Atmospheric Boundary Layer

THE ATMOSPHERIC BOUNDARY LAYER (ABL) is the bottom layer of the atmosphere. Its defining characteristic is that it interacts with the Earth's surface on a time scale of a few hours or less. Therefore, all constituents emitted at or near the surface rapidly diffuse throughout the ABL. This rapid interaction is a direct result of turbulence, which is an essential feature of the ABL. Over land, its depth can vary from a few miles in the daytime to a few dekameters at night.

The atmosphere near the earths surface is almost always turbulent; that is, the air is continually undergoing seemingly random motions, in addition to whatever wind may exist. The sources of turbulence are wind shear (the change in wind speed and direction with height) and convection (motions driven by air density differences resulting from surface heating or latent heating from water phase changes). Defining characteristics of turbulence are its chaotic fluctuations over a broad range of scales, and its diffusiveness. Therefore, trace constituents released into a turbulent fluid are rapidly diffused and the small-scale patterns of this diffusion cannot be predicted. Because of the randomness and the large range of scales of ABL turbulence, processes in the ABL are often described in terms of statistical averages of fluctuations. This means that most measurements of ABL structure need to be spatially or temporally averaged before they can be quantitatively interpreted.

The top of the ABL is characterized by an increase in temperature (such as a decrease in density) with height, which also caps the level to which turbulence extends. The ABL grows by entraining air from the overlying non-turbulent air by turbulent eddies. That is, the kinetic energy of the ABL turbulence is used to overcome the lower density of the overlying air as it diffuses into the ABL. The lowest few yards to dekameters of the ABL is known as the surface layer (SL). This region, roughly up to about 10 percent of the ABL, is characterized by nearly constant vertical turbulent transport and relatively large vertical gradients; in contrast to the rest of the ABL, where the transport can vary significantly, but the gradients are typically small. In this layer, wind shear caused by the interaction of the average wind with the surface, and with obstacles such as plants, buildings, hills, and ocean swell, is the dominant source of turbulence.

If turbulence generation by convection occurrs in the ABL, it is known as an unstable or convective boundary layer (CBL); if the hydrodynamic stratification of the ABL acts to suppress or dissipate turbulence, it is known as a stable boundary layer (SBL). In the course [Page 87]of a typical diurnal cycle over land, the surface becomes warmer than the overlying air within a few hours after sunrise, and warms the lowest layer of air so that it is warmer and less dense than the air higher up. This relatively warm air rises and thereby converts its potential energy to kinetic energy in the form of turbulent fluctuations that deepen the boundary layer through the morning. By mid-afternoon, as solar heating begins to decrease, the boundary layer reaches a plateau of one to several miles in depth and turbulence decreases.

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