用小波系数谱方法分析湍流湿度脉动的相干结构

Analysing Coherent Structures of Humidity Time Series by the Spectral Analysis of the Wavelet Transform Coefficients

小波系数谱分析方法是结合小波分析和高分辨率谱分析的一种统计方法，可以用来同时识别时间序列中相干结构的生命尺度和出现周期，可以很好地描述相干结构的演变过程。基于此方法，作者分析了2004年11月在河北省白洋淀地区的陆地和岛上两个观测点（分别代表陆地和水面两种不同下垫面）湍流湿度脉动的相干结构特征，结果表明陆地和水上湿度序列的相干结构尺度分布相似，并且尺度与周期之间的关系一致：小于5s的相干结构不连续出现，而且通常伴有更大尺度的相干结构，而5～30s的相干结构有与其尺度差不多的周期。在寻找更大尺度的相干结构时发现存在一个尺度，当大于某个周期时，在各个周期上这个尺度的相干结构都显著；与正交小波变换识别相干结构主尺度的方法识别的相干结构主尺度一致。另外，小尺度结构不连续出现也可以解释小尺度湍流能量变化比较大。

The spectral analysis of the wavelet transform coefficients （spavelet analysis）, a statistical method combining wavelet analysis and a high-resolution spectral technique, recently developed by Petenko （2001）, can identify coherent structures in time series and their durations and periods. Comparing with wavelet transformation and spectral analysis respectively, in a way, the method describes the evolution of coherent structures by durations and periods of coherent structures, but cannot describe the frequency character of various coherent structures, which shows there are coherent structures of some sizes in some time. By this method, more than two hundred data, which are collected by an infrared gas analyzer （LiCor7500） for water vapor and carbon dioxide fluctuations at two locations overlaying the different surfaces （water and land） during 16- 22 November 2004, are analyzed. The conclusions show that the distributions of durations of coherent structures in humidity time series at land and water are similar, and the common relationship between durations and periods of coherent structures in all time series is that coherent structures whose durations are smaller than 5 s occur intermittently and are often accompanied by coherent structures of larger durations while there are periods close to durations for coherent structures of duration 5 - 30 s there is a significant duration for every larger period than a certain period, which can result from the interaction of coherent structures of different sizes. Comparing this method with the method determining principal time scales of coherent structures using orthonormal wavelets, the authors find that they agree with each other, which shows that the spectral analysis of the wavelet transform coefficients can identify the principal coherent structures accurately. In addition, the fact that coherent structures of small durations occurring intermittently may be a reason that change of turbulent energy at smaller scales is larger.