涡旋相关法测定湍流通量偏低的研究

针对野外实验所发现的不同观测法测定地表能通量不平衡问题,进行了均匀加热大气边界层的大涡模拟实验.用模拟的湍流风、温度和湿度涨落的时间序列证实,对流边界层低频涡普遍存在,并经常以一簇一簇热泡的形式出现.风速较小时,有限时长的取样不足以捕捉低频涡的贡献,可造成涡旋相关法测量的统计量异常偏低.仿照涡旋相关法的步骤进行数据处理发现,经去除平均或趋势计算的温度和湿度通量偏低程度在边界层下部随观测高度的增高而显著,其中尤以湿度通量为甚.其结果在一定程度上可以解释低风速条件下地表能通量测量的不闭合问题,但是尚不能完全解释诸如青藏高原实验出现的严重不闭合.文中对此作了探讨性的讨论.

The Underestimation of the Turbulent Fluxes in Eddy Correlation Techniques

A series of large eddy simulations is performed for convective boundary layer(CBL) with homogeneously heated surface.The research aims at investigating the lack of energy budget closure on field energy flux measurements reported in recent years.Time series of simulated turbulent wind,temperature and humidity show the frequent occurrence of low-frequency eddies,which appear as the cluster-like thermals in the CBL.Following data processing procedures adopted in eddy correlation techniques,the covariances with linear tendency removal or mean removal only are calculated based upon the data produced from the large eddy simulation(LES).The results show that the failure to capture low-frequency eddies due to finite sampling length is responsible for the great loss of fluxes evaluated by eddy correlation techniques in low wind CBL.The underestimation of energy fluxes increases with increasing of the height and decreases with increasing of the wind speed.The results may enhance the understanding of the inadequate sampling problem in eddy correlation techniques.