Path-averaged surface fluxes determined from infrared and microwave scintillometers

Methods are needed to measure the surface fluxes of sensible (H) and latent heat (E) at large scales. A promising method is scintillometry. Over pasture, a near-infrared scintillometer was most sensitive to temperature fluctuations whilst a microwave scintillometer was unduly affected by both humidity fluctuations and correlated temperature–humidity fluctuations. Slower changes in path-averaged humidity caused additional signal variance and an overestimation of H and E. Log-amplitude spectra of the microwave scintillometer signal showed inertial-convective subrange behavior. In combination, path-averaged E and H could be determined over 3 km. Independent corroborative measurements of H and E were made at the path midpoint using the eddy covariance technique. For sensible heat, agreement was within 4% over a measured range 0–300 W m−2, with a residual standard deviation of 45 W m−2. Latent heat agreed at best to within 12% over the range 0–450 W m−2 (residual standard deviation of 94 W m−2) and an offset of 30 W m−2.