Future tropical cyclone rainfall constrained by increased atmospheric dryness

Tropical cyclone rain rates are projected to increase under warming, yet many climate models produce weaker rain-rate responses than expected from Clausius–Clapeyron scaling, which predicts that atmospheric moisture-holding capacity increases by approximately 7% per degree of warming. Here we use large-ensemble simulations, satellite observations and reanalysis data to identify what suppresses these rain-rate increases below Clausius–Clapeyron expectations. We show that future increases in tropical cyclone rain rate are constrained by the amplified absolute dryness of the warming atmosphere, quantified by the saturation deficit. Whereas atmospheric moistening elevates column water vapour at near-Clausius–Clapeyron rates and storm intensification further boosts rain rate, these gains are systematically offset by saturation-deficit-driven reductions in precipitation efficiency through enhanced evaporation. The negative relationship between precipitation efficiency and column saturation deficit is corroborated by satellite-based precipitation observations. Although precipitation efficiency correlates positively with storm intensity, attribution analyses reveal that increases in column saturation deficit can dominate intensity effects under warming, leading to net declines in precipitation efficiency and smaller rain rates. Our results highlight increased absolute dryness as the principal thermodynamic limiter of future tropical cyclone rain rate, uncovering a mechanism overlooked by prior projections focused solely on storm intensification and moisture increases. Climate model ensembles suggest tropical cyclones will bring more rain as the world warms, but increases may be smaller than expected. Atmospheric dryness reduces how much rain storms produce, offsetting extra moisture and stronger storms.