Varying Partitioning of Surface Turbulent Fluxes Regulates Temperature‐Humidity Dissimilarity in the Convective Atmospheric Boundary Layer

Experimental evidence shows that temperature‐humidity (θ−q) similarity in the atmospheric surface layer (ASL) is reduced as Bowen ratio (β) increases over land. However, underlying physical mechanisms remain not well understood. With large‐eddy simulations, θ−qdissimilarity is investigated in the steady‐state, convective boundary layer (CBL) over homogeneous landscape with varying β. As βincreases from 0.4 to 2.0, the entrainment ratio for θslightly decreases but that for qlargely increases. As a result, local production of humidity variance is substantially enhanced in the upper CBL and transported to the lower CBL by vigorous large eddies, contributing significantly to nonlocal fraction. However, the increased temperature variance in the ASL associated with strong heat flux is larger than that transported from the upper CBL. Such asymmetry in vertical diffusion induced by varying partitioning of surface fluxes strongly regulates θ−qdissimilarity even under perfect conditions valid for Monin‐Obukhov similarity theory. 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 such assumption of θ−qsimilarity is not satisfied as evaporation decreases (i.e., increased Bowen ratio, β). In order to understand the intrinsic physical mechanism, we investigate θ−qsimilarity in the steady‐state convective boundary layer (CBL) using the high‐resolution model and analyze the results in various βcases. We confirm that θ−qsimilarity is reduced across the CBL with increasing or decreasing βfrom 0.4, with the lowest similarity appearing in the middle or upper CBL. The disproportional variations of σθ2and σq2associated with asymmetric contributions by top‐down and bottom‐up transport of θand qunder varying βconditions explain the θ−qdissimilarity in the CBL. The results suggest that varying degrees of validity of similarity assumption with changes in βshould 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. Temperature‐humidity (θ−q) similarity varies with Bowen ratio (β) in the convective boundary layer (CBL) over homogeneous surfaceθ−qdissimilarity with varying βis mostly linked to the large‐scale eddiesDisproportional variations of σθ2and σq2by asymmetric top‐down and bottom‐up transport of θand qexplain the dissimilarity with varying β Temperature‐humidity (θ−q) similarity varies with Bowen ratio (β) in the convective boundary layer (CBL) over homogeneous surface θ−qdissimilarity with varying βis mostly linked to the large‐scale eddies Disproportional variations of σθ2and σq2by asymmetric top‐down and bottom‐up transport of θand qexplain the dissimilarity with varying β