Comparisons of some parametrizations of wind direction variability with observations

An understanding of wind direction variability is important in predicting the dispersion of pollutants in the atmosphere. Parametrizations of wind direction variability over different time scales are compared with turbulence observations made at the Meteorological Research Unit (MRU) at Cardington. Observations of the variability averaged over one hour in stable conditions are compared with the parametrizations proposed by Hanna and by Joffre and Laurila for light winds and show broad agreement with these parametrizations. The formula proposed by Hanna, suitably matched to a similarity approach at strong winds, can be recommended for general use. The direction variability over longer time periods (up to 12 h) has also been investigated. Results show only a weak dependence on averaging time with the wind direction variability roughly proportional to averaging time to the power .