The behavior of potential temperature (θ) and specific humidity (q) in the atmospheric surface layer (ASL) is assumed to be similar over homogeneous landscape. However, abundant experimental evidence shows that temperature-humidity (θ−q) similarity in the ASL is reduced as Bowen ratios (β) increases over land. In order to understand the underlying physical mechanism, θ−q dissimilarity is investigated in the steady-state convective boundary layer (CBL) over homogeneous landscape with varying β by using the high-resolution large-eddy simulations. As β increases, the entrainment ratio for θ slightly decreases but that for q largely increases. As a result, local production of humidity variance (σ_q²) is substantially enhanced in the upper CBL and can be efficiently transported to the middle and lower CBL by vigorous large eddies, contributing significantly to nonlocal fraction. However, the largely local-produced temperature variance (σ_θ²) in the ASL associated with strong heat flux is larger than that transported from the upper CBL. Such asymmetry in vertical diffusion of θ and q induced by varying partitioning of surface fluxes strongly regulates θ−q dissimilarity in the CBL even under perfect conditions valid for Monin-Obukhov similarity theory (MOST). The results suggest that varying degrees of validity of similarity assumption with changes in β should be noted in applying MOST and interpreting eddy covariance data even over homogenous landscapes, and highlight the influence of the CBL processes on the ASL turbulence structures.

Bowen ratios,convective atmospheric boundary layer,large-eddy simulations,Monin-Obukhov similarity theory,temperature-humidity dissimilarity,turbulent fluxes