Â鶹ÒùÔº

When people hear about hurricanes, they often focus on the category rating: Category 1 through 5, based on maximum wind speeds. But not all hurricanes with the same wind speeds are alike. Some are compact storms while others can span the size of entire states. Larger hurricanes bring far greater damage, generating wider footprints of high winds, heavier rainfall and more dangerous storm surge.

A new study led by Purdue University researchers has uncovered why some hurricanes grow significantly larger than others and why this growth occurs rapidly under certain ocean conditions. The research shows, for the first time, that hurricanes grow in size much faster when traveling over locally warm waters where the ocean surface is significantly warmer than the rest of the tropical oceans.

"This discovery can be put directly into use for daily forecasting of hurricane size and impacts," said Danyang Wang, postdoctoral researcher in Purdue's Department of Earth, Atmospheric, and Planetary Sciences (EAPS). "It can also be used to better model hurricane size in long-term risk models used by industry to evaluate property risks."

The discovery, led by Wang with guidance from professor Dan Chavas of Purdue's EAPS department, was in the Proceedings of the National Academy of Sciences (PNAS).

Wang developed the underlying theory, extracted and analyzed data from historical records and climate simulations, and wrote the manuscript. Chavas provided high-level feedback on how to connect the theory to real-world storms.

They were joined by collaborator Ben Schenkel, a research scientist at the Cooperative Institute for Severe and High-Impact Weather Research and Operations at the University of Oklahoma. Schenkel provided a tropical cyclone size database used in the analysis and helped clarify results across multiple datasets.

Before this work, scientists knew that some hurricanes expanded significantly during their lifetimes while others stayed compact. But the factors behind that difference were not well understood. Wang and Chavas showed that the rapid growth of storms is tied to "hot spots" in the ocean. These are localized areas where the water is significantly warmer than the surrounding tropical waters.

The results also suggest a surprising silver lining in a warming world. The study found that hurricane size growth rates do not change much with global mean warming, though global temperatures continue to rise.

The 2024 Atlantic hurricane season gave a striking example of why storm size matters. Hurricane Helene expanded rapidly before making landfall, ballooning into one of the largest storms in U.S. history at an estimated width of over 400 miles and causing unprecedented damage.

"Two hurricanes with the same maximum wind speed can be two very different sizes," Wang said. "But think of one donut the size of South Carolina and another the size of Texas."

Chavas compared the process to popcorn kernels in a pan. "The hurricanes see the tropical ocean like popcorn heated on an uneven pan—turning up the heat everywhere may make them pop a little faster, but it's over the hot spots where the hurricanes will pop the fastest."

Modern satellites provide high-quality, daily estimated measurements of sea surface temperatures worldwide. By applying this new understanding of how hurricanes respond to local ocean hot spots, forecasters may be able to better predict how large storms will become at landfall.

"A larger storm has a larger footprint of damaging winds, generates higher storm surge and over a larger area, and produces more rainfall—all greater risks to society," Wang said. "Better predictions of storm size at landfall translate to better predictions of the hazards that pose risks to life and property."

The Chavas lab at Purdue specializes in understanding extreme weather, from tropical cyclones to severe thunderstorms and tornadoes. Wang focuses on the physics of hurricane structure, particularly storm size.

The team tapped into Purdue's Rosen Center for Advanced Computing, which gave them the ability to analyze global data in fine detail and uncover patterns that would have been impossible to see otherwise. These resources helped ensure that their findings about tropical cyclone growth are both accurate and comprehensive.

They also used the National Center for Atmospheric Research's Cheyenne and Derecho supercomputers, some of the fastest in the world, to run experiments that mimic how storms behave in different warming scenarios. This powerful combination of Purdue and NCAR computing resources let the researchers explore what-if questions about our climate and deliver insights that can improve forecasts and preparedness for future storms.

The findings pave the way for improvements in both daily storm forecasting and long-term risk assessment used by industries such as insurance and infrastructure planning. The research also highlights the importance of integrating theoretical science with high-resolution data and advanced computing power.