Near-surface Gust Factor Characteristics in Several Disastrous Winds over Zhejiang Province

Studies on near-surface gust characteristics in high winds are necessary for weather services. Using the daily 10 min data from automatic weather stations in Zhejiang Province during 2011-2013, characteristics of near-surface gust factors in several kinds of high winds caused by cold air masses, tropical cyclones and abruptly severe convections, are investigated over the offshore and in-land areas of Zhejiang Province. Spatial distributions of wind velocities and gust factors are especially considered, as well as the relationship among gust factors, geographical elements and mean wind speeds. The fuzzy cluster mean (FCM) and stepwise regression methods are applied as well to do the weather station clusters under different weather patterns and set up gust factor forecast models. Result shows that the gust factor distribution displays similarly both in cold air and tropical cyclone strong winds although spatial speed distributions might be different from each other, and wind directions show no effects on gust factor distributions. Disastrous winds usually happen over the offshore seas and coastal areas, with gust factors less than 1.5 and the isolines paralleling to the coastline and descending eastwards. However, over in-land areas of Zhejiang Province, gust factors are generally greater than 2.0 and even more than 3.0 over the hilly regions with gentle wind speeds, indicating enhancing effects of hilly terrain. The average gust factor is more than 1.8 under severe convective systems, which is greater than operational regulations. The convective gale events could occur at any locations within Zhejiang Province, but stations with occurring probabilities more than 10% mainly lay in the coastal and offshore Zhejiang Province, and the terrain roughness doesn't show much influence. Gust factors perform well related to 10 min mean wind speeds and altitudes in high winds by cold air masses and tropical cyclones. FCM analysis indicates that there are few differences in station distributions between clod air mass and tropical cyclone gale events, stations located in the northern and coastal regions often differ from those in the middle and southern areas in Zhejiang Province, and stations with altitudes more than 400 m are different from those with altitudes lower than 70 m. Stepwise regression is carried out to set up forecasting models between gust factors and mean winds and station altitudes before and after FCM clusters, verifications imply that FCM could help improve forecast ability of the models. The regression model for type Ⅰ tends to overestimate gust factors at stations with relative high altitudes, on the contrary, the model for type Ⅱ tends to underestimate gust factors at stations with relative low altitudes.