A Global-Regional-Unified Atmospheric Dynamical Core On The Yin-Yang Grid

A global-regional-unified nonhydrostatic dynamical core was constructed on the Yin-Yang grid using a semi-implicit semi-Lagrangian solver.Arbitrary coordinate rotation was possible for both global and limited-area models with a multi-level nesting capability.Significant flexibility is available when configuring the model’s horizontal mesh using coordinate rotation.The equations of the dynamic core considers the Coriolis force component in the three-dimensional direction,ensuring that all grids use the same equations,and the equations does not need to change before and after coordinate rotation.Using bi-cubic Lagrangian interpolation in space and temporal interpolation to determine the boundary conditions of nested grids,the coefficients of the Helmholtz equation need to be redefined for nested grids and finite region models.The performance of the dynamical core was assessed using a series of numerical tests in spherical coordinates.The results illustrate the reasonable ability of the piecewise rational method to manage sharp gradient advection and the capability of the dry dynamical core to simulate fine structures in target systems in the spherical configurations.An average convergence rate of 2.43 was confirmed for the dry dynamical core in the balanced flow test.The nested grid improved the fine structure simulation of the baroclinic wave development without affecting wave propagation.The propagating speed of the vortex remained unchanged in the nested grid in the colliding modons test,although the vorticity amplitude decayed more slowly than that in the coarse grid.Proper development of topographic waves was achieved for both large-and small-scale mountains with a clear damping effect in association with the off-center semi-implicit average.A deepened trough in the nesting region,in comparison with that in the parent grid,was simulated in a topographic Rossby wave test.Nested grids can analyze more refined features in real-case simulations.Triple nesting showed good performance in a rainstorm case caused by the northeast cold vortex.The nested grids of each layer can roughly capture the distribution of rain areas.With the improvement of resolution,the precipitation area and magnitude are closer to observations.In the 720 extremely heavy rainstorm test,the high-resolution nested results can capture more detailed precipitation characteristics,and the long wave radiation shows the model’s ability to generate small and medium-sized features.The above results indicate that the implementation of coordinate rotation and grid nesting improved the numerical performance when managing atmospheric dynamical problems at relatively low cost.