Displaced-beam small aperture scintillometer test. Part II: CASES-99 stable boundary layer experiment

The performance of the Scintec displaced-beam small aperture scintillometer (DBSAS) in the stable boundary layer (SBL) is investigated using data gathered during the CASES-99 experiment in Kansas, U.S.A. The DBSAS is superior to the eddy-covariance method in determining vertical fluxes of sensible heat and momentum close to the ground and/or over short (< 1 min) averaging intervals. Both aspects are of importance in the shallow and non-stationary SBL. The friction velocity, u*, the temperature scale, θ*, and from these the sensible heat flux, H, were calculated from the indirectly determined dissipation rate, ε, and the structure parameter of temperature, CT2, by the DBSAS, which was operated over a path length of 112 m. All these variables are compared with eddy-covariance data for 10-minute time averages. Previously reported systematic errors in the DBSAS, overestimation of u* for low u* values and underestimation of u* for high u* values, have in part been dealt with by adjusting the beam displacement distance from 2.7 mm to 2.6 mm in the calculations. The latter adjustment is presented as a working hypothesis, not a general solution.