Tropical cyclone rainfall is shaped by storm intensity, atmospheric moisture and how efficiently that moisture is converted into rain. Illustration for conceptual purposes only.
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 analyse climate model projections, they encounter a puzzling phenomenon: some models project rainfall increases that are far lower than what 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) has 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.
Widening Atmospheric Unsaturation Obstructs Cloud Formation and Accelerates Evaporation
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, analysed 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 vapour in the atmosphere and its level at complete saturation (the threshold for precipitation). Generally, rainfall occurs as water vapour condenses into cloud droplets, coalesces into raindrops, and falls to the ground. Under 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 possibly trigger two effects:
- Pre-landing Evaporation: Raindrops that condense at high altitudes in a typhoon are rapidly evaporated by dry air in the lower and middle troposphere during their descent, preventing them from reaching the ground.
- Inhibition of Condensation: As dry environmental air is entrained into the typhoon's updraft, it dilutes the moisture supply, suppressing cloud and rain formation at the source.
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.
Pioneering a "Unified Assessment Framework"
The study also proposes a unified framework for understanding tropical cyclone rainfall. It shows that rainfall depends not only on storm intensity and the amount of water vapour in the atmosphere, but also on precipitation efficiency—how efficiently that moisture is converted into rain. Two opposing effects in a warming climate shape this efficiency: greater storm intensity tends to boost it, while increased atmospheric dryness tends to suppress it. Although atmospheric dryness dominates in some climate models, this framework does not rule out an increase in precipitation efficiency if future storm intensification outweighs the suppressive effect of atmospheric dryness.
The findings could have important practical implications. For coastal communities, disaster managers, and infrastructure planners, more accurate projections of rainfall from future hurricanes and typhoons are critical for flood protection, evacuation planning, and climate resilience. By accounting for the effect of atmospheric dryness, the new framework could improve rainfall and flood-risk assessments and support better-informed climate adaptation planning.
The study also notes that global climate models do not fully capture some fine-scale processes. Future high-resolution simulations will therefore be needed to refine the projections. Nevertheless, multiple datasets and models consistently show that greater atmospheric dryness reduces rainfall efficiency. This robust negative correlation underscores that atmospheric dryness is a critical thermodynamic constraint that must be incorporated into future climate projections.
For details of the research, please refer to the journal paper "Future tropical cyclone rainfall constrained by increased atmospheric dryness".
