Wind Characteristics Study in Surface Layer of Typhoon Morakot (0908)

The observations from 33 anemometer tower within Typhoon Morakot (0908) affected area are analyzed. As the typhoon moves closer, the wind asymmetry becomes more apparent. Morakot has a counterclockwise shift of wind direction on the left flank, and a clockwise shift of wind direction on the other flank. Near the landing area, the wind reaches maximum and the direction is northerly before landing. When landing, the wind slows down, its direction rapidly changes and air pressure drops rapidly. Southerly gale appears when landing and northerly gale appears to the north of the location where it enters the sea.In the place far away from Morakot, wind direction is steady and the turbulence intensity changes are correspondingly stable. Its value is decided by sub-layer attribute, complex surface conditions lead to strong turbulence intensity.Near the landing area, the wind direction, the wind speed and the turbulence intensity may all changes suddenly. In the mountainous region, turbulence intensity change is most complex. Influenced by Morakot, the turbulence intensity curves do not look like the IEC standard curve which reduces stably along with the wind speed increasing. The turbulence intensity reaches a peak with wind speed of 7—17 m·s^-1when I₁₅achieves Level A, B or above. No matter southerlies or northerlies, each layer turbulence intensity difference tends to reduces with wind speed. Given the same wind speed and height, the turbulence intensity of southerly gale is bigger than that of the northerly gale. Turbulence intensity decreases first then increases with wind speed increasing at Morakot landing area northern flank, finally surpassing IEC the standard Level A curve. But to the south of landing location, turbulence intensity along with the wind speed changes much smaller than the north side and increases along with the wind speed, showing the standard Level A curve tendency. On the north side turbulence intensity is bigger than the south side and at each height the turbulence intensity difference reduces remarkably compared to south side. It indicates that the north side vertical direction the perturbation is stronger.Morakot gust coefficient is between 1.2 and 1.7. The average gust coefficient changes with height and regional terrain, and it also becomes larger when the typhoon center draws near. In general cases, the gust coefficient reduces with height. But over complex terrain especially the knoll mountainous region it reduces with height below 50 meters, and increases with height above 50 meters.