THE RELATIONSHIP BETWEEN THE GUST RATIO, TERRAIN ROUGHNESS, GUST DURATION AND THE HOURLY MEAN WIND-SPEED

A large volume of 1 min mean wind data is available at the United Kingdom Meteorological Office. Until recently, very little scientific use has been made of the data. This study provides up-to-date information, using this data, upon the relationships between the gust ratio, the terrain roughness and the hourly mean wind speed by gust duration. The ratio of the maximum wind averaged over a period of a few seconds, 1 and 10 min and the hourly mean is obtained after careful quality control of many hours of data for each of eight wind direction sectors at 14 Meteorological Office anemograph stations. The ratio of the peak recorded gust to the IO min mean wind is also derived using two alternative derivations of the 10 min mean wind. The sector median gust ratio is successfully correlated with the estimate of terrain roughness derived from the best estimate of the sector aerodynamic roughness length. The form of the gust ratio-terrain roughness dependence is very similar to previous relationships for gust averaging periods ranging from a few seconds to 10 min but the median gust ratios obtained for averaging periods of 1 and 10 min are somewhat lower than those given in previous guidance documents. The ratio of the 3 s gust and the 10 min mean wind is obtained. A comparison is made between the observed median values of the above gust ratio and various predictive relationships in the literature. The dependence of the median gust ratio upon hourly mean wind speed is obtained. The median gust ratios for gust averaging periods of 1 and 10 min averaged over the set of stations do show a statistically significant decline with increasing wind speed. The 3 s gust ratio shows a much wider range of wind speed dependence from station to station and it is not possible to confidently define a general pattern of change with wind speed.