Decreased dissimilarity of turbulent transport attributed to large eddies
Changxing Lan, Baomin Wang, Dan Zheng, Ye Wang, Zhijie Zhang, Renzhi Fang
Abstract
Abstract A number of studies have brought up that turbulent transport dissimilarity between momentum and scalars, and even among scalars, is a distinct feature of the unstable boundary layer, affecting the performance of the Monin–Obukhov similarity theory. Although the source/sink distribution of scalars, as well as the top‐down entrainment processes, are identified as being responsible for affecting transport dissimilarity, less is known about the role of bulk‐shear‐generated non‐local large eddies. Using data collected by an eddy covariance system mounted on a 70‐m meteorological tower over a complex rural area, decreased transport dissimilarity with enhanced wind shear is observed during daytime unstable conditions, prompting our interest in the underlying governing mechanisms. Enhanced wind shear increases the intensity of coherent large eddies, with spectra analysis confirming that large eddies with scales larger than 0.3 (in the normalized frequency) are mainly responsible for the enhanced correlation of fluxes. Moreover, this process is operating on large eddies with minimal phase differences. With enhanced wind shear, fluctuations of scalars within these non‐local large eddies become synchronous. Due to the enhanced ejections of large eddies under strong wind shears, fluxes are transported evenly, thereby resulting in decreased transport dissimilarity.