Impact of two-way interactive nesting on high-resolution hurricane simulations

High resolution is required for a successful numerical simulation of extreme weather events such as hurricanes and tornados. Despite advances in computer speed and memory, it is still impractical to run a numerical weather prediction (NWP) model with a very high resolution on all scales for providing real-time forecasts, given limited resources. Therefore, grid nesting technology is widely used in the NWP models to make simulations and forecasts to a reasonable level of accuracy using static or moveable high resolution nests embedded in a relatively coarser large-scale domain. Accurate specification of the interactions between nest and parent grids is critical to resolve the effects of multi-scale processes on simulations of significant weather events of interest. We have made several numerical experiments using National Centers for Environmental Prediction (NCEP) operational regional Hurricane Weather and Forecast model (HWRF) to illustrate the importance of the nest-parent grid interactions on hurricane forecast skills. Using the operational HWRF configured with a parent grid (27 km resolution) and two telescopic, movable and two-way interactive nested grids (9km and 3km resolutions), several land-falling hurricanes have been simulated with different weights for the feedback between nest and parent grids. Results show that simulations with full two-way nesting (100% feedback) produced better hurricane track and intensity forecasts and improved large-scale flow than those with one-way nesting (0% feedback). This implies the importance of multi-scale interactions of dynamic and physical processes in hurricane simulations. To further analyze the importance of nest-parent interactions on hurricane simulations, we simulated a hurricane using 3km resolution (a full cloud resolving mode) over the entire parent domain of the operational HWRF with identical initial and boundary conditions to the 3-domain run, and obtained detailed structure of the simulated hurricane. The hurricane structure from the two-way nested run with 100% feedback is much closer to the full cloud resolving run than that from the one-way nested simulation in terms of size, intensity, structure and microphysical features such as simulated radar reflectivity and precipitation. Additional experiments were also made to investigate effects of the nest domain size on the simulations of different types of hurricanes (weak vs strong, and small and big size). Results from these experiments suggest that the two-way interactive nests could provide a more practical approach for the next generation global models to provide more accurate high-resolution non-hydrostatic forecast solutions for significant weather events anywhere in the world.