Building a new atmospheric model: testing a nonhydrostatic dynamical core on unstructured variable resolution hexagonal C-grids

Spherical centroidal Voronoi tessellations (SCVTs) are proving themselves a viable alternative to latitude-longitude grids for numerical weather prediction but require the development of new numerical schemes. SCVT-based variable-resolution grids are another area of research interest which can help produce more efficient runs. These, along with increasing computer resources, have the potential to make regional and global atmospheric models more robust. However, finer grids mean the hydrostatic approximation traditionally used for global modeling is no longer valid. In this research, we test an approach to discretizing the nonhydrostatic equations developed for rectangular meshes that has been adapted to planar, hexagonal (SCVT) meshes. This discretization employs a split-explicit time integration method in addition to a finite-volume spatial discretization. An initial simulation of a squall line using a constant-resolution grid is performed to verify the validity of the dynamical core. The core is then tested on variable-resolution unstructured SCVTs. The tests performed on the variable-resolution grids include additional squall line simulations and gravity current simulations. Final results from variable-resolution grids show a notable improvement over using grid nesting to simulate fine-resolution regions in coarse domains. The results suggest two important areas for further research. First, the impact of variable-resolution grids on solution accuracy must be further studied, as the transition region of a variable grid can impact accuracy if it is abrupt. Second, diffusion can have a considerable impact on solutions, and model filter formulations need further study. These preliminary results are promising and suggest further research.