Variations in gust factor with wind direction and height based on the measurements from a coastal tower during three landfalling typhoons

Abstract

Using high-frequency onshore wind data from four different heights of a coastal tower, the variations in gust factor with turbulence intensity, height and wind speed were studied under typhoon conditions. The gust factor increases with increasing turbulence intensity and, most often, can be described by a linear relationship with the turbulence intensity. The gust factor decreases with height and is relatively small compared with those presented in the national codes and other studies. A value of 2.5 is acceptable for the peak factor, which is close to the recommended value of the national code in China. The gust factor increases with increasing wind speed and is also affected by the wind direction. The gust factor has a value to that of previously published results when the wind flows roughly perpendicular to the shoreline, and has a smaller value when the wind flows roughly parallel to the shoreline. The phenomenon is caused by the confinement of shoreline on the sea wave development. Sea waves tend to propagate normal to the shoreline because of the refraction effect. As a result, a shorter roughness length exists in the parallel direction to the shoreline. It can be further explained by the weakness in the momentum flux exchange between the air and sea based on the wave form drag theory when the wind flows parallel to the shoreline.

Keywords

• Gust factor
• Landfalling typhoon
• Peak factor
• Roughness length
• Wind direction