SURFACE-LAYER SCINTILLATION MEASUREMENTS OF DAYTIME SENSIBLE HEAT ANDMOMENTUM FLUXES

Line-averaged measurements of the structure parameter of refractive in dex (C(n)2) were made using a semiconductor laser diode scintillometer above two markedly different surfaces during hours of positive net ra diation. The underlying vegetation comprised in the first instance a h orizontally homogeneous, pasture sward well-supplied with water, and i n the second experiment, a sparse thyme canopy in a semi-arid environm ent. Atmospheric stability ranged between near neutral and strongly un stable (-2 less-than-or-equal-to xi less-than-or-equal-to 0). The temp erature structure parameter C(T)2 computed from the optical measuremen ts over four decades from 0.001 to 2 K2 m-2/3 agreed to within 5% of t hose determined from temperature spectra in the inertial sub-range of frequencies. Spectra were obtained from a single fine thermocouple sen sor positioned near the midway position of the 100 m optical path and at the beam propagation height (1.5 m). With the inclusion of cup anem ometer measurements, rule-of-thumb assumptions about surface roughness , and Monin-Obukhov similarity theory, path-averaged optical scintilla tions allow calculation of surface fluxes of sensible heat and momentu m via a simple iterative procedure. Excellent agreement was obtained b etween these fluxes and those measured directly by eddy correlation. F or sensible heat, agreement was on average close to perfect over a mea sured 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 in crease the range of validation obtained in previous field experiments. The potential of this scintillation technique and its theoretical fou ndation are briefly discussed.