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, theta(*), and from these the sensible heat flux, H, were calculated from the indirectly determined dissipation rate, epsilon, and the structure parameter of temperature, C-T(2), 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.