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Abstract Surface moisture heterogeneity degrades temperature‐humidity (‐) similarity in the atmospheric surface layer, yet the underlying physical mechanisms driving this dissimilarity remain underexplored. This study employs large‐eddy simulations coupled with a land‐surface model to investigate ‐ similarity in the convective boundary layer (CBL) over surfaces with varying scales of surface moisture heterogeneity. Results reveal that as the heterogeneity scale increases, patch‐scale thermally induced circulations develop and interact with cellular turbulent organized structures, significantly altering scalar transport and turbulence dynamics. The patch‐scale thermally induced circulations enhance horizontal advection, modify the production and transport of scalar variances, and lead to a disproportionate increase in the standard deviations of temperature () and humidity (), accompanied by a reduction in ‐ covariance (). As a result, ‐ similarity is substantially reduced throughout the CBL. Spectral analysis reveals that ‐ dissimilarity is most strongly influenced by turbulent motions at scales corresponding to patch lengths. The findings offer insights into the role of surface heterogeneity in shaping scalar similarity in the CBL, with implications for land‐atmosphere interactions and parameterization in numerical models. 

Abstract Top‐down entrainment shapes the vertical gradients of sensible heat, latent heat, and CO₂fluxes, influencing the interpretation of eddy covariance (EC) measurements in the unstable atmospheric surface layer (ASL). Using large eddy simulations for convective boundary layer flows, we demonstrate that decreased temperature gradients across the entrainment zone increase entrainment fluxes by enhancing the entrainment velocity, amplifying the asymmetry between top‐down and bottom‐up flux contributions. These changes alter scalar flux profiles, causing flux divergence or convergence and leading to the breakdown of the constant flux layer assumption (CFLA) in the ASL. As a result, EC‐measured fluxes either underestimate or overestimate “true” surface fluxes during divergence or convergence phases, contributing to energy balance non‐closure. The varying degrees of the CFLA breakdown are a fundamental cause for the non‐closure issue. These findings highlight the underappreciated role of entrainment in interpreting EC fluxes, addressing non‐closure, and understanding site‐to‐site variability in flux measurements. 

Abstract It is well‐established that large eddies significantly influence the turbulent transport of heat and scalars in the atmospheric surface layer. However, the mechanistic understanding of how large eddies originating from both the ground (updrafts) and aloft (downdrafts) regulate flux convergence (FC) and divergence (FD) remains relatively unexplored. Based on turbulence data measured at 12 levels, spanning from 1.2 to 60.5 m above the ground, we observe a notable increase in the variability of sensible heat flux magnitudes with height. Our results show that FC and FD of sensible heat are primarily linked to variations in the respective transport efficiencies () at different heights. Using the cross‐wavelet transform, we find that in FC cases, the regions with high wavelet coherence expand with height, resulting in higher at higher levels compared to low ones. Conversely, in FD cases, the regions with high wavelet coherence decrease with height, leading to lower at higher levels. Large eddies with length scales of approximately 120–500 m have a significant impact on amplifying or attenuating at higher levels compared to lower levels. Using conditional sampling to extract the updrafts and downdrafts of large eddies, distinct patterns are observed in the characteristics of updrafts and downdrafts between FC and FD groups especially in their flux contribution and transport efficiencies. This work emphasizes the significant contribution of asymmetric turbulent transport by updrafts and downdrafts to the discrepancy between the observed turbulent fluxes and those predicted by the Monin‐Obukhov similarity theory. 

Abstract How convective boundary‐layer (CBL) processes modify fluxes of sensible (SH) and latent (LH) heat and CO₂(F_c) in the atmospheric surface layer (ASL) remains a recalcitrant problem. Here, large eddy simulations for the CBL show that whileSHin the ASL decreases linearly with height regardless of soil moisture conditions,LHandF_cdecrease linearly with height over wet soils but increase with height over dry soils. This varying flux divergence/convergence is regulated by changes in asymmetric flux transport between top‐down and bottom‐up processes. Such flux divergence and convergence indicate that turbulent fluxes measured in the ASL underestimate and overestimate the “true” surface interfacial fluxes, respectively. While the non‐closure of the surface energy balance persists across all soil moisture states, it improves over drier soils due to overestimatedLH. The non‐closure does not imply thatF_cis always underestimated;F_ccan be overestimated over dry soils despite the non‐closure issue.