Journal Article

Is There a Scalar Atmospheric Surface Layer Within a Convective Boundary Layer? Implications for Flux Measurements

Katul, Gabriel G., Zhang, Qianyu, Huang, Jianping, Liu, Cheng, Desai, Ankur R., Liu, Heping, Zhou, Yanzhao, Ghannam, Khaled

2025

Overview

Top‐down entrainment shapes the vertical gradients of sensible heat, latent heat, and CO2 fluxes, 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. Plain Language Summary In the atmosphere over a heated surface, water vapor, carbon dioxide, and heat are transported from both the ground (bottom‐up) and the top of the air column (top‐down). The swirling motion of air within the column helps to even out the distribution of these quantities, known as “scalars.” Scalar fluxes measure how many molecules of these substances cross a unit area over time. At the surface, energy balance and plant processes control heat, water vapor, and carbon dioxide fluxes. However, fluxes at the top of the air column do not follow the same rules and abide by the same constraints as their ground counterpart. This study uses numerical simulations to show that when the temperature difference across a layer at the top of the boundary layer decreases, the boundary layer becomes deeper, increasing the transport of heat from the top. This causes changes in the slopes of flux profiles, disrupting the assumption that fluxes remain constant with height even close to the ground surface. As a result, measurements near the surface often underestimate or overestimate true surface fluxes, contributing to the much‐debated surface energy balance non‐closure problem. Key Points Entrainment‐modulated top‐down transport influences the slopes of the scalar flux profiles in the unstable atmospheric surface layer Variations in scalar flux profiles lead to differing degrees of failure in the constant flux layer assumption (CFLA) for different scalars The failure of the CFLA explains the non‐closure issue in the surface energy balance

• Publisher: John Wiley & Sons, Inc,American Geophysical Union (AGU),Wiley
• Subject: Air / Boundary layer flow / Boundary layers / Breakdown / Carbon dioxide / Carbon dioxide flux / Convergence / Divergence / Eddy covariance / eddy covariance fluxes of scalars / Energy balance / Enthalpy / Entrainment / entrainment processes / Fluctuations / flux vertical gradients / Fluxes / Heat / Large eddy simulation / Large eddy simulations / Latent heat / Mixed layer / non-closure of surface energy balance / Numerical simulations / Scalars / Sensible heat / Surface boundary layer / Surface energy / Surface energy balance / Surface fluxes / Surface layers / Surface properties / Swirling / Temperature differences / Temperature gradients / Water vapor / Water vapour