possible to define a single layer in which interface effects remain significant. In Section 6.1 we shall deal first with the classic Ekman treatment of the steady-state motion field above and below the boundary of two incompressible, rotating laminar fluids. This will be followed by a discussion of transients and of integral horizontal transports. Section 6.2 deals with coherent structures—longitudinal rolls, thermal plumes, convection cells, and so on—that are common phenomena, particularly in the gravitationally unstable PBL. This is followed in Section 6.3 by a discussion of various parameterization schemes and models that have been used either to represent vertical fluxes or vertical profiles of conservative properties. Mixed-layer models, which are the topic of Section 6.4, are distinguished from these parameterization schemes by use of vertical integrals of the conservative properties. The resulting gain in simplicity is offset to some extent by a loss of detail. In Section 6.5 we shall discuss the merits and drawbacks of the different approaches in the two preceding sections. During his arctic expeditions on the ship Fram, late in the nineteenth century, Nansen noticed that the pack ice drifted at an angle of about 20-40° to the right of the surface wind. He interpreted this correctly as being due to the ...
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