Why Some Hurricanes Rapidly Intensify and Others Do Not, According to New Study

A team of researchers from NOAA just published a study explaining why hurricanes rapidly intensify while other hurricanes do not. 

The study is timely, considering that during 2017 hurricane season, six hurricanes underwent rapid intensification, an increase in speed of at least 35 mph in 24 hours or less. 

The authors, led by Hua Leighton at NOAA's Atlantic Oceanographic and Meteorological Laboratory, identified several factors that may allow hurricanes to rapidly intensify even in wind shear: 

- Increased moisture downwind: The area downwind of a well-sheared hurricane is generally where thunderstorms will have the most success growing stronger and taller. Thunderstorms are the beating hearts of a hurricane, and without strong thunderstorms, the hurricane will weaken. For more on what wind shear is, please see the bottom of this article.

- Thunderstorms migrating around the eye of a hurricane: If a hurricane can wrap thunderstorms around most of its core, it has the best chance of rapid intensification. This is because taller thunderstorms reduce the tilt of the eye and eyewall and more thunderstorms reduce the asymmetries around the storm. 

- Complimentary flow: If the wind flow on the outer periphery of the hurricane is in the same direction that the hurricane’s wind moves it promotes rapid intensification. That is counter-clockwise in the north Atlantic Ocean basin.  This flow promotes the movement of thunderstorms into upwind quadrants of a sheared hurricane, which should increase moisture and reduce tilt in a hurricane.

- An increase in spin: Hurricanes that are more efficient at pulling spin inward are more likely to rapidly intensify because more spin leads to higher winds, which help build a more vertical eye and eyewall, which further promotes intensification. This is similar to a figure skater on the ice that spins faster when they pull their arms inward.

Using an ensemble of forecasts of one hurricane model, called the Hurricane Weather Research and Forecast (HWRF) model, the team discovered that the conditions listed above allowed Hurricane Edouard to rapidly intensify in 2014.  

An ensemble, in this case, was used to see how Edouard would react to slightly different atmospheric conditions such as moisture, wind shear, temperature and spin in the atmosphere. 

(MORE: Four Things You Should Know About Spaghetti Models)

Rapid intensification is tricky to forecast because the steps that lead to this process are usually on a small scale, making it hard for both computer model guidance and for humans to predict. Therefore, it is one of the biggest areas of research right now. 

Three of 2017's hurricanes that underwent rapid intensification landed on the list of the top five costliest hurricanes on record.  

Hurricane Harvey, the second costliest hurricane in the Atlantic basin on record, rapidly intensified for two days in the Gulf of Mexico before crashing into the middle Texas coast. Winds increased from 45 mph to 130 mph before landfall. This period of rapid intensification "was not well-anticipated", according to the National Hurricane Center, and studies like this one could help us better forecast extreme changes in wind speed. 

(MORE: Harvey's Rapid Intensification)

Wind shear initially caused Harvey to weaken to a tropical wave in the Caribbean before it reached less hostile conditions in the Gulf of Mexico, likely with some of the ingredients identified in this study.

Vertical wind shear, or simply wind shear in the case of hurricanes and tropical storms, is defined as the difference in wind speed from the ground to cloud top level. Wind speeds generally increase with height through the canopy level of hurricanes, which can top out as high as 50,000 feet. 

When this difference between wind speeds near the surface and winds higher aloft is larger, wind shear becomes detrimental to even the strongest hurricanes. Thunderstorms are generally strewn downwind of a well-sheared hurricane as higher winds aloft "push" over the top of the hurricane and tilt it over. This hinders development and intensification of hurricanes.

(MORE: Water, Not Wind, the Deadliest Factor in U.S. Hurricanes and Tropical Storms)

Increased moisture in the area (downwind) where thunderstorms are pushed into helps those sheared thunderstorms grow taller and gives them a better chance to survive elsewhere in the hurricane. If that area has plentiful moisture to work with, this also boosts the chances of additional thunderstorms being able to blossom.