A Modification of Monin–Obukhov Similarity Theory with a Double-Layer Turbulence Model for the Convective Surface Layer and Bifurcation Hypothesis

A modification of the Monin–Obukhov theory is proposed, introducing a two-layer model of the convective atmospheric surface layer (ASL) with a fixed interlayer boundary. The convective ASL is assumed to consist of a frictional–heat sublayer, adjacent to the underlying surface, and a forced-convection sublayer. Approximations of the turbulent moments are derived independently for each sublayer in the form of truncated Taylor expansions in the modified dimensionless height. The interlayer boundary is fixed and defined on the basis of the bifurcation hypothesis. This hypothesis postulates the existence within the ASL of a domain of strong turbulence, represented by a coherent structure formed by an ensemble of small convective eddies (thermals), and a domain of weak turbulence, represented by eddy-wave motion. According to the bifurcation hypothesis, the strong-turbulence domain corresponds to the forced-convection sublayer, whereas the weak-turbulence domain corresponds to the frictional–heat sublayer. The proposed approach makes it possible to construct a one-parameter family of approximations for a wide class of turbulent moments. The one-parameter model, validated against available experimental data, shows that the proposed piecewise-linear approximation is accurate across the entire convective ASL and demonstrates a high degree of universality.