Model Responses to Sub−Filter Scale (SFS) Topographic Information

It is commonly thought that having high-resolution data will improve the performance of a Numerical Weather Prediction (NWP) model. In a NWP model, usually a numerical dissipation scheme works to damp out signals at the wavenumbers smaller than the effective resolution (ER), about 7 times the grid spacing. The numerical dissipation is needed not to have an unrealistic build-up of energy at those high wavenumbers (Skamarock 2004). The wavenumber range where numerical dissipation takes place is between the ER and the Nyquist wavenumber (two times the grid spacing). This fact poses the following question: what are the effects of the SFS information on model performance? We answer the questions through numerical experiments with a fine-resolution large eddy simulation (LES) nested in a coarse-resolution LES. We hypothesize that when and/or where there is up-scale energy cascade, the erroneous SFS information due to the numerical dissipation may significantly contaminate model results even on the scales larger than the ER. This study will provide useful information to the NWP community in order to improve model performance.