Configuration and evaluation of a global unstructured mesh atmospheric model (GRIST-A20.9) based on the variable-resolution approach

Abstract

Abstract. Targeting a long-term effort towards a variable-resolution (VR) global weather and climate model, this study systematically configures and evaluates an unstructured mesh atmospheric model based on the multiresolution approach. The model performance is examined from dry dynamics to simple physics and full physics scenarios. In the dry baroclinic wave test, the VR model reproduces comparable fine-scale structures in the refined regions as a fine-resolution quasi-uniform (QU) mesh model. The mesh transition zone does not adversely affect the wave pattern. Regional kinetic energy spectra show that the fine-scale resolving ability improves as the fine resolution increases. Compared to a QU counterpart that has equivalent degrees of freedom, the VR model tends to increase the global errors, but the errors can be reduced when the resolution of the coarse region is increased. The performance over the coarse region is generally close to that of a low-resolution QU counterpart. Two multi-region refinement approaches, the hierarchical and polycentric refinement modes, further validate the model performance under the multiresolution refinement. Activating hyperdiffusion for horizontal velocity is helpful with respect to VR modeling. An idealized tropical cyclone test is further used to examine its ability to resolve fine-scale structures. In the simple physics environment, the VR model can have the tropical cyclone stably pass the transition zone in various configurations. A series of sensitivity tests examines the model performance in a hierarchical refinement mode. The simulations exhibit consistency even when the VR mesh is slightly perturbed by one of the three parameters that control the density function. The tropical cyclone, starting from the second refinement region and passing through the inner transition zone, gets intensified and covers a smaller area in the refined regions. Such variations are consistent with the behavior that one may observe when uniformly refining the QU mesh. In the full physics environment with a highly variable mesh that reaches sub-10 km resolution, the VR model also produces a reasonable evolution for the tropical cyclone. The explicit diffusion shows its usefulness in terms of suppressing some unrealistic isolated-scale structures that are far away from the initial vortex and does not adversely affect the physically important object. The fine-scale structure is determined mainly by the fine-resolution area, although the systems may have larger differences before they move into the fine-resolution area. Altogether, this work demonstrates that the multiresolution configuration is a reliable and economic alternative to high-resolution global modeling. The adverse impact due to mesh transition and the coarse region can be controlled well.

Topography-based local spherical Voronoi grid refinement on classical and moist shallow-water finite-volume models

“…A fourth-order hyperdiffusion is necessary to stabilize the velocity field error. These results are consistent with Zhou et al (2020), wherein the authors show that the employment of horizontal hyperdiffusion removes numerical noise in the transition zone of a VR SCVT grid.…”

“…Finally, the grids developed in this work, which are available in the open repository https://github.com/pedrospeixoto/ imodel (last access: 11 November 2021), can be applied directly in the MPAS model or other models that use unstructured grids, and their impact may be investigated considering the full three-dimensional model, along similar lines as Zhou et al (2020). From one side, we expect that the unstable modes could be challenging to mitigate due to other already existing unstable modes (e.g., Peixoto et al, 2018).…”

“…This maximum is caused by the use of the VR grid and is not observed in the uniform-resolution grid. Also, in the absence of hyperdiffusion, Zhou et al (2020) report gridscale oscillations in a variable-resolution grid considering baroclinic wave and tropical cyclone test cases. While MPAS allows a model that is suitable to work with grids with local refinement in arbitrary shapes by only defining a density function, wave representation and interaction in these grids are still not well understood.…”

AMIP Simulations of a Global Model for Unified Weather‐Climate Forecast: Understanding Precipitation Characteristics and Sensitivity Over East Asia

“…Some preliminary simulations from two distinctively different packages were reported by Zhou et al. (2020) and Li et al. (2020).…”

Multiscale Simulation of Precipitation Over East Asia by Variable Resolution CAM‐MPAS

“…Recently, variable-resolution global climate models (VR-GCMs) with unstructured grids have been extensively tested for high resolution modeling at grid spacing comparable to those of RCMs over specific regions while a coarser resolution is used away from the refinement region to reduce the number of grid points. Previous studies have shown that VR-GCMs can produce comparable results with high-resolution GCMs at lower computational cost (Gettelman et al, 2018;Huang et al, 2016;Tang et al, 2019;Wu et al, 2017;Zhou et al, 2020). Rahimi et al (2019) found that the VR-GCM with 1/8° resolution refined over TP can alleviate precipitation biases over TP compared with the 1° resolution counterpart.…”