Structure and intensity changes in Hurricanes Ophelia and Katrina (2005): Coupled atmosphere-ocean modeling and RAINEX observations

Intensity and intensity change remain one of the most challenging issues in tropical cyclone (TC) prediction. Complex interactions between the storm and its atmosphere-ocean environment are difficult to both observe and model. Hurricanes Ophelia and Katrina (2005) were excellent examples of how these complex interactions affect changes in storm structure and intensity. The large-scale environmental flow and upper ocean conditions were distinct for the two storms and facilitated unique interactions between the storms and their environments. The storm structure and intensity were well observed by unprecedented airborne Doppler radar on board the NOAA and U.S. Navy P3 aircraft during the Hurricane Rainband and Intensity Change Experiment (RAINEX) in 2005. In this study, we investigate the storm-environment interaction and structure and intensity change using a high-resolution coupled atmosphere-wave-ocean model and RAINEX observations. Hurricane Ophelia developed from a weak tropical storm east of the Florida coast and became a slow-moving TC with weak atmospheric steering flow. It induced a strong upper ocean cooling that remained close to the inner core of the storm, which may have contributed to an unusually shallow vertical structure and asymmetry in convection. By contrast, Katrina went over the warm eddy and loop current in the Gulf of Mexico with a translation speed faster than Ophelia. It intensified into a major hurricane with deep and symmetric inner core. The RAINEX observations are used to evaluate and verify the University of Miami Unified Wave Interface-Coupled atmosphere-wave-ocean Model (UWIN-CM) simulations of both storms. The atmosphere, surface wave, and ocean circulation component models in UWIN-CM are the Weather Research and Forecasting (WRF) model, the University of Miami Wave Model (UMWM), and the Hybrid Coordinate Ocean Model (HYCOM). The model is configured with nested grids of WRF with 12-, 4-, and 1.3 km grid spacing and 44 vertical levels. Both UMWM and HYCOM use 4-km grid spacing over the entire UWIN-CM domain. The initial and lateral boundary conditions for UWIN-CM are from the 6-hourly 0.5° resolution NCEP and the daily 1/12 degree global HYCOM analysis fields. In addition to storm structure observation from RAINEX, the coupled model simulations will also be evaluated by satellite observations including SST, surface winds, and other fields.