Abstract
Abstract. Locally refined grids for global atmospheric models are attractive since they are expected to provide an alternative to solve local phenomena without the requirement of a global high-resolution uniform grid, whose computational cost may be prohibitive. Spherical centroidal Voronoi tessellation (SCVT), as used in the atmospheric Model for Prediction Across Scales (MPAS), allows a flexible way to build and work with local refinement. In addition, the Andes Range plays a key role in the South American weather, but it is hard to capture its fine-structure dynamics in global models. This paper describes how to generate SCVT grids that are locally refined in South America and that also capture the sharp topography of the Andes Range by defining a density function based on topography and smoothing techniques. We investigate the use of the mimetic finite-volume scheme employed in the MPAS dynamical core on this grid considering the nonlinear classic and moist shallow-water equations on the sphere. We show that the local refinement, even with very smooth transitions from different resolutions, generates spurious numerical inertia–gravity waves that may even numerically destabilize the model. In the moist shallow-water model, wherein physical processes such as precipitation and cloud formation are included, our results show that the local refinement may generate spurious rain that is not observed in uniform-resolution SCVT grids. Fortunately, the spurious waves originate from small-scale grid-related numerical errors and can therefore be mitigated using fourth-order hyperdiffusion. We exploit a grid geometry-based hyperdiffusion that is able to stabilize spurious waves and has very little impact on the total energy conservation. We show that, in some cases, the clouds are better represented in a variable-resolution grid when compared to a respective uniform-resolution grid with the same number of cells, while in other cases, grid effects can affect the cloud and rain representation.
Highlights
The Andes, the world’s longest mountain range, located along entire South American western coast, act as an obstacle to atmospheric circulation and play a key role in the weather of South America
When we compare the potential vorticity at day 7 for the grids variable-resolution grid of level 7 (VR7) and UR7, both of level 7, with the UR8 grid, we can observe that the potential vorticity in the VR7 grid (Fig. 14c), considering numerical hyperdiffusion, better represents the vortex formed near the Andes region than in the uniform-resolution grid with the same number of cells (Fig. 14d); i.e., the vortex formed near the Andes region in the VR7 grid with numerical fourth-order hyperdiffusion is more similar to the vortex formed in the same area in the UR8 grid (Fig. 14e) compared to the vortex generated in the UR7 grid
We developed spherical centroidal Voronoi tessellation (SCVT) grids with local refinement based on topography that refines and captures the Andes Range well and is smoothly transitioned to a circular grid region centered in South America
Summary
The Andes, the world’s longest mountain range, located along entire South American western coast, act as an obstacle to atmospheric circulation and play a key role in the weather of South America. The finitevolume method for the shallow-water equations proposed by Thuburn et al (2009) and Ringler et al (2010), hereafter named TRSK, was designed to work on arbitrary orthogonal Voronoi C-grids This scheme has many desirable mimetic problems, such as mass and total energy conservation, as well as preservation of stationary geostrophic modes on the f sphere, among others. Our locally refined grids will be designed to capture the Andes and the South American continent well with smooth transitions to the coarse grid regions, focusing on obtaining a grid that may allow better precipitation forecasts in South America.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
