Gibbs, Jeremy A., and Evgeni Fedorovich, University
of Oklahoma
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
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 grid spacing (20 m to 80
m) are investigated.
Horizontal slices of
velocity fields are presented and discussed to enable comparison of CBL flow
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 focus slightly more energy in larger-scale 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 are overly dissipative on small scales of motion.