Mixed layer

From Glossary of Meteorology
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mixed layer

  1. (Abbreviated ML; sometimes called convective mixed layer, convective boundary layer, or mixing layer in air-pollution meteorology.) A type of atmospheric boundary layer characterized by vigorous turbulence tending to stir and uniformly mix, primarily in the vertical, quantities such as conservative tracer concentrations, potential temperature, and momentum or wind speed.

    Moisture is often not so well mixed, showing a slight decrease with height. The vigorous turbulence can be caused by either strong winds or wind shears that generate mechanical turbulence (called forced convection), or by buoyant turbulence (called free convection) associated with large thermals. The buoyantly generated mixed layers are usually statically unstable, caused by heating at the bottom boundary such as the earth's surface or radiative cooling at the tops of cloud or fog layers within the mixed layer. The terms mixed layer, convective mixed layer, and convective boundary layer commonly imply only the buoyantly stirred layer. During fair weather over land, mixed layers are usually daytime phenomena generated buoyantly, with growth caused by entrainment of free-atmosphere air into the mixed-layer top.
    See mixed-layer depth, entrainment zone, radix layer, uniform layer.

  2. In oceanography, a fully turbulent region of quasi-isopycnal water (i.e., virtually uniform potential density) that, in the case of the surface mixed layer, is bounded above by the air-sea interface and below by the transition layer.

    Mixed layer depth is often defined as the depth at which potential density differs from that of the surface by 0.01 kg m-1.

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