Martí et al. (2025) Revisiting Stability Functions of Surface Heat Fluxes in Semi-arid Environments

Identification

• Journal: Boundary-Layer Meteorology
• Year: 2025
• Date: 2025-11-22
• Authors: Belén Martí, Daniel Martínez‐Villagrasa, J. D. Price, Joan Cuxart
• DOI: 10.1007/s10546-025-00947-4

Research Groups

• Department of Physics, University of the Balearic Islands (UIB), Spain.
• Met Office, Exeter, UK.

Short Summary

This study evaluates the accuracy of standard Businger-Dyer stability functions for estimating heat fluxes in semi-arid regions, finding they significantly fail under wet-surface conditions and high Bowen ratios. The research proposes new, Bowen-ratio-dependent stability functions and demonstrates that mixing efficiencies for heat and moisture are rarely equal in these environments.

Objective

• To assess the validity of standard Monin-Obukhov Similarity Theory (MOST) flux-gradient relationships for sensible ($H$) and latent ($LE$) heat fluxes in complex, semi-arid Mediterranean environments.
• To investigate the variability of the turbulent Lewis number ($L_w$) and the relative transport efficiency of heat versus moisture.

Study Configuration

• Spatial Scale: Two semi-arid sites in Spain: ECUIB (Mallorca, foothills with heterogeneous vegetation/buildings) and Els Plans (Ebro Basin, rainfed agriculture and bare soil). Measurements were taken in the surface layer within the first 3.1 m above ground.
• Temporal Scale: Long-term datasets including 22 months at ECUIB (2020–2021) and 13 months at Els Plans (2021–2022), focusing on daytime periods (11:00–14:00 UTC).

Methodology and Data

• Models used: Monin-Obukhov Similarity Theory (MOST) using flux-gradient relationships; Businger-Dyer stability functions; Orthogonal Distance Regression (ODR) for fitting new empirical coefficients.
• Data sources:
  • Eddy-covariance (EC) systems: CSAT3A and HS-50 sonic anemometers; EC150 and KH20 gas analyzers for reference flux measurements.
  • Gradient measurements: Temperature and humidity sensors (EE181, HygroVUE5) at two vertical levels (ranging from 0.26 m to 2.5 m).
  • Soil data: Volumetric water content (VWC) sensors at depths of 0.04 m to 0.21 m.
  • Radiation: NR01, CRG4, and CM21 radiometers.

Main Results

• Sensible Heat Flux ($H$): Standard Businger functions are accurate for dry soils but fail (overestimating $H$) when the surface is wet and solar irradiation is high ($Bo \le 1$). New coefficients for these "wet" cases were derived ($\alpha \approx 1.79, \beta \approx 29.69$).
• Latent Heat Flux ($LE$): Standard functions are only applicable over dry soils where $H \ge LE$. In moisture-dominant or extremely dry conditions ($Bo \ge 10$), standard functions fail significantly.
• Lewis Number ($Lw$): The assumption that $Lw = 1$ (equal mixing efficiency) is invalid for most conditions. $L_w$ increases with the Bowen ratio, exceeding 1.0 when $Bo > 2.5$, indicating heat is mixed more efficiently than moisture in dry regimes.
• Transport Dissimilarity: Deviations in humidity gradients are linked to surface heterogeneity and patchy vegetation, which decorrelate temperature and water vapour signals.

Contributions

• Demonstrates that the Bowen ratio is a critical parameter for selecting appropriate stability functions in semi-arid land surface modeling.
• Provides a new set of empirical coefficients for stability functions specifically tuned for semi-arid, heterogeneous terrain.
• Challenges the universal application of the $L_w = 1$ assumption in the atmospheric surface layer, showing a clear dependence on soil moisture and surface heating.

Funding

• Spanish Ministry of Science, Innovation and Universities (MICIU/AEI) and European Regional Development Fund (FEDER, UE): Projects RTI2018-098693-B-C31 and PID2021-124006OB-I00.
• Vicepresidència i Conselleria de Innovació, Recerca i Turisme del Govern de les Illes Balears and European Social Fund: Grant FPI-CAIB (FPI/2165/2018).
• Comunitat Autònoma de les Illes Balears: Tourist Stay Tax Law ITS 2017-006 (PRD2018/67).

Citation

@article{Martí2025Revisiting,
  author = {Martí, Belén and Martínez‐Villagrasa, Daniel and Price, J. D. and Cuxart, Joan},
  title = {Revisiting Stability Functions of Surface Heat Fluxes in Semi-arid Environments},
  journal = {Boundary-Layer Meteorology},
  year = {2025},
  doi = {10.1007/s10546-025-00947-4},
  url = {https://doi.org/10.1007/s10546-025-00947-4}
}

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