Boundary layer climates
Meteorological phenomena encompass a wide range of space and timescales, from gusts of wind that swirl up leaves and litter to the global-scale wind systems that shape the planetary climate. Their time and length scales, and their kinetic energy, are illustrated in Figure 12.1 in comparison with those for a range of human activities. Small-scale turbulence, with wind eddies of a few meters dimension and lasting only for a few seconds, represents the domain of micrometeorology, or boundary layer climates. Small-scale climates occur within the planetary boundary layer (see Chapter 5) and have vertical scales in the order of 103m, horizontal scales of some 104m, and timescales of about 105 seconds (i.e., one day). The boundary layer is typically 1km thick, but varies between 20m and several kilometers in different locations and at different times in the same location. Within this layer mechanical and
convective diffusion processes transport mass, momentum and energy, as well as exchanging aerosols and chemicals between the lower atmosphere and the earth’s surface. The boundary layer is especially prone to nocturnal cooling and diurnal heating, and within it the wind velocity decreases through friction from the free air velocity aloft to lower values near the surface, and ultimately to the zero-velocity roughness length height (see Chapter 5).