This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Under global warming, scientists have widely expected tropical cyclones (including typhoons and hurricanes) to bring more intense and frequent rainfall. The underlying physics seems intuitive: Rising temperatures allow the atmosphere to hold more moisture, which, combined with intensifying storms, should theoretically trigger more destructive downpours.
However, when researchers analyze climate model projections, they encounter a puzzling phenomenon. Some models project rainfall increases far lower than thermodynamics alone would predict. This uncertainty has long hindered the scientific community's ability to accurately project future tropical cyclone precipitation and assess associated flood risks.
Recently, a new study led by the University of Hong Kong (HKU) and Imperial College London (ICL) uncovered a key missing piece of the puzzle: increasing atmospheric dryness. Published in Nature Geoscience, the work reveals that while a warmer atmosphere can indeed hold more moisture, it also becomes drier in a way that suppresses rainfall—effectively acting as a "brake" on tropical cyclone precipitation. The team, consisting of Professor Dazhi Xi and Dr.
Jianan Chen from the HKU Department of Earth and Planetary Sciences, and Professor Ralf Toumi from ICL, analyzed climate simulations, satellite observations and reanalysis data. They found that as the climate warms, tropical cyclones become less efficient at converting moisture into rainfall. The team pointed out that the key lies in a physical mechanism known as the "column saturation deficit"—the gap between the actual amount of water vapor in the atmosphere and its level at complete saturation (the threshold for precipitation).
Generally, rainfall occurs as water vapor condenses into cloud droplets, coalesces into raindrops and falls to the ground. In a warming climate, however, the atmosphere's moisture-holding capacity increases exponentially. Consequently, even if relative humidity remains constant, the gap to "complete saturation" widens significantly, meaning the air becomes substantially drier.
This dryness can trigger two effects: This constraining effect of atmospheric dryness is potent enough to offset the rainfall increases driven by storm intensification. This offers a robust physical explanation for why many climate models project rainfall increases that are consistently lower than traditional theoretical calculations. The study also proposes a unified framework for understanding tropical cyclone rainfall.
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