Diagnosing Potential Climate Change Impacts on Recent Major Hurricanes in Variable-Resolution CAM

With a total of 17 named tropical systems and six major hurricanes, the 2017 Atlantic hurricane season was exceptionally active. Due to different combinations of strong winds, storm surge, and extreme rainfall, these powerful storms caused over $200 billion in damages and resulted in the most expensive hurricane season ever. The question addressed here is if the characteristics of these major tropical cyclones would have been different had they occurred in a pre-industrial climate. The Community Atmospheric Model is setup in a variable-resolution configuration (CAM-VR) with a base grid spacing of 100 km and a refined region over the North Atlantic ocean basin with a grid spacing of 28 km. CAM-VR is initialized with atmospheric analyses from NOAA’s Global Forecast System (GFS) 4-7 days before the time of interest (e.g., landfall) for each individual storm (e.g., Hurricane Irma). A suite of ensemble members with slightly different perturbations in their physics parameterizations is created to account for model uncertainty in storm characteristics. This ensemble of hindcasts is referred to as “the world that was” simulations. A second suite of ensembles will then be run using alternate initial conditions where the large-scale climate change signal has been removed, a “world that could have been” scenario. Through comparison of the “world that was” and “world that could have been” ensembles for each individual storm, the impact of climate change on the storm’s size, rainfall, and intensity will be quantified.