Future Changes in Atlantic Tropical Cyclone Rainfall and Size in Convection-Permitting Regional Model Simulations

Heavy rainfall is one of the greatest threats associated with landfalling tropical cyclones (TCs), causing flooding that accounts for many TC-related fatalities. TC rainfall is projected to increase in a warmer future climate, although the magnitude and whether its scaling with temperature will exceed the theoretical Clausius-Clapeyron (CC) rate remains uncertain. In this study, we investigated ten TC events that impacted the United States and Caribbean Islands. We performed four-member ensembles of convection-permitting regional model simulations for each TC under historical conditions and under a high-emissions scenario for the mid-21^st century to quantify future changes in TC rainfall and its scaling with temperature. We found that future TC rainfall in a reference frame following the TC increased by up to ~1.5 times the historical for all ten TCs considered, while peak 10-meter wind speeds increased by 5–22 kts. Future rainfall increased at super-CC rates of 8–19% K^-1 for nine of the ten TCs. We then evaluated environmental moisture, vertical velocity, wind-based TC size, and TC translation speed to identify potential causes for super-CC rainfall increases. For environmental moisture, we found that mean precipitable water increased at near-CC rates for seven of ten TCs, and alone did not explain the future TC rainfall changes. For vertical velocity, the mean 700 hPa omega indicated increased ascent in the future climate by 3-58% for eight of ten TCs when over land and by 4-45% for all ten TCs over ocean. In addition, seven of ten TCs experienced slowdowns in median translation speed in the future, which can contribute to increased regional TC precipitation. For TC size, we found that the peak radius of maximum wind decreased for nine of ten TCs by 1-40%. Our results suggest that TCs in a mid-21^st-century climate may pose an enhanced flooding risk, with rainfall enhancements that exceed simple theoretical thermodynamic estimates.