Hurricanes, which rapidly intensify for mysterious reasons, pose a serious threat, especially to those at risk. Forecasters have struggled for years to understand why a seemingly ordinary tropical depression or tropical storm sometimes develops into a powerful hurricane, producing catastrophic winds and sending potentially deadly water ashore.

Scientists now shed light on why this prediction problem is so difficult to overcome: There are multiple mechanisms that cause rapid condensation. A new study by scientists at the US National Science Foundation’s (NSF) National Center for Atmospheric Research (NCAR) uses cutting-edge computer modeling techniques to identify two completely different rapid condensation regimes. The resulting data can help better understand and predict these dangerous events.

The new study was published at: Monthly Weather Review. It was co-authored by NCAR scientists Rosimar Rios-Berrios and George Bryan.

“Trying to find the holy grail behind rapid condensation is the wrong approach because there is more than one holy grail,” said NCAR scientist Falco Judt, lead author of the new study. “There are at least two different modes or types of rapid intensification, and each has a different set of conditions that must be met for a storm to intensify that quickly.”

One mode discussed by Judt and his co-authors occurs when a hurricane intensifies symmetrically, fueled by favorable environmental conditions such as warm surface waters and weak wind shear. This type of intensification is associated with some of the most destructive storms in history, such as Hurricanes Andrew, Katrina, and Maria. Meteorologists were stunned this week when Hurricane Otis defied predictions and exploded at 180 mph in just 24 hours, slamming into Mexico’s west coast as a Category 5 hurricane.

Judt and his co-authors also identified a second mode of rapid intensification that had previously been overlooked because it did not cause maximum winds to reach such destructive levels. In the case of this regime, strengthening can be associated with major storm outbreaks occurring far from the center of the storm. These fluctuations cause a restructuring of the cyclone’s circulation, allowing it to rapidly intensify and reach Category 1 or 2 intensity within hours.

This second mode is more unexpected because it usually occurs under adverse conditions, such as upper-level headwinds shifting the storm and the top blowing in a different direction than the bottom.

“These storms are not that memorable and they’re not that important,” Judt said. “But forecasters should be aware that even a strong, asymmetrical storm can intensify quickly.”

Forlorn

Rapid intensification occurs when the winds of a tropical cyclone increase to 30 knots (about 35 miles per hour) within 24 hours. While working on an unrelated project, Judt encountered two rapid modes of concentration.

The discovery was made possible by Judt performing a 40-day, very high-resolution computer simulation of the global atmosphere using the NCAR-based Multiscale Forecast Model (MPAS). This simulation, performed at the NCAR-Wyoming Supercomputing Center, was developed for an international project to compare results from leading atmospheric models that have achieved unprecedented levels of detail thanks to increasingly powerful supercomputers.

When Judt created the model, he wanted to investigate rapidly intensifying storms in the simulation. By examining a series of cases in the world’s ocean basins, he realized that rapid intensification occurred in two different ways. This was not evident in earlier models; This was because previous simulations only captured individual regions and did not allow scientists to track the spectrum of hurricanes and typhoons in the world’s oceans.

Judt and his co-authors then analyzed actual observations of tropical cyclones and found a number of real-world cases of both forms of rapid intensification.

“It was kind of an accidental discovery,” Judt said. “Just by looking at the storms in the simulation and creating the plot, I realized that the rapidly intensifying storms fell into two different camps. One of them is the canonical mode, where there is a tropical storm when you go to bed and it is category 4 when you wake up. However, there is another mode that goes from a tropical storm to a category 1 or 2 and fits the definition of rapid intensification. Since no one had these storms on their radar, this rapid intensification pattern went unnoticed until I ran the simulation.”

Meteorologists have long known that favorable environmental conditions, including very warm surface waters and minimal wind shear, can cause rapid intensification and bring on a Category 4 or 5 cyclone with sustained winds of 130 mph or higher. In their new paper, Judt and his co-authors call this rapid intensification regime a marathon because the storm continues to intensify symmetrically at moderate speed while the primary vortex continues to intensify.

Judt described Hurricane Otis as a fast-paced marathon; It intensified symmetrically but extremely quickly, with wind speeds increasing by 80 mph over a 12-hour period.

The research team called another method of rapid condensation a sprint, because condensation occurs extremely quickly but is usually short-lived; storms peak at Category 1 or 2 with sustained winds of 180 mph or less. In such cases, explosive storm outbreaks cause the cyclone to regroup and form a new center, allowing the storm to grow stronger even in adverse environmental conditions.

The paper concludes that the two regimes may represent opposite ends of the spectrum, and many cases of rapid amplification fall somewhere in the middle. For example, rapid intensification may begin with a chain of discrete events, such as a thunderstorm burst, characteristic of sprint mode, but then progress towards a more symmetrical intensification pattern, characteristic of marathon mode.

One question for future research is why storm outbreaks can cause about 10 percent of storms in unfavorable environments to intensify rapidly, while the other 90 percent cannot, Judt said.

“Maybe there’s a mechanism we haven’t discovered yet that would allow us to identify 10 out of 90,” he said. “My working hypothesis is that this is random, but it is important for forecasters to know that rapid intensification is a typical process even in adverse environments.”