JP1.8 Numerical Accuracy of the Regional Spectral Method used in NCEP RSM

Monday, 1 June 2009
Grand Ballroom Center (DoubleTree Hotel & EMC - Downtown, Omaha)
Jun Wang, NOAA/NWS/NCEP, Camp Springs, MD; and H. M. H. Juang

The numerical properties of the method used in the NCEP (National Centers for Environmental Prediction) Regional Spectral Model (RSM) including the nesting strategy and boundary conditions are analyzed in this paper. Through a simple linear advection equation, the boundary effect and scale dependent accuracy in this method are clarified, two different time schemes and two differencing methods are investigated, and the nesting frequency in time and the nesting density in space are tested. The numerical results show that there are no significant lateral boundary problems and no unusual scale dependent inaccuracy. The negligible root mean square error (RMSE) of the numerical solution when a wave travels inside the domain has the same magnitude as it does when passing out of the boundary; and the large as well as the small spatial scale phenomena have good degree of accuracy. The effects of the time step and grid point number on the results in regional domain show that the RMSE from the NCEP RSM numerical solution decreases with increasing grid number and shortened time step, and the optimal NCEP RSM numerical solutions are always better than the numerical solutions from the 4th order and 6th order finite difference methods with leap frog time scheme without surprise. The results show that there is an optimal grid number in the base field where enough information is provided to get the best solution in the regional domain; beyond that other error sources may dominate the total error in the numerical solutions. In summary, the NCEP RSM method has no lateral boundary and scale dependence problems
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