Abstract:
To investigate the effects of atmospheric stability conditions on the near-surface characteristics of wind fields during the passage of a typhoon, field experiments of typhoon Morakot (2009) were conducted at three wind towers equipped with ultrasonic anemometers. The occurrence frequencies of the neutral, stable, and unstable atmosphere conditions are first analyzed. It is found that the non-neutral atmosphere has a occurrence frequency about 0.44 and the atmosphere tends to become more stable further away from the ground. Then, wind-field characteristics including the mean (mean wind velocity and direction) and statistics (turbulence intensity, gust factor, integral length scale, and power spectra) under different atmospheric stability conditions are explored. The atmospheric stability condition affects the mean wind velocity, turbulence intensity, and gust factor considerably. However, its effect on the mean wind direction and integral length scale is less significant. The most probable mean wind velocity falls in the ranges of 10-12 m/s and 14-16 m/s under the non-neutral atmosphere condition, which is different from that under the neutral atmosphere condition (12-14 m/s). The most representative turbulence intensity associated with the non-neutral condition is about 8%-10%, while it is distributed in a wider range of 8%-16% regarding the neutral condition. The applicability of the von-Karman spectrum into the typhoon-induced power spectra under different atmospheric stability conditions is verified. Results show that the von-Karman spectrum fits the power spectrum associated with the neutral atmosphere very well. However, it overestimates the turbulence energy in the low-frequency range and underestimates the turbulence energy in the higher-frequency part under non-neutral conditions. In summary, the influence of the non-neutral atmospheric stratification needs to be considered when conducting the typhoon resistance design of civil structures. In addition, the variation of atmospheric stability conditions along the vertical direction should also be taken into account, since wind loads exerted on the structure are closely related to wind velocity, turbulence intensity, and power spectrum.