Investigating How Tropical Cyclone Expansion Rate Changes With Warming

The size of a tropical cyclone (a.k.a. hurricane) leading up to landfall determines the footprint of its hazards, including wind, storm surge, inland flooding, and tornadoes. Hence, understanding how storm size responds to warming is important for both forecasting storm impacts and for assessing long-term risk. Tropical cyclones have been shown in observations and models to be intensifying more rapidly with warming, which makes forecasting and emergency planning more difficult. Could the tropical cyclone wind field expand more quickly in a warmer world, too? This outcome is predicted by recent theory, but it has yet to be tested in observations or real-world models. This represents a critical gap in our knowledge that is directly relevant to future changes in societal risk, as more rapid expansion rates would make storm impacts even more difficult to forecast and mitigate in the future. The outcomes of this work will help improve forecasts of tropical cyclone hazards needed for communities to prepare and evacuate ahead of a landfalling storm, and it will help improve our understanding of how the risk of impacts may change regionally as the climate warms. This project systematically investigates how the expansion rate of tropical cyclones on Earth depends on sea surface temperature in three settings: 1) in historical data; 2) in existing climate model simulations under both real-Earth conditions and idealized aquaplanet conditions; and 3) in simplified limited-area cloud-permitting model simulation experiments. The first two tasks quantify the dependence in the real-world, including both global-mean warming and regional warming relative to the mean. The third task more carefully tests the underlying physics of the outcomes of the first two tasks to raise our understanding of the fundamental mechanisms of tropical cyclone size expansion under climate warming. This work seeks to bridge the gap between theory and the real-world need to better forecast tropical cyclones and their impacts. The project explicitly foregrounds real-world observational and climate-model data in order to make the results as usable to real-world interests as quickly as possible. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.