Distinct Structure and Intensity of Hurricanes Katrina and Ophelia (2005) in Coupled WRF-HYCOM Model

Although many previous studies have shown that a large area of warm water provides a favorable condition for intensification of hurricanes, storm development and intensity over the warm oceans varies in a broad range. Various factors contribute to the storm intensity including the atmospheric environmental conditions and internal dynamic of each individual storm. In the 2005 hurricane season, Hurricanes Ophelia and Katrina both developed over the warm water near the east coast of South Florida. However, they evolved differently with a distinct structure and intensity. Katrina became one of the most intense Category 5 hurricanes in the Gulf of Mexico, whereas Ophelia remained a relatively weak Category 1 hurricane over several days near the Gulf Stream. In this study, we aim to investigate the relationship between the hurricanes and their environmental conditions over the warm water. We focus on analyzing the structures of the two hurricanes to shed light on why they both move over the warm water, one intensified quickly and the other did not. To best resolve the storm structure, we use a high-resolution, coupled atmosphere-ocean model. The components of the coupled model system are the non-hydrostatic Weather Research and Forecasting (WRF) model with 1.33 km grid horizontal spacing on the finest nested mesh and the HYbrid Coordinate Ocean Model (HYCOM). The WRF model includes the vortex-following nested grids similar to that developed at the University of Miami and tested in various hurricane studies. There are 4 nested domains for the simulation of Hurricane Katrina and 3 for Hurricane Ophelia. The initial and lateral boundary conditions of WRF for Katrina are from the NCAR's ensemble Kalman filter (EnKF) data assimilation, and for Ophelia are from the 1°×1° National Centers for Environmental Prediction (NCEP) global analysis fields at 6-h intervals. The extensive observations from the Hurricane Rainbands and Intensity Change Experiment (RAINEX) are used to evaluate and validate the model simulations. Multiple airborne radar reflectivity and 3D Doppler wind observations during both hurricanes provided unprecedented data sets for this high-resolution modeling study. A comprehensive analysis of model output in comparison with RAINEX observations will help to understand the physical and dynamic processes that contributed to the distinct evolution of the two hurricanes.