Study Finds Atmospheric Dryness Limits How Much Typhoon and Hurricane Rainfall Increases

Climate scientists have long expected global warming to bring heavier rainfall from typhoons and hurricanes, since a warmer atmosphere can hold more moisture. But climate model projections have shown a puzzle: many models predict rainfall increases far smaller than basic thermodynamics alone would suggest.

A study published in the journal Nature Geoscience, led by researchers at the University of Hong Kong (HKU) and Imperial College London (ICL), says it has identified a missing piece: increasing atmospheric dryness. The team, including Professor Dazhi Xi and Dr. Jianan Chen of HKU's Department of Earth and Planetary Sciences and Professor Ralf Toumi of ICL, analyzed climate simulations, satellite observations and reanalysis data.

The researchers point to a mechanism called the column saturation deficit, the gap between the actual water vapor in the air and the amount needed for complete saturation, the threshold at which rain forms. As the climate warms, the atmosphere's capacity to hold moisture rises exponentially, so even when relative humidity stays constant, this gap widens, making the air effectively drier. That dryness evaporates raindrops as they fall through the lower and middle troposphere, the atmospheric layer nearest Earth, before they can reach the ground, and it dilutes moisture drawn into a storm's updraft, suppressing cloud and rain formation at the source.

Together, the two effects act as a brake strong enough to offset rainfall increases driven by storm intensification, the study says, which explains why many climate models project smaller rainfall increases than theory alone predicts. The researchers propose a framework in which tropical cyclone rainfall depends on storm intensity, atmospheric moisture and precipitation efficiency, how effectively that moisture converts into rain, with storm intensity boosting efficiency and atmospheric dryness suppressing it.

The study notes that global climate models do not fully capture some fine-scale atmospheric processes, and says future high-resolution simulations will be needed to refine rainfall projections.