A Note on the Use of Cup Anemometers in Wind Profile Experiments

Abstract

In the study of wind profiles in the atmospheric surface layer it is important to distinguish between the speed of the mean vector wind |V|, which is equivalent to u WHEN THE x-axis is oriented in the downwind direction, and the mean horizontal wind speed |Vh|, which is equivalent to (u2 + ?)½ or simply ?h, and which is the quantity measured by a cup anemometer. The difference between these two ?wind speeds? may be significant, especially when quantities such as the Reynolds stress, the roughness parameter, and the zero-plane displacement are determined from the wind profile. This difference is shown to be related to the standard deviation of wind direction fluctuations, σ?, which tends to be greatest close to the ground and under unstable conditions with light winds. Due to the non-availability of simultaneous profiles of mean wind speed and σ?, a set of estimates of σ? at White Sands is used in conjunction with a ?typical? wind profile measured at a very flat site in Australia to illustrate qualitatively the nature of this effect.