Towards improved understanding on flow decoupling at eddy covariance sites with the aid of a universal coupling metric

Eddy covariance (EC) flux observations deviate from the fluxes at the ecosystem-atmosphere interface when the turbulent flow is decoupled from the surface. This problem severely limits the applicability of the EC technique to monitor ecosystem-atmosphere interactions including trace gas exchange. Despite some progress on understanding vertical coupling processes over the past years, the role and interplay of dynamic stability, canopy drag, and the strength of vertical turbulent mixing remains insufficiently understood. Furthermore, the commonly used metric to identify decoupling, friction velocity, does not represent these processes.

In this work we use the recently developed decoupling metric Omega to detect decoupling at 45 contrasting EC sites across a broad range of canopy architectures and biomes (Peltola et al. 2025, https://doi.org/10.1016/j.agrformet.2024.110326). Omega encapsulates the main processes controlling decoupling in a single dimensionless metric, thus providing a unified framework for studying coupling at all sites. We focus on evaluating the applicability of Omega to detect decoupling at these sites and use it to evaluate the processes controlling decoupling across sites.

The results show that Omega was able to identify coupling at all tested sites satisfactorily. The vertical turbulent carbon dioxide flux showed a similar Omega dependence at all sites, although there was some site-to-site variability. In contrast, when the change in storage flux term was added to the analysis, the similarity between sites disappeared. This suggests that the storage flux term depends on parameters other than those controlling vertical turbulent mixing. Canopy drag played an important role in the formation of decoupling at dense forest sites, and at such sites decoupling was observed even during the day.

Based on these findings, we delineate different Omega regimes in which different mass balance terms (vertical turbulent flux, storage flux and advective components) are important, and discuss improved approaches for detecting the regime where the sum of vertical turbulent flux and storage flux equals the surface gas exchange.