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 earth's 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 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.

At night, infrared cooling of the surface suppresses turbulence, while wind shear near the surface can still generate it. As a result, the nocturnal SBL is much shallower—typically a few dekameters or less—and the temperature decreases less rapidly with height than the daytime CBL. The overlying air that was previously within the CBL, and therefore well mixed, is no longer turbulent. This residual layer may again become turbulent the following day when solar heating of the surface resumes.

This diurnal cycle is strongly dependent on the surface characteristics. A bare, dry surface, for example, will become warmer than a moist or vegetated surface. Therefore, the turbulence is likely to be stronger and the boundary layer deeper over a bare, dry surface. The moisture given off by vegetation will likely cause clouds to form at lower heights than over a bare, dry surface. If these differences occur over a large enough area, there may be significant differences in the mean temperature, humidity, and cloudiness. Horizontal variations in the surface can induce corresponding horizontal variability in the ABL. For example, on clear

In the surface layer, which makes up about 10 percent of the ABL, turbluence is increased by surface features such as hills.

summer days, cumulus clouds are often observed to form preferentially over land, but a nearby lake may remain clear. Similarly, in mountainous terrain, clouds often preferentially form over ridges and elevated plateaus, rather than adjacent valleys.

Over the ocean, the diurnal cycle is quite different, and often scarcely noticeable. Because the ocean has a much larger effective heat capacity and heat conductivity than land, its surface temperature is hardly perturbed by the daily solar cycle. As a consequence, the ABL depth, typically about a kilometer, is much less variable. Furthermore, in daytime the boundary layer air is heated directly by the sun, which typically more than compensates for long-wave (infrared) radiational cooling. In this case, the boundary layer may be more stable (with less convection) in the daytime than at night.

When relative humidity reaches 100 percent within the ABL, clouds form, which can have dramatic effects on its subsequent evolution through generation of turbulence by release of the latent heat of condensation, shading of the Earth's surface, and precipitation. The resulting clouds may be confined to the ABL, or they may penetrate into the overlying atmosphere, depending on the change of temperature (density) with height across the ABL top; a large increase in temperature across the top suppresses cloud penetration and results in stratiform or layered clouds, while penetration leads to cumuliform or vertically-developed puffy clouds. Cumulus convection is an important mechanism for transporting ABL air, and, thus, trace constituents released at the surface, into the overlying atmosphere, thereby ventilating the ABL.

SEE ALSO: Atmospheric Absorption of Solar Radiation; Clouds, Cumulus; Clouds, Stratus; Convection; Ekman Layer; Radiation, Infrared.

BIBLIOGRAPHY. J.R. Garratt, The Atmospheric Boundary Layer (Cambridge University Press, 1992); J.C. Kaimal and J.J. Finnigan, Atmospheric Boundary Layer Flows: Their Structure and Measurement (Oxford University Press, 1994); Zbigniew Sorbjan, Structure of the Atmospheric Boundary Layer (Prentice Hall, 1989); R.B. Stull, An Introduction to Boundary Layer Meteorology (Kluwer Academic Publishers, 1988).

Donald H. Lenschow National Center for Atmospheric Research

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