Displaced-beam small aperture scintillometer test. Part I: The WINTEX data-set

The friction velocity (u(*)) and the sensible heat flux density (H) determined with a displaced-beam small aperture scintillometer (DBSAS) and a hot-film eddy correlation system are compared. Random errors in the DBSAS are relatively small, compared to scatter found with two eddy-correlation systems. Assuming that the hot-film system yields the `true' fluxes, the DBSAS appears to overestimate u(*) when u(*) is less than approximate to0.2 m s(-)1 and to underestimate u(*) at high wind speeds. This implies that the DBSAS measurements of the inner scale length of turbulence, l(0), a direct measure for the dissipation rate of kinetic turbulent energy, are biased. Possible causes for these results are discussed in detail. A correction procedure is presented to account for effects of random noise and of so-called inactive turbulence or sensor vibrations. The errors in u(*) cause errors in the DBSAS measurements of the structure parameter of temperature C-T(2). The derived H appears to be less sensitive to errors in l(0) and C-T(2), because errors in these quantities tend to cancel out.