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

Journal Article · Fri Feb 28 00:00:00 EST 2025 · Geophysical Research Letters

DOI:https://doi.org/10.1029/2024GL112619· OSTI ID:2530670

Liu, Heping ^[1]; Liu, Cheng ^[2]; Zhou, Yanzhao ^[3]; Zhang, Qianyu ^[1]; Desai, Ankur R. ^[4]; Ghannam, Khaled ^[5]; Huang, Jianping ^[6]; Katul, Gabriel G. ^[7]

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1. Department of Civil and Environmental Engineering Washington State University Pullman WA USA
2. Jiangxi Provincial Key Laboratory of Genesis and Remediation of Groundwater Pollution/School of Water Resources and Environmental Engineering East China University of Technology Nanchang China
3. School of Geographical Sciences/Hebei Technology Innovation Center for Remote Sensing Identification of Environmental Change/Hebei Key Laboratory of Environmental Change and Ecological Construction Hebei Normal University Shijiazhuang China
4. Department of Atmospheric and Oceanic Sciences University of Wisconsin‐Madison Madison WI USA
5. Department of Civil and Environmental Engineering Northeastern University Boston MA USA
6. Environmental Modeling Center NOAA National Centers for Environmental Prediction College Park MD USA
7. Department of Civil and Environmental Engineering Duke University Durham NC USA

Abstract

Top‐down entrainment shapes the vertical gradients of sensible heat, latent heat, and CO ₂ 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.

• Sponsoring Organization: USDOE
• Grant/Contract Number: SC0022072
• OSTI ID: 2530670
• Journal Information: Geophysical Research Letters, Journal Name: Geophysical Research Letters Journal Issue: 5 Vol. 52; ISSN 0094-8276
• Publisher: American Geophysical Union (AGU)
• Country of Publication: United States
• Language: English