A GRID NESTING METHOD FOR LARGE-EDDY SIMULATION OF PLANETARY BOUNDARY-LAYER FLOWS

A method for performing nested grid calculations with a large-eddy sim ulation code is described. A common numerical method is used for all m eshes, and the grid architecture consists of a single outer or coarse grid, and nested or fine grids, which overlap in some common region. I nter-grid communication matches the velocity, pressure and potential t emperature fields in the overlap region. Resolved and sub-grid scale ( SGS) turbulent fluxes and kinetic energy on the fine grid are averaged to the coarse grid using a conservation rule equivalent to Germane's identity used to develop dynamic SGS models. Simulations of a slightly convective, strong shear planetary boundary layer were carried out wi th varying surface-layer resolutions. Grid refinements in the (x, y, z ) directions of up to (5, 5, 2) limes were employed. Two-way interacti on solutions on the coarse and fine meshes are successfully matched in the overlap region on an instantaneous basis, and the turbulent motio ns on the fine grid blend smoothly into the coarse grid across the gri d interface. With surface-layer grid nesting, significant increases in resolved eddy fluxes and variances are found. The energy-scale conten t of the vertical velocity, and hence vertical turbulent fluxes, appea r to be most influenced by increased grid resolution. Vertical velocit y spectra show that the dominant scale shifts towards higher wavenumbe rs (smaller scales) and the magnitude of the peak energy is increased by more than a factor of 3 with finer resolution. Outside of the neste d region the average heat and momentum fluxes and spectra are slightly influenced by the fine resolution in the surface layer. From these re sults we conclude that fine resolution is required to resolve the deta ils of the turbulent motions in the surface layer. Al the same time, h owever, increased resolution in the surface layer does not appreciably alter the ensemble statistics of the resolved and SGS motions outside of the nested region.