2009年“莫拉克”台风登陆过程阵风特征分析

Analysis of the characteristics of gusts during the landing of Typhoon Morakot (2009)

  • 摘要: 利用上海台风研究所移动观测车获取的“莫拉克”台风登陆过程中超声风、温等观测资料对地面阵风特性进行了诊断分析。结果表明,在风速时间序列中叠加有周期为3—7 min的阵风扰动,显现出明显的相干结构,即沿顺风方向阵风风速峰期有下沉运动,谷期有上升运动;阵风扰动的各向异性特征明显,沿顺风方向的阵风扰动能量最大,其次是沿侧向和垂直方向的扰动能量;沿顺风方向的阵风垂直动量通量向下传播,而沿侧风方向阵风扰动动量垂直通量总体贡献接近于0。阵风扰动沿顺风方向的积分空间尺度和时间尺度最大,沿侧风方向和垂直方向其次,均明显大于湍流的积分空间和时间尺度。此外,阵风扰动的其他特征还包括:感热垂直通量极小;当平均风速较大时阵风风向变化幅度较小,而风速较小时阵风风向变化幅度则较大;动力学分析表明,阵风扰动主要表现出重力内波的一些特性。

     

    Abstract: The dynamic nature of gust and turbulence is similar to each other, both containing seemingly random and complex motion with multi-scale interaction. However, there is a fundamental difference between these two. The gust scale both on time and space is far greater than that of turbulence, so generally the gust looks like a kind of weather phenomena. So far there have been some researches on turbulence for typhoon, but less on gust for typhoon which has yet to be explored. From 7 to 10 August 2009 an observational experiment with the mobile vehicle for Typhoon Morakot near the coast of Ningde City in Fujian Province was carried out by Shanghai Typhoon Institute. In this paper, the diagnostic analysis for gusts at the 10 m height is made based on the ultra-sound anemometer thermometer monitoring data with 0.1 s sampling frequency. The results show that the gust along the direction of mean wind flow with 3-7 min period is superimposed on the basic wind current, which has obvious coherent structures along the direction of mean air flow, i.e., there is descending (ascending) motion in the period of wind speed peak (wind speed valley). Different from the isotropic characteristics of turbulent kinetic energy, the kinetic perturbation energy of gust is anisotropy with larger value on the direction of mean air flow and smaller one on the lateral and vertical. And the change of wind direction of gust during stronger wind is small, while it shows big variations during weaker wind. The vertical momentum flux by gust on the direction of mean air flow near the ground spreads downward, while its overall contribution by gust on the lateral direction of mean air flow is close to zero. The vertical sensitive heat flux has also a very small value. The integral scales both on space and time are larger in the principal wind direction compared to smaller one along the lateral direction and vertical wind direction, which are much larger than the scales of turbulent. The gust disturbance shows some characteristics of iner-gravity wave as shown by dynamic analysis. The convergence and divergence movement is one of the main features of gust disturbances. The period of iner gravity wave estimated by the GPS sounding data is consistent with the observed gust period.

     

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