Abstract
AbstractAir flows may be decoupled from the underlying surface either due to strong stratification of air or due to canopy drag suppressing cross‐canopy mixing. During decoupling, turbulent fluxes vary with height and hence identification of decoupled periods is crucial for the estimation of surface fluxes with the eddy covariance (EC) technique and computation of ecosystem‐scale carbon, heat, and water budgets. A new indicator for identifying the decoupled periods is derived using forces (buoyancy and canopy drag) hindering movement of a downward propagating air parcel. This approach improves over the existing methods since (1) changes in forces hindering the coupling are accounted for, and (2) it is based on first principles and not on ad hoc empirical correlations. The applicability of the method is demonstrated at two contrasting EC sites (flat open terrain, boreal forest) and should be applicable also at other EC sites above diverse ecosystems (from grasslands to dense forests).
Highlights
Understanding of air flows and mixing in the very stable boundary layer often observed for example, during clear sky, weak wind nights persists to be incomplete (Mahrt, 2014)
Turbulent fluxes vary with height and identification of decoupled periods is crucial for the estimation of surface fluxes with the eddy covariance (EC) technique and computation of ecosystem-scale carbon, heat, and water budgets
A new indicator for identifying the decoupled periods is derived using forces hindering movement of a downward propagating air parcel. This approach improves over the existing methods since (1) changes in forces hindering the coupling are accounted for, and (2) it is based on first principles and not on ad hoc empirical correlations
Summary
Understanding of air flows and mixing in the very stable boundary layer (vSBL) often observed for example, during clear sky, weak wind nights persists to be incomplete (Mahrt, 2014). This issue poses problems for all scientific studies enquiring into surface-atmosphere interactions including mass and energy budgets, since they rely on turbulence observations or boundary layer theories, both of which tend to fail under strong stratification. Under strong enough stratification and weak turbulence production via wind shear, the turbulent eddies become detached from the surface, that is, they are not coupled to the surface. As eddies detach from the surface, they lose their immediate connection to the exchange of momentum, heat and gases at the surface resulting in vertical variability of turbulent flux of these constituents with height (Mahrt et al, 2018)
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