Intrinsic Constraints on Asymmetric Turbulent Transport of Scalars Within the Constant Flux Layer of the Lower Atmosphere

A widely used assumption in boundary layer meteorology is the z independence of turbulent scalar fluxes F-s throughout the atmospheric surface layer, where z is the distance from the boundary. This assumption is necessary for the usage of Monin-Obukhov Similarity Theory and for the interpretation of eddy covariance measurements of F-s when using them to represent emissions or uptake from the surface. It is demonstrated here that the constant flux assumption offers intrinsic constraints on the third-order turbulent transport of F-s in the unstable atmospheric surface layer. When enforcing z independence of F-s on multilevel F-s measurements collected above different surface cover types, it is shown that increasing instability leads to a novel and universal description of (i) the imbalance between ejecting and sweeping eddy contributions to F-s and (ii) the ratio formed by a dimensionless turbulent transport of F-s and a dimensionless turbulent transport of scalar variance. When combined with structural models for the turbulent transport of F-s, these two findings offer a new perspective on closing triple moments beyond conventional gradient diffusion schemes. A practical outcome is a diagnostic of the constant flux assumption from single-level F-s measurements.