Atmospheric turbulence and energy spectra in mesoscale models

Kinetic energy spectra obtained in short range high-resolution runs using the NCEP Nonhydrostatic Mesoscale Model (NMM) are examined and compared with observed kinetic energy spectra. The NMM is one of two currently existing Weather Research and Forecasting (WRF) Model cores. Horizontal finite differencing employed in the NMM 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 and controls the nonlinear energy cascade.

The results of tests reveal that the NMM dynamics can reproduce very well the observed Nastrom and Gage spectrum derived from commercial aircraft measurements in the upper troposphere and lower stratosphere over a broad range of horizontal scales. The model generated energy spectra agreed very well with observations in both the –3 and –5/3 ranges, provided there was a sufficiently strong energy source on the meso scales. Moreover, on the cloud resolving scales, the model dynamics successfully reproduced the theoretical –5/3 slope corresponding to the decaying three-dimensional turbulence excited by moist convection. It is believed that the nonlinear energy cascade controlled by the conservation of energy and enstrophy plays an important role in reproducing the observed spectral energy distributions.