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What causes the rapid intensification of hurricanes?

Hurricanes with the ability to rapidly intensify under unknown conditions have long been a cause for concern. Swift escalations from simple tropical storms or depressions to major hurricanes can be catastrophic, given the violent winds and deadly surges they bring. 

However, new research suggests that hurricane unpredictability is due to not just one, but two distinct mechanisms that cause rapid intensification. 

This is the conclusion of a computer modeling study conducted by scientists from the National Science Foundation (NSF) and the National Center for Atmospheric Research (NCAR).

Study significance 

The knowledge of two unique modes associated with hurricane intensification will enhance our understanding and prediction of these hazardous events.

“Trying to find the holy grail behind rapid intensification is the wrong approach because there isn’t just one holy grail,” said NCAR scientist Falko Judt, lead author of the new study. “There are at least two different modes or flavors of rapid intensification, and each one has a different set of conditions that must be met in order for the storm to strengthen so quickly.”

Modes of rapid intensification

The first type is when a hurricane intensifies symmetrically due to optimal environmental conditions, such as warm surface waters and low wind shear. Historical examples of this type include Hurricanes Andrew, Katrina, and Maria. 

The most recent example is Hurricane Otis, which unexpectedly grew by 110 miles per hour within a day, evolving into a category 5 hurricane before hitting Mexico’s west coast.

The second type, which previously went unnoticed due to the peak winds not being as destructive, is linked to significant bursts of thunderstorms distant from the hurricane’s center. 

These bursts rearrange the cyclone’s circulation, enabling it to rapidly amplify, often reaching category 1 or 2 intensity in mere hours. Astonishingly, this form of intensification can occur even under unfavorable conditions, such as opposing upper-level winds.

“Those storms are not as memorable and they’re not as significant. But forecasters need to be aware that even a storm that’s strongly sheared and asymmetric can undergo a mode of rapid intensification,” said Judt.

Unexpected discovery 

Judt’s discovery emerged unexpectedly during an unrelated project, where he conducted a comprehensive 40-day simulation of the global atmosphere. 

Upon analyzing the data, Judt discerned that the rapid intensification occurred in two distinctive manners. Such findings were previously elusive due to models focusing on isolated regions, rather than a global perspective.

Real-world tropical cyclone observations further corroborated both modes of rapid intensification. 

Serendipitous finding

“It was kind of a serendipitous finding,” Judt said. “Just by looking at the storms in the simulation and making plots, I realized that storms that rapidly intensify fall into two different camps. One is the canonical mode in which there’s a tropical storm when you go to bed and when you wake up it’s a category 4.”

“But then there’s another mode that goes from a tropical storm to a category 1 or 2, and it fits the definition of rapid intensification. Since nobody has those storms on their radar, that mode of rapid intensification went undetected until I went through the simulation.”

Study implications 

Concluding their research, the team theorized that many cases of rapid intensification could fall between these two modes. A storm might start with the “sprint” mode but gradually transition to the “marathon” mode. 

According to Judt, a question for future research is why bursts of thunderstorms can cause about 10% of storms in an unconducive environment to rapidly intensify, even though the other 90% do not.

“There could be a mechanism we haven’t discovered yet that would enable us to identify the 10 from the 90,” said Judt. “My working hypothesis is that it’s random, but it’s important for forecasters to be aware that rapid intensification is a typical process even in an unfavorable environment.”

The research, funded by the U.S. Navy Office of Naval Research and the U.S. National Science Foundation, appeared in the Monthly Weather Review, a journal of the American Meteorological Society.

The study is published in the journal Monthly Weather Review.

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