Influence of Large Eddy Generation Mechanisms on the Turbulent Flux Transport in the Unstable Atmosphere Boundary Layer

Abstract

The turbulent transport dissimilarity between momentum and scalars and the transport similarity among scalars have been widely investigated in unstable atmospheric boundary layers (ABLs). Although buoyancy and mechanically driven turbulence, along with variations in scalar sources and sinks, are recognized as key factors influencing transport similarity, the specific roles of local thermal plume-generated and nonlocal bulk shear-generated large eddies under varying stability conditions are less explored. This study utilized over four years of eddy covariance data sampled 50 m above a complex suburban canopy to characterize the influence of buoyancy and wind shear on flux transport similarity in an unstable ABL. The time threshold τ method was applied to detect large coherent events, with wind shear enhancing their intensity, while buoyancy primarily affected the ejection–sweep asymmetry of scalars. The dynamics between buoyancy and wind shear were analyzed through separate momentum, heat, and joint transport events. The results show that strong wind shear enhances nonlocal large eddies, reducing momentum–heat transport similarity, whereas strong buoyancy supports localized turbulence. As stability varies, the shift between nonlocal and local eddies alters the trends in co-transport duration and intensity, revealing distinct patterns in the water vapor intensity from that of the sensible heat owing to local sources and sinks.