On the use of Taylor's translation hypothesis for diffusion in the mixed layer

Diffusion measurements in a modelled mixed layer support the equivalence of an instantaneous line‐source concentration field without a mean wind and a continuous point source with a mean wind when use is made of Taylor's translation hypothesis. Taylor's hypothesis as applied to mixed‐layer diffusion appears to break down in this type of flow for values of s̀u/U > 0.5, where s̀u is the horizontal turbulence velocity component and U is the uniform mean wind. Variations in expected levels of cross‐wind integrated concentration are presented as a function of height and distance downstream from the source for a sampling duration of 4zi/U, where zi is the mixed‐layer depth.