Turbulence Influenced by Mesoscale Motions in the Stable Boundary Layer over Complex Terrain of the Loess Plateau
黄土高原复杂地形受中尺度运动影响的稳定边界层湍流特征

Turbulent characteristics in the stable boundary layer are evaluated by using data collected by the eddy covariance approach at the Semi-Arid Climate and Environment Observatory of Lanzhou University (SACOL). The influence of mesoscale motions in the averaging time is the chief reason for the large scattering of turbulent fluxes obtained by using the eddy covariance technique. The time interval to define the turbulence ranges from several tens of seconds to several minutes. The multiresolution cospectra of the heat flux reveal a gap scale from 112.4 s to 449.9 s for strong stability when gradient Richardson number (Ri) is greater than 0.3. The motions at averaging time scales greater than the gap scale lead to large random heat flux errors and may even change the sign of heat flux cospectra. The gap scale for momentum flux occurs between 112.4 s and 224.9 s. In a weak wind regime in which the submeso velocity scale is greater than the mean flow, standard deviation of vertical velocity depends systematically on the submeso velocity scale and increases with increasing submeso velocity scale at an average rate of approximately 0.1. The standard deviation of vertical velocity correlates well with the generalized velocity scale, and approaches to zero with the generalized velocity scale vanishing. The standard deviations of w, u, and v normalized by friction velocity equal 1.35, 2.54, and 2.21 respectively. Analysis of turbulent kinetic energy reveals that persistent turbulence exists even under condition of Ri > 0.3. Stationarity time based on turbulent kinetic energy changes with stability ranged from 133.5 s to 856.2 s during December 7-11, 2008.