Horizontal grid size selection and its influence on mesoscale model simulations

The use of two-dimensional spectral analysis applied to terrain heights in order to determine characteristic terrain spatial scales and its subsequent use for the objective definition of an adequate grid size required to reso lve terrain forcing are presented in this paper. In order to illustrate the influence of grid size, atmospheric flow in a complex terrain area of the Spanish east coast is simulated by the Regional Atmospheric Modeling System (RAMS) mesoscale numerical model using different horizontal grid resolutio ns. In this area, a grid size of 2 km is required to account for 95% of ter rain variance. Comparison among results of the different simulations shows that, although the main wind behavior does not change dramatically, some sm all-scale features appear when using a resolution of 2 km or finer. Horizon tal flow pattern differences are significant both in the nighttime, when te rrain forcing is more relevant, and in the daytime, when thermal forcing is dominant. Vertical structures also are investigated, and results show that vertical advection is influenced highly by the horizontal grid size during the daytime period. The turbulent kinetic energy and potential temperature vertical cross sections show substantial differences in the structure of t he planetary boundary layer for each model configuration.