Climatology of Wind Direction Fluctuations at Risø

Standard deviations of wind direction fluctuations at 76 m at Risø for the first half year of 1975 have been analyzed as functions of wind speed and temperature lapse rate, either measured near the surface or near the level of the azimuth variations. Between 31 and 37% of the variance of the standard deviations (σA) is accounted for by the predictors. For strong winds, σA approaches a constant, about 3.5°. This is consistent with the value expected for overwater trajectories. For lower speeds, σA generally increases with decreasing hydrostatic stability. Largest values are found with weakest winds. In unstable air, σA always decreases with increasing wind speed. In stable air, there is a minimum σA for a particular wind speed. These properties are compared with a new theory of fluctuations of horizontal wind components. An analysis of independent data (July–October, 1975) showed the same pattern but significantly higher values of σA with strong winds. This is presumably due to the fact that in this period a significantly larger fraction of air reached the tower after an overland trajectory.