Grid nesting in a global atmospheric model

by Lucas Harris and Shian-Jiann Lin

Manuscript submitted to Monthly Weather Review

• A global-to-regional nested-grid version of the Finite-Volume dynamical core is presented.
• Simple grid coupling methods are used for both nested-grid boundary conditions and for performing mass-conservative two-way updating.
• Despite the simplicity of the methods used, little distortion is apparent at the nested-grid boundary.
• A nested grid preserves the large-scale flow well while improving the representation of smaller-scale disturbances, such as baroclinic waves and mountain lee vortices.

While limited area models allow high-resolution simulations to be done with reasonable computational resources, they have several disadvantages for regional climate simulation and medium-to-long-range weather forecasting. Boundary conditions of the limited-area domain must be supplied from another model, which will likely have different numerics and possibly even different governing equations. Further, disturbances resolved on the limited-area domain are unable to affect their large-scale environment, which is a major concern for hurricanes and for studying the effects of orographic drag and convection on the general circulation. Global models do not have lateral boundary conditions, and are able to simulate the entire large-scale simulation consistently. However, running a uniform-resolution global model which can resolve mesoscale features is not always practical with today’s computers.

We introduce a two-way nested model, similar to the nesting used in many mesoscale models (but very rare in global models), using the Finite-Volume dynamical core. Both the coarse, global grid and the nested, limited-area grid use this core, ensuring a consistent simulation throughout. The coupling between the coarse and nested grids is deliberately kept simple, while remaining consistent with the finite-volume discretization. The nested-grid boundary condition is simple interpolation from the coarse grid, while two-way updating uses simple averages of nested-grid values to update the coarse grid. In particular, mass conservation is achieved on the coarse grid by simply not updating the mass field, in contrast to the delicate flux boundary conditions many mass-conserving nested models must use.

The nested-grid model was tested using a battery of common test cases. Despite the abrupt refinement at the nested grid boundary, the large scale flows in the test cases was well preserved in nested-grid simulations. Further, simulations of smaller-scale phenomena were improved by the nested grid, and there is evidence that the nested grid’s improved solution can improve the coarse-grid solution, even beyond the boundaries of the nested grid.

So far, two-way nested global-to-regional models have only rarely been used. Forthcoming improvements to the model, such as adding physical parameterizations and moving grids which can track features of interest, will allow this nested-grid model to be used for a range of applications, including hurricane forecasting, regional climate simulation, studying scale interactions of orography and convection, and modeling long-range pollutant transport.