Global-to-regional grid nesting in GFDL HiRAM for very-high resolution climate simulation and storm-scale prediction

Abstract:

Forecasts at lead times greater than three days, and any regional climate simulation, require a correct and consistent simulation of the large-scale flow to be able to properly represent the small-scale phenomena desired from expensive, high-resolution models. Further, representing both the large and small scales must be done efficiently enough so as to meet an operational forecast requirement or so that multi-year climate simulations can be performed. To this end, two-way global-to-regional grid nesting has been implemented in the GFDL finite-volume cubed-sphere (FV3) dynamical core to consistently and efficiently represent the global large-scale circulation and the convection-permitting or convection-resolving scales. We demonstrate recent applications of grid nesting in the global GFDL High-Resolution Atmosphere Model (HiRAM), including simulations using nonhydrostatic dynamics. In present-day climate simulations, an 8-km nested grid over the continental US permits explicitly-resolved propagating convective systems, greatly improving the diurnal cycle of warm-season precipitation. A similar 8-km nest over the tropical Atlantic permits resolution of eyewalls and rainbands within tropical cyclones, opening the way for efficient seasonal and sub-seasonal prediction of intense hurricanes. On smaller scales, a 1.5-km nest over the central US allows simulation of intense supercell thunderstorms with realistic convective and boundary-layer structures.