Improving Atmospheric Turbulence Models
Scientists update a 30-year-old turbulence model; validation against ARM observations shows improved simulations of boundary layer wind speed.
Large-eddy simulation (LES) models are numerical models used to simulate atmospheric turbulence and atmospheric boundary layers for applications in wind energy, weather prediction, and climate. Atmospheric LES models are detailed numerical models, but directly resolving all scales of atmospheric turbulence is too computationally expensive for most applications. Consequently, LES models reduce the computational complexity by neglecting the smallest length scales, which requires approximation of higher-order moments known as turbulence “closure.”
A commonly used turbulence closure method for LES models of atmospheric boundary layer flows was developed more than 30 years ago. In a recent study, researchers developed a modified turbulence closure method, which was validated with Atmospheric Radiation Measurement (ARM) program observations. The new model produces improved simulations of vertical profiles of wind speed, which could be important for wind energy, weather, and climate applications.
The Deardorff 1980 (D80) subgrid turbulence model is perhaps the most ubiquitous scheme used in LES studies of atmospheric boundary-layer flows. This model is often included as the default closure scheme in a variety of codes and numerical weather prediction models. In this study, researchers investigated the three commonly employed corrective adjustments of the D80 closure model. These include a stability-dependent length scale, formulation for the subgrid turbulent Prandtl number, and enhancement of near-surface dissipation. They implemented a modified formulation of the D80 closure, then compared simulated flow statistics in the lower portion of a representative nocturnal stable boundary layer (SBL) case from LES with realistic forcing using the original D80 scheme and the modified version of the scheme. LES data were compared with observations from the ARM program’s Southern Great Plains (SGP) site in Lamont, Oklahoma. The modified scheme shows overall improvement in reproducing vertical profiles of wind speed and potential temperature in the SBL near-surface region. Conclusions regarding turbulence kinetic energy and friction velocity are not as definitive, although there are signs of improved agreement with measurement data. Examination of the stability parameter and near-surface sensible heat flux suggests the modified scheme better captures effects of stability in the considered flow case. The proposed modification offers a more straightforward and interpretable framework for the parametrization of subgrid turbulence in LES of atmospheric boundary layers.
University of Utah
This research was supported by the National Science Foundation. Data were obtained from the Department of Energy’s ARM Climate Research Facility.
Gibbs, J. A., and E. Fedorovich. 2016. “Sensitivity of Turbulence Statistics in the Lower Portion of a Numerically Simulated Stable Boundary Layer to Parameters of the Deardorff Subgrid Turbulence Model,” Quarterly Journal of the Royal Meteorological Society, DOI: 10.1002/qj.2818.