The Importance of Storm-storm Interactions in the Real-time Tropical Cyclone Forecast System

In the past two decades, tropical cyclone (TC) track forecasts have been improved significantly around the world. The improvement of track forecasts can be largely attributed to advances in global model development. However, intensity forecasts from global models are still unreliable. In particular, the relatively coarse resolution is inadequate to simulate the TC inner core and its evolution. To achieve the ten-year track and intensity forecast improvement goals outlined in the Hurricane Forecast Improvement Project (HFIP), the National Oceanic and Atmospheric Administration's Hurricane Research Division (HRD) along with collaborators started developing the basin-scale Hurricane Weather Research and Forecasting (HWRF) system in 2012. With a large outer domain and nesting down to fine resolution, the basin-scale HWRF system was designed to retain the track forecast skill from global models and to improve upon the intensity forecast skill from regional models with a better representation of the vortex-scale environment.The basin-scale HWRF system is especially capable of tracking multiple storms with enhanced resolution. In this study, we investigate the role of storm-storm interactions on track and intensity forecasts using this system. Based on retrospective forecasts from four seasons (2011-14), we have identified cases for which the basin-scale HWRF and the operational HWRF have significant track forecast differences. The impacts of storm-storm interactions are diagnosed through sensitivity experiments on the initial conditions and storm vortex initialization, with a focus on how these interactions influence track and intensity forecasts. These results demonstrate that the inclusion of storm-storm interactions in the basin-scale HWRF improves both the track and intensity forecasts of TCs. The basin-scale HWRF system can simulate more realistic environmental circulations that steer the motions of TCs in addition to the thermal/dynamic structures that determine TC intensity changes. This study suggests that the basin-scale HWRF system can provide valuable guidance on track and intensity forecasts when multiple TCs present in the forecast basins.