ANALYSIS ON BOUNDARY LAYER TURBULENT FEATURES OF LANDFALLING TYPHOON

Typhoons are one of severe weather systems inducing severe catastrophe in the coastal regions in China.Investigation on characteristics of near-surface turbulence of landfalling typhoons is vitally important for better understanding of inner structure and movement of typhoons, characters of disasters so arisen, as well as sustaining period of typhoons. Furthermore, near-surface turbulence activities of landfalling typhoons have enormous impact on buildings, safety of lives and properties in the typhoon affected regions. Based on a number of observations on landfalling typhoons, three representative cases of “Vonfong”, “Dujuan” and “Hagupit” were chosen in the article to analyze near surface turbulence characteristics in the typhoon center, severely affected areas near typhoon center and their peripheral areas of typhoon. It is expected through this study to improve our understanding of near -surface turbulence characteristics of landfalling typhoons. In-situ observations were carried out using CSAT3d type of three dimensional supersonic anemometer manufactured by Campbell Science in USA, with data sampling frequency of 10Hz. The supersonic anemometer is capable of automatically providing data quality codes to eliminate invalid data. Therefore, interference on data accuracy by rainfall can be avoided. In this study, multiple samples were chosen to analyze for each typhoon processes with each sampling length of 30 minutes, and over 98% valid data for each sample. Each sample is processed to satisfy the requirement of stable stochastic process by applying linear interpolation technique to interpolate at data void points, and second order polynomials fitting to remove trend in the time series.Through computational analysis on the observational data of typhoons “Vonfong”, “Dujuan” and “Hagupit”, Preliminary results may be obtained as following:(1) In the center and its adjacent areas of a landfalling typhoon, horizontal wind speeds and directions change dramatically, with significant downdraft in the vertical.(2) In the center or its adjacent areas of a landfalling typhoon, turbulence intensities increase enormously, and may increase over two times in the principal wind direction. (3) In the center area of a landfalling typhoon, the turbulence integral scale can be expanded significantly in the principal wind direction, closer to the center, more significant the scale expansion. In the center of a landfalling typhoon, the turbulence integral scale l may be expanded by one order in horizonta, while no evident change of turbulence integral scale can be manifested in vertical.(4) In the centre or its adjacent areas of a landfalling typhoon, the turbulence spectral in the inertial range does not satisfy the assumption of isotropy. Neither do the slope rates of turbulence spectra in u, v and w directions satisfy the -5/3 law. The deviation amplitude generally exhibits in vertical＞side＞principal wind direction, i.e. the slope rates in the inertial range were 0.6—0.8 in vertical wind direction, 0.9—1.0 in side wind direction and 1.0—1.4 in principal wind direction. However, in the peripheral circulation of typhoon, the assumption of isotropy is basically satisfied, with the slope rates in accordance with the rule of “-5/3” power.(5) In the centre or its adjacent areas of a landfalling typhoon, turbulent energy can increase by 1 to 2 orders in both low and high frequency domains. The amplitude of increase is less in vertical than in horizontal. However, in the periphery of typhoon circulation, no evident change of turbulent energy was observed.