Characteristics Of Turbulent Structure And Transfer Over The South China Coastal Region

Based on the turbulence data observed on platform in the South China Sea at Maoming from 11 August to 20 October,2009, the statistical characters of turbulent are analyzed, and air-sea fluxes are calculated by the method of eddy correlation. The study focuses on the statistical characters of turbulent structure and transfer of different weather conditions. With the wavelet transform the coherent structures can be detected and isolated from the turbulence signals. Moreover the spectral analysis of the wavelet transform coefficients gives the evolution process of the coherent structure. Finally, combining with the weather process and the statistical characteristics of turbulence during observation period, the eddy energy distribution and transformation of the coherent structure are investigated as well. Main concludes are as following:(1) Averaged turbulent intensity (TI) are Iu=0.120,Iv=0.115,Iw=0.059, respectively. Longitudinal and lateral TI is mostly less than 0.5. Vertical TI is normally less than 0.3, which is smaller than that of the horizontal direction. There is strong relationship between TI and horizontal wind velocity.(2) The variance and covariance of wind and temperature obey the Monin-Obukhov similarity theory. Whereas it is basically inapplicable of the water vapor, and predicted that the complicacy of the vapor turbulence as well as the sensible heat flux.(3)All of the turbulence spectra and co-spectrum comprise the integrated structure of classical spectrum (co-spectrum):the energy-containing eddies of low frequencies and the inertial subrange of high frequencies. The normalized spectrum of wind speed and temperature comply with -2/3 exponential rule and converge to a single universal curve in the inertial subrange. Whereas the normalized co-spectrum of wind speed and temperature aren't full comply with the -4/3 law of slope, and dispersed obviously in the inertial subrange. Comparing with other underlying surface turbulence over the sea meets the isotropic condition approximately well. Besides, the upwarping phenomenon in low frequency of spectra can also be found over the sea which was considered cause by the topographic effect.(4) Under the influence of the thermal characteristics of underlying surface, the diurnal variation of the sensible heat flux and the latent heat flux is not obvious. Latent heat flux is mainly from surface to atmosphere, only a few is transported to the sea from the atmosphere and the value is small. Momentum flux will be stronger with the increase of the square of the horizontal velocity, and thus calculated the drag coefficient Cd≈1.4×10-3. It is smaller than the result over the Nansha sea region which based on wind conditions.(5)The results of wavelet analysis based on the wind fluctuation data show that the principal time scale of the coherent structures (CS) is tens of seconds over South China Sea coastal region, and the CS display the characteristics of multi-scale. The scale of CS is in agreement with the results under other underlying surface conditions in the literature, and three orders of magnitude lager than that of laboratory turbulence. The primary structure of original signal can be well reflected by the quasi-periodic structures in numeric value and waveform, which is represent the eddy of intermediate scales. The magnitudes of the quasi-periodic structures reflect the strong transport parts of the CS. While non-coherent signals associated with the small-scale, irregular, strong intermittent parts of the turbulence.(6)The results of the spectral analysis of the wavelet transform coefficients ('spavelet' analysis) show that period of the CS close to their durations in a certain frequency range, while in the small scale CS occur intermittently whose periods are large than durations.(7)The generation and development of CS implies a close relationship with weather conditions, especially with atmospheric stability. Rainy weather corresponding to unstable stratification, convective exchange impels the development of the eddy in different scales, turbulence develops strongly, and the turbulent eddy of large intensity as well as wide scales. In the condition of sunny night with stable stratification, atmospheric turbulence development is suppressed, while the turbulent eddies are intermittent and low-intensitv.