Surface layer

The surface layer is the layer of a turbulent fluid most affected by interaction with a solid surface or the surface separating a gas and a liquid where the characteristics of the turbulence depend on distance from the interface. Surface layers are characterized by large normal gradients of tangential velocity and large concentration gradients of any substances (temperature, moisture, sediments et cetera) transported to or from the interface.

The term boundary layer is used in meteorology and in physical oceanography. The atmospheric surface layer is the lowest part of the atmospheric boundary layer (typically the bottom 10% where the log wind profile is valid). The ocean has two surface layers: the benthic, found immediately above the sea floor and the marine surface layer, at the air-sea interface.

A simple model of the surface layer can be derived by first examining the turbulent momentum flux through a surface. Using Reynolds Decomposition to express the horizontal flow in the ${\displaystyle x}$ direction as the sum of a slowly varying component,${\displaystyle {\overline {u}}}$, and a turbulent component,${\displaystyle \ u'}$,:

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