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.