One of the traditional uses of scintillation measurements is to infer path-averaged values of the turbulent surface fluxes of momentum (tau) and sensible (Hs) and latent (HL) heat. Many scintillometer set-ups, however, measure only the path-averaged refractive index structure parameter, Cn2; the wind information necessary for inferring the fluxes comes from point measurements or is entirely absent. In this latter case, Hs and HL can be obtained only under the assumption of free convection; and a path-averaged tau is not available. The Scintec SLS20 surface-layer scintillometer system, on the other hand, measures both Cn2 and the inner scale of turbulence, l0, where l0 is related to the dissipation rate of turbulent kinetic energy, epsilon. As a result, the SLS20 is presumed to provide path-averaged estimates of both tau and Hs.
I have used an SLS20 in two experiments: one over Arctic sea ice during SHEBA (the experiment to study the Surface Heat Budget of the Arctic Ocean), and a second over a mid-latitude land site during spring (the so-called Rapid Forcing Experiment). In each deployment, we also made eddy-covariance measurements of tau, Hs, and HL near the scintillometer path. In this presentation, I will compare the scintillometer and eddy-covariance measurements of tau and Hs for these two diverse sites. For both tau and Hs, the correlation between eddy-covariance and scintillometer-derived values is reasonable, but the scintillometer results tend to be biased low. Moreover, estimates of both tau and Hs degrade as Cn2 decreases. Essentially, the signal-to-noise ratio for the scintillometer-derived fluxes decreases as Cn2 decreases.
An essential question that arises during these comparisons is what similarity functions to use for nondimensionalizing Cn2 and epsilon. From the combined scintillation and eddy-covariance measurements, I will evaluate which of several candidate similarity functions are consistent with the data.