小波检测湍流相干结构方法的改进与其应用

Improvement on the Method of Wavelet Transform for the Detection of Turbulence Coherent Structures and its Usage

相干结构(coherent stnicture,记为"CS")在湍流物质、能量的输送中占有重要地位°鉴于目前以小波变换检测CS的方法存在的问题,本文基于温度在不稳定与稳定条件下分别呈现岀缓升陡降与缓降陡升的斜坡模态特征与完整的理想CS波形,对之进行了改进。该改进方法可以提取具有变动时间尺度的单个CS与连发CS的信号。由此改进方法,利用中国气象局兰州干旱气象研究所定西干旱气象与生态环境试验基地的湍流观测资料,经过质量控制与滤除线性重力波,分析了马铃薯下垫面上CS的特征。主要结论如下:⑴白天(夜间)CS的典型特征是空气辐合上升,温度升高(降低),水汽密度增大;紧接着,空气辐散下沉,温度降低(升高),水汽密度减小。⑵CS以每30 min出现15个、持续时间18.2 s、长度尺度M 15 m、间隔18 s、间歇因子55%(稳定)与85%(不稳定)、且两个并发的特征最为典型;(3)上扬支与下扫支的持续时间呈现极为显著的、斜率小于1的线性关系,CS并不对称;⑷Taylor长度尺度与稳定度和摩擦速度呈极为显著的线性关系,动力因素倾向于产生大而少的CS,热力因素则产生更多的小的CS;(5)内外半径比揭示,CS在不稳定条件下比稳定条件下相对集中,标量相对集中在CS的中部,矢量相对分散。

Coherent structures(CS)are important to the turbulent transport of matter and energy.The whole and idea CS waves and the temperature ramp modes show a gradual rise followed by a sudden fall under an unstable atmospheric condition and a gradual fall followed by a sudden rise under a stable condition.Based on this and in view of the problems existing in the detection of CS by wavelet transform technique,an improved approach was presented.The approach could extract the signals of single CS and consecutive CSs at the varied time scales.The characteristics of CS over a potato field were then analyzed based on the turbulence observation data collected from Dingxi Experimental Base of Arid Meteorology and Ecological Environment,Arid Meteorological Institute,China Meteorological Administration,Lanzhou.The main results were as follows:(1)The typical features of CS during the day(night)were that temperature and water vapor density were increased(decreased)with the air convergent rise,and then temperature and water vapor density decreased(increased)with the air divergent sink.(2)In the semiarid zone,the CSs commonly appeared with 15 every 30 min and had a spatial scale of 15 m or less,duration of 18.2 s,and separation time of 18 s.And the intermittent factor of CS was 55%under the stable condition and was 85%under the unstable condition,and two CSs are concurrent;(3)The durations of burst arm and sweep arm were very significant linear correlation with the linear fitting slope less than 1,which meant the CS was not symmetrical;(4)Taylor length scale was also extremely significantly linearly correlated with Obukhov length and friction velocity.Dynamic factor tended to generate big and samll CSs while thermal factor tended to produce more but small CSs;(5)The ratio of inner radius to outer radius revealed that CSs was relatively concentrated under the unstable conditions relative to the stable condition,and scalar relatively concentrated in the central part of a CS while vector relatively scattered.