Impacts of HAFS physics sensitivities on track, intensity and structure prediction of hurricane Ian

There is a significant gap in our understanding of the physical processes in tropical cyclones, which results in inaccurate representations of these processes in numerical weather prediction (NWP) models and, consequently, forecast uncertainties. To mitigate these uncertainties, forecasters rely on multiple models to understand the range of possibilities and enhance the accuracy of their guidance. Conducting physics sensitivity experiments in these NWP models is crucial for identifying model biases and improving probabilistic forecasts. Additionally, these studies provide valuable insights into how changes in physical parameters influence the evolution of tropical cyclones.

The Hurricane Analysis and Forecasting System (HAFS) is NOAA’s next-generation, ocean-atmosphere-coupled multi-scale numerical model and data assimilation system. HAFS is expected to provide reliable and skillful guidance of Tropical Cyclone (TC) track and intensity (including rapid intensification, storm size, genesis, storm surge, rainfall, and tornadoes associated with TCs, all within the framework of the Unified Forecast System (UFS). Two regional versions of HAFS with a parent domain at 6-km and storm-following nest at about 2-km horizontal resolution to cover a TC’s inner core and its immediate surrounding were implemented in operations in 2023 and upgraded to V2 in 2024. In order to provide diversity, each version ran with different microphysics and cumulus physics parameterization schemes.

Hurricane Ian (2022) originated from a tropical wave in the Eastern Caribbean and became a tropical depression on September 23. It strengthened into Tropical Storm Ian the next day southeast of Jamaica. Rapid intensification began on September 25 due to a ridge to the southwest. By September 26, Ian moved towards Cuba, benefiting from warm waters and low vertical wind shear. It reached Category 5 strength with 160 mph (260 km/h) winds on September 28, making landfall just below peak intensity on Cayo Costa Island in Southwest Florida. There were significant uncertainties in Ian's track and intensity forecasts, particularly on September 24, varying intensities of Hurricane Ian result in different interactions with the nearby synoptic scale environment, thereby modulating the steering currents that influence the storm's movement and challenges in predicting the landfall location. To address these uncertainties, this study used a combination of physics parameterization schemes to create a mini-ensemble of HAFS simulations, aiming to better understand the forecast uncertainties associated with Hurricane Ian.