Comparison of convective boundary layer velocity spectra calculated from large eddy simulation and WRF model data

The Weather Research and Forecasting (WRF) model has evolved toward a self-contained numerical weather prediction system, capable of modeling atmospheric motions ranging from global to microscales. The promise of such capability is appealing to both operational and research environments where accurate prediction of turbulence is increasingly desirable. However, the ability of the WRF model to adequately reproduce small-scale atmospheric motions in the range of scales of the order of 100 m and smaller remains questionable. In this study, turbulent flow in the dry atmospheric convective boundary layer (CBL) is reproduced using a traditional large eddy simulation (LES) code and the WRF model applied in an LES mode. The simulations use almost identical numerical grids and are initialized with the same idealized vertical profiles of velocity, temperature, and moisture. The respective CBL forcings are set equal and held constant. The effects of CBL flow types (with and without shear) and of varying grid spacing (from 20 m to 100 m) are investigated. Horizontal slices of velocity fields are presented and discussed to enable comparison of CBL flow patterns obtained with each simulation method. Two-dimensional spectra calculated from the turbulent velocity fluctuations are used to characterize the planar turbulence structure. One-dimensional velocity spectra are also calculated, both by one-directional Fourier transform and subsequent averaging, and by integrating the two-dimensional spectra. Results show that the WRF model tends to attribute slightly more energy to larger-scale flow structures as compared to the CBL reproduced by the traditional LES. Consequently, the WRF model fails to adequately reproduce spatial variability of velocity fields within broader scale ranges. Spectra from the WRF model have narrower inertial spectral subranges and indicate enhanced dissipation of turbulence on small scales of motion.