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Geosciences LibreTexts

18.7: Review

  • Page ID
    9650
  • We live in a part of the atmosphere known as the boundary layer (ABL) — the bottom 200 m to 4 km of the troposphere. Tropospheric static stability and turbulence near the Earth’s surface combine to create this ABL, and cap it with a temperature inversion.

    Within the ABL are significant daily variations of temperature, winds, static stability, and turbulence over land under mostly clear skies. These variations are driven by the diurnal cycle of solar heating of the ground during daytime and infrared cooling at night.

    Above the boundary layer is the free atmosphere, which is not turbulently coupled with the ground (except during stormy weather such as near low pressure centers, fronts, and thunderstorms). Thus, the free atmosphere does not normally experience a strong diurnal cycle.

    In the daytime ABL under fair weather conditions (i.e., in anticyclonic or high-pressure regions), vigorous turbulence mixes potential temperature, humidity, wind speed, and pollutants such that they become nearly uniform with height. This turbulence creates a well-mixed layer that grows due to entrainment of free-atmosphere air from above.

    At night in fair weather, there is a shallow stable boundary layer near the ground, with a nearly neutral residual layer above. Turbulence is weak and sporadic. Winds often become calm near the surface, but can be very fast a few hundred meters above ground.

    The bottom 5 to 10% of the ABL is called the surface layer. Surface drag causes the wind to be zero near the ground, and to increase with height. The shape of this wind profile is somewhat logarithmic, but depends on the roughness of the surface, and on convection.

    Turbulence is a quasi-random flow phenomenon that can be described by statistics. Covariance of vertical velocity with another variable represents the vertical kinematic flux of that variable. Heat fluxes, moisture fluxes, and momentum fluxes (stress) can be expressed as such an eddy-correlation statistic.

    Velocity variances represent components of turbulence kinetic energy (TKE) per unit mass, a measure of the intensity of turbulence. TKE is produced by wind shear and buoyancy, is advected from place to place by the mean and turbulent winds, and is dissipated into heat by molecular viscosity.

    The relative magnitudes of the shear and buoyant production terms determine whether convection is free or forced. The sum of those terms is proportional to the intensity of turbulence. The ratio gives the flux Richardson number for determining whether turbulence can persist.

    Turbulence is so complex that it cannot be solved exactly for each swirl and eddy. Instead, parameterizations are devised to allow approximate solutions for the net statistical effect of all turbulent eddies. Parameterizations, while not perfect, are acceptable if they satisfy certain rules.

    One type of local parameterization, called K-theory, neglects the large eddies, but gives good answers for special regions such as the surface layer in the bottom 10% of the atmospheric boundary layer. It is popular because of its simplicity. Another type of parameterization is called transilient turbulence theory (T3), which is a nonlocal closure that includes all eddy sizes. It is more accurate, more complicated, and works well for free convection.