不同大气稳定度下台风“莫拉克”(2009)的近地风场特性

Near-surface wind-field characteristics of typhoon Morakot(2009)under different atmospheric stability conditions

为了探究台风影响期间不同大气稳定度条件对近地层风场特性的影响,基于2009年台风“莫拉克”移动路径上的三座测风塔测得的高频三维风速时程,分析了台风影响期间中性、稳定、不稳定三种大气层结状况的分布频率,以及不同稳定度条件下近地层平均风场特性(平均风速和风向)和脉动风场特性(湍流强度、阵风因子、湍流积分尺度和脉动风速谱)。研究结果表明,台风外围雨带区影响期间非中性层结发生概率约44%;在非中性层结影响下,平均风速出现频率最高的区间(10~12 m/s,14~16 m/s)不同于中性层结(12~14 m/s),且湍流强度最高频数分布较为集中(8%~10%);验证von-Karman谱描述不同大气稳定度下台风脉动风速谱的适用性,发现该经验谱对中性层结下的风速谱刻画较好,对非中性层结下的风速谱存在低频段高估、高频段低估谱能量的现象。进行工程结构抗风设计时,需注意非中性大气层结对风速、湍流强度、脉动风速谱等与结构风荷载确定密切相关的参数的影响。

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.