Time Scales in the Unstable Atmospheric Surface Layer |
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| Authors: | Meredith Metzger Heather Holmes |
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| Institution: | (1) Department of Mechanical Engineering, University of Utah, Salt Lake City, UT 84112, USA |
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| Abstract: | Calculation of eddy covariances in the atmospheric surface layer (ASL) requires separating the instantaneous signal into mean
and fluctuating components. Since the ASL is not statistically stationary, an inherent ambiguity exists in defining the mean
quantities. The present study compares four methods of calculating physically relevant time scales in the unstable ASL that
may be used to remove the unsteady mean components of instantaneous time signals, in order to yield local turbulent fluxes
that appear to be statistically stationary. The four mean-removal time scales are: (t
c
) based on the location of the maximum in the ogive of the heat flux cospectra, ( ) the location of the zero crossing in the multiresolution decomposition of the heat flux, (t
*) the ratio of the mixed-layer depth over the convective velocity, and ( ) the convergence time of the vertical velocity and temperature variances. The four time scales are evaluated using high quality,
three-dimensional sonic anemometry data acquired at the Surface Layer Turbulence and Environmental Science Test (SLTEST) facility
located on the salt flats of Utah’s western desert. Results indicate that  and  , with t
c
achieving values about 2–3 times greater than t
*. The sensitivity of the eddy covariances to the mean-removal time scale (given a fixed 4-h averaging period during midday)
is also demonstrated. |
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| Keywords: | Atmospheric surface layer Atmospheric turbulence Averaging time Mean removal Time scales |
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