Spectra of Scalar-Flux and Stress Fluctuations in the Atmospheric Surface Layer

Abstract

In a turbulent flow, the instantaneous turbulent flux and stress fluctuate in time and space. In large-eddy simulations (LES), the subgrid-scale (SGS) flux and stress are also time and space dependent, and their fluctuations are important in the modeling of SGS turbulence. In the present work, spectra of turbulent scalar-flux (temperature-flux) and stress fluctuations in the atmospheric surface layer are studied both by experiment and analysis. LES data show that horizontal two-dimensional (fourth-order) spectrum of temperature-flux and stress fluctuations peak at a scale comparable to the height from the surface z. The quasi-normal assumption is used to predict the variance and model the spectra of temperature-flux and stress fluctuations. The calculated one-dimensional spectra are in good agreement with experimental results. Under convective conditions, the temperature-flux and stress spectra both have a ??5/3 dependence in the inertial range. The normalized stress spectrum is very close to the vertical velocity spectrum at all wavenumbers for the stability conditions considered, whereas the temperature-flux spectrum peaks at a slightly higher wavenumber and has larger inertial-range values. The two-point coherence of flux and stress fluctuations is also similar to that of the vertical velocity.