Dr. John D. Wilson

Measuring the Turbulent Schmidt Number (2010, 2011)

• The turbulent Schmidt number (Sc) is the ratio of the eddy viscosity to the eddy diffusivity for a given gas. This ratio features as an input in most environmental models that encompass the atmospheric boundary layer and parameterize turbulent mixing using the eddy-diffusion paradigm. It is typically assumed the Schmidt number is unity (Sc=1). The objective here was to determine the value of the Schmidt number in the neutral limit.

• This supervised fairweather experiment ran during daytime conditions with a long fetch of uniform wheat canopy upwind of the instruments

• A Campbell Scientific CSAT3 sonic anemometer and LICOR LI-7500A gas analyzer measured the vertical fluxes of momentum, sensible heat, latent heat and carbon dioxide at a height of 2.55 m above ground (1.5 metres above a wheat canopy)

• Simultaneously the differences in mean humidity and carbon dioxide concentration were measured between two levels above the crop, using a LICOR LI-7000 with reversing air intakes.

• Cup anemometers at four heights (one coinciding with the sonic anemometer) determined the mean wind profile

• These data permit to deduce the ratio of the eddy viscosity to the eddy diffusivity for each scalar, i.e. the Schmidt numbers for water vapour and for carbon dioxide. Note that the daytime fluxes of these species are opposite in sign, and the sources/sinks not exactly coincident. Furthermore interpretation of the flux-gradient relationship in terms of an eddy diffusivity demands that the measurements influence of the near field of the nearest sources be negligible. Interpretation of the measurements -- ongoing -- is subtle.

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Last Modified: 7 Nov., 2011