Surface-layer scintillation measurements of daytime sensible heat and momentum fluxes

A. E. Green*, K. J. McAneney, M. S. Astill

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

37 Citations (Scopus)

Abstract

Line-averaged measurements of the structure parameter of refractive index (Cn 2) 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 parameter CT 2 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.

Original languageEnglish
Pages (from-to)357-373
Number of pages17
JournalBoundary-Layer Meteorology
Volume68
Issue number4
DOIs
Publication statusPublished - Mar 1994
Externally publishedYes

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