2026/06/09 by Mengjuan Liu, Wei Huang, Bowen Zhou
Earth and Planetary Sciences · Environmental Science · Engineering · #Meteorological Phenomena and Simulations #Wind and Air Flow Studies #Wind Energy Research and Development
paper · doi:10.1175/jas-d-25-0111.1
Abstract The dominant turbulence length scale λ in the atmospheric boundary layer characterizes the size of the most energetic turbulent eddies. It plays a key role in scale-adaptive planetary boundary layer (PBL) schemes for numerical weather prediction models operating at kilometer-scale resolutions. However, the vertical profile of λ for the entire boundary layer has remained elusive due to deficiencies in its quantitative definition. Specifically, conventional definitions of λ rely on the magnitude of the Reynolds-averaged flux T , hence fail when T is close to zero, leading to discontinuities and fluctuations in the λ profile. To overcome this deficiency, this study proposes an alternative definition of λ that does not depend on the value of T . Instead, it determines λ as the grid resolution that maximizes the intergrid variance of the subgrid-scale flux. The idea behind the proposed definition is illustrated with a wavenumber analysis. A comparative evaluation among λ profiles extracted from large-eddy simulations (LESs) of the convective boundary layer (CBL) with the new and the conventional definitions is conducted. While achieving close agreement with the conventional definitions for the most part, the new definition provides physically meaningful and vertically continuous estimates of λ for the entire CBL. Applying the new definition, self-similar profiles of the normalized λ for the vertical sensible heat and momentum fluxes are obtained for use in scale-adaptive PBL schemes. Significance Statement Turbulence in the atmospheric boundary layer plays a major role in weather and climate processes. Its representation in numerical weather prediction models relies on the knowledge of the size of the most energetic turbulent eddies, denoted as λ . Vertical profiles of λ for the entire boundary layer have remained elusive. This is because at heights where the turbulent flux is close to zero, conventional definitions would often fail to produce reliable estimates of λ . We propose a new definition based on the variation (i.e., the error bars) of the turbulent fluxes. It enables the derivation of both vertically continuous and physically meaningful λ profiles for the daytime convective boundary layer.