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 intrinsic physical mechanism, we investigate θ−q similarity in the steady-state, convective boundary layer (CBL) using the high-resolution model over homogeneous landscape with varying β. We confirm that θ−q similarity is reduced across the CBL as β increases, with the lowest similarity appearing in the middle or upper CBL. As β increases, the largely local-produced temperature variance (σ_θ²) in the ASL associated with strong surface heat flux dominate the variation of σ_θ² in the CBL. However, local production of humidity variance (σ_q²) is significantly increased in the upper CBL because of enhanced entrainment flux and can be efficiently transported by large eddies to the lower CBL. The 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. The results suggest that varying degrees of validity of similarity assumption with changes in β should be noted in applying Monin-Obukhov similarity theory 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