Abstract
Line-averaged measurements of the structure parameter of refractive index (C 2 n ) were made using a semiconductor laser diode scintillometer above two markedly different surfaces during hours of positive net radiation. The underlying vegetation comprised in the first instance a horizontally homogeneous, pasture sward well-supplied with water, and in the second experiment, a sparse thyme canopy in a semi-arid environment. Atmospheric stability ranged between near neutral and strongly unstable (−2≤ζ≤0). The temperature structure parameterC 2 T computed from the optical measurements over four decades from 0.001 to 2 K2 m−2/3 agreed to within 5% of those determined from temperature spectra in the inertial sub-range of frequencies. Spectra were obtained from a single fine thermocouple sensor positioned near the midway position of the 100m optical path and at the beam propagation height (1.5m).
With the inclusion of cup anemometer measurements, rule-of-thumb assumptions about surface roughness, and Monin-Obukhov similarity theory, path-averaged optical scintillations allow calculation of surface fluxes of sensible heat and momentum via a simple iterative procedure. Excellent agreement was obtained between these fluxes and those measured directly by eddy correlation. For sensible heat, agreement was on average close to perfect over a measured range of 0 to 500 W m−2 with a residual standard deviation of 30 W m−2. Friction velocities agreed within 2% over the range 0–0.9 m s−1 (residual standard deviation of 0.06 m s−1). The results markedly increase the range of validation obtained in previous field experiments. The potential of this scintillation technique and its theoretical foundation are briefly discussed.
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Green, A.E., McAneney, K.J. & Astill, M.S. Surface-layer scintillation measurements of daytime sensible heat and momentum fluxes. Boundary-Layer Meteorol 68, 357–373 (1994). https://doi.org/10.1007/BF00706796
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DOI: https://doi.org/10.1007/BF00706796