Energy Spectra in Mesoscale Models

Zavisa Janjic


Energy spectra obtained in short range high resolution runs using the dynamics of the NCEP Nonhydrostatic Mesoscale Model (NMM) are examined and compared with observed data. The NMM is one of the two currently existing Weather Research and Forecasting Model cores. In the NMM dynamics the "isotropic" horizontal finite differencing is employed that conserves a variety of basic and derived first order and quadratic quantities. Among these, the conservation of energy and enstrophy improves the accuracy of nonlinear dynamics of the model.

The results of the tests reveal that the NMM dynamics can reproduce very well the observed atmospheric spectrum in both the -3 and -5/3 ranges provided there is a sufficiently strong energy source on the small scales and given enough time. Moreover, on the cloud resolving scales, the model successfully reproduced the -5/3 slope corresponding to decaying 3D turbulence. It is believed that the nonlinear energy cascade controlled by the conservation of energy and enstrophy played an important role in reproducing the observed spectral shape. Namely, another dynamics formulation that relied only on dissipation for controlling the nonlinear energy cascade had difficulties in reproducing the spectra. The possibility of employing the observed atmospheric spectra for validation of mesoscale models is discussed.