New simulations reveal how vortex tubes intensify tornadoes in typhoons

The study shows that horizontal vortex tubes have a dual role in tornado evolution. They enhance rotation by transferring horizontal vorticity into vertical spin, and they strengthen upward motion by increasing vertical pressure gradient forces. Together, these processes help explain why tornadoes in typhoon rainbands can intensify rapidly.

Unlike the more studied core of typhoons, where instability and dynamics are better understood, the outer rainbands often combine atmospheric instability and vertical shear in subtle ways. This makes their tornadoes harder to detect, more sudden in onset, and more dangerous.

The Foshan tornado, rated EF-3, left a 31.7 km (19.7 miles) track and caused seven fatalities. By analyzing it in unprecedented detail, the researchers identified physical processes that could improve future warnings.

The team led by Prof. Lingkun Ran of the Institute of Atmospheric Physics at the Chinese Academy of Sciences used a large-eddy simulation within the Weather Research and Forecasting model at 49 m (161 feet) resolution. This approach allowed them to capture fine-scale turbulent processes that traditional numerical models often miss.

They rederived the vertical pressure gradient force equation to include horizontal vortex contributions. This revealed that vortex tubes directly influence both the tilting term in the vorticity equation and the vertical gradient of horizontal divergence in the pressure gradient force. The interactions amplified vertical vorticity and strengthened vertical lifting, confirming that horizontal vortex tubes are critical in the development stage of tornadoes.

The study also examined storm relative helicity, a parameter describing how inflow winds align with environmental shear. Persistent positive values were observed throughout the Foshan tornado’s lifecycle. This alignment favored tilting and stretching of horizontal vorticity, sustaining tornado intensification.

The researchers suggest that monitoring storm relative helicity in the lowest 500 m (1 640 feet) of the atmosphere could provide a practical early-warning indicator for tornadoes forming in typhoon rainbands.

Traditional forecasting models struggle to capture typhoon-borne tornadoes because they cannot resolve fine-scale vortex interactions. By showing how horizontal vortex tubes control both spin and vertical acceleration, this study provides a framework for more accurate simulations.

The ultimate goal is to translate these findings into improved operational forecasts for tornadoes in South China, where the combination of high population density and frequent typhoon activity creates heightened risk.

Accurate tornado forecasts could help reduce casualties in densely populated coastal regions.

The tornado struck on October 4, 2015, during the landfall of Super Typhoon Mujigae. It was embedded in the storm’s outer rainbands, where vertical wind shear and buoyant updrafts combined to create favorable conditions. The researchers simulated the event with five nested model domains, reaching 49 m (161 feet) horizontal resolution, and compared the results with radar observations.

They found that horizontal vortex tubes were most influential during the developing stage, when tilting and stretching sharply increased vertical vorticity. At maturity, convergence sustained the vortex even as torsion weakened.

References:

¹ Horizontal vortex tubes have a significant impact on tornado development – Institute of Atmospheric Physics, Chinese Academy of Sciences – September 24, 2025

² Investigating the outer spiral tornado mechanism in Typhoon Mujigae – Yuchen Liu et al. – Atmospheric and Oceanic Science Letters – July 2, 2025 – https://doi.org/10.1016/j.aosl.2025.100671 – OPEN ACCESS

Reet Kaur

I’m a science journalist and researcher at The Watchers, contributing to the Epicenter edition, where I cover peer-reviewed scientific research and emerging discoveries across Earth and space sciences. With a background in astronomy and a passion for environmental science, I’ve worked in shark and coral conservation in Fiji, conducting reef and shark-behavior research, contributing to mangrove restoration, and earning PADI Open Water and Coral Reef Certifications. I bring a blend of scientific rigor and storytelling to illuminate the discoveries shaping our planet and beyond.

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