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(Also called coefficient of diffusion.) The ratio of the flux of a conservative property through a specified surface by turbulence to the gradient of the mean property normal to the surface.

In the special case of isotropic turbulence and no mean motion the Fickian diffusion equation (or Fick's equation) takes the form
where Ks represents the diffusivity, ∇2 is the Laplacian operator, and
is the mean value of the property
. This equation describes decreasing
where the Laplacian is negative and increasing
where the Laplacian is positive. The general case is more complex. In the statically stable atmosphere or ocean, the horizontal scale of turbulence is much greater than the vertical scale and turbulent diffusion in the horizontal may greatly exceed diffusion in the vertical. On the other hand, in the case of buoyancy, vertical diffusion may be greater than horizontal diffusion.
See also mixing, eddy flux, turbulence length scales.

Fleagle, R. G., and J. A. Businger 1980. An Introduction to Atmospheric Physics. 2d ed., Academic Press, . 64– 66.

Hinze, J. O. 1975. Turbulence. 2d ed., McGraw–Hill, . 48–55.

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