A Critical Evaluation of the Aerodynamical Error of a Turbulence Instrument

Abstract

An instrument, intended for tower-borne measurements of atmospheric turbulence, constructed at the Department of Meteorology, Uppsala University, has been carefully tested for the errors induced by the local flow around the instrument itself, including the supporting boom. The instrument is a wind-vane-based three-axial hot wire system. It is equipped with electronic tilt sensors, which enable very accurate alignment of the instrument. The local flow around the instrument was carefully mapped in a wind tunnel, with the aid of a highly sensitive flow direction probe. The wind tunnel test results were used, together with Wyngaard's (1981) theory for probe-induced flow distortion, to evaluate the corrections to be applied to the various turbulence statistics. For the particular instrument studied the corrections for the turbulent fluxes, τ H and E are about 10% and for w2 about 20%. It is shown that this error cannot be removed simply by tilt correction. Earlier versions of the instrument had much bigger errors, and it is concluded that this may be the case for other instruments currently in use for atmospheric turbulence measurements.