Towards Global Large Eddy Simulation: Super-Parameterization Revisited

抄録

　This paper argues that a global large eddy simulation can be achieved though the application of the super-parameterization (SP) methodology on massively parallel computers. SP was proposed over 15 years ago to improve representation of deep convection and accompanying small-scale processes in large-scale models of weather and climate. The main idea was to embed in all columns of the large-scale model (featuring horizontal gridlengths of the order of 100 km) a two-dimensional convection-permitting small-scale model with about 1 km horizontal gridlength and periodic lateral boundaries. We propose to expand this methodology by applying a high-spatial-resolution three-dimensional (3D) large-eddy simulation (LES) model as the SP model, and by embedding it in all columns of a large-scale model with the horizontal gridlength in the range of 10 to 50 km. The outer model can apply hydrostatic equations as typical global numerical weather prediction and climate models today, and can simulate atmospheric processes down to the mesoscale, including organized convection. Small-scale processes, such as boundary layer turbulence and convective drafts, can be simulated by embedded nonhydrostatic (e.g., anelastic) LES models. Although significantly more expensive than the traditional SP, the SP LES is ideally suited to take advantage of parallel computation because of the minimal communication between LES models when compared to traditional domain-decomposition methodologies in parallel simulation. Moreover, as illustrated through the idealized 2D mock-Hadley cell simulations, LES models can feature different horizontal and vertical grids in various columns of the large-scale model and thus target dominant cloud regimes in various geographical regions. Such a system allows an unstructured-grid simulation with no additional model development.