Impact of Large Eddies on Flux-Gradient Relations in the Unstable Surface Layer Based on Measurements over the Tibetan Plateau

The Monin-Obukhov similarity theory (MOST) is widely used for surface layer parameterization. Discrepancies in MOST highlight the need to account for large eddy effects. A possible solution is to introduce the boundary layer depth zi as a new scaling parameter, which may enhance the applicability of the theory. A novel similarity scheme has recently been proposed to explicitly account for large eddy effects under unstable conditions. In this study, we estimated the impact of large eddies on the unstable surface layer using field measurements from a summer experiment on the Tibetan Plateau. Furthermore, we evaluated the proposed scheme and suggested simplifications for its improvements. In this study, the non-dimensional wind shear, ϕm, exhibited greater scatter and larger deviations from MOST than the non-dimensional temperature gradient, ϕh. Additionally, the normalized wind gradient ϕm is found to depend on both z/L and zi/L, where z is height above ground and L is the Monin-Obukhov length. The additional dependence on zi/L suggests that it may serve as a crucial missing scaling parameter in the MOST under unstable conditions. Ultimately, we observed that the zi-scaling parameter Cm derived from the proposed scheme maintains a linear correlation with the stability parameter (−zi/L), confirming the scheme’s effectiveness. Moreover, vertical wind gradients, friction velocity, and momentum flux predicted by this new scheme align more closely with observations than those estimated using the classical similarity function, thereby strengthening its feasibility and offers valuable insights for its simplification for Earth System modeling.