What is an eyewall replacement cycle?
Updated: Jul 24, 2020
Hurricane Florence completed an eyewall replacement cycle today. What is that?
Hurricane Florence underwent an eyewall replacement cycle today in which a new ring of intense convection developed outside of the original eyewall, cutting off the core of the storm from heat and moisture flowing in from around the storm. That lead to the collapse of the original eyewall as it could no longer be sustained by the limited amount of warmth and moisture within the outer ring of convection. Additionally, as the outer ring strengthened, outflow from the top of those storms further disrupted the storms within the original eyewall. The new ring of storms has since taken over and strengthened. However, since the new eyewall is larger in diameter than the original one, maximum winds are weaker than earlier but spread out over a larger area. With time, this new eyewall will strengthen further, gradually shrink in diameter, and winds will increase again. Major hurricanes often undergo multiple eyewall replacement cycles if they have enough time over warm water to do so, and it is possible that Florence could undergo another such cycle prior to landfall, but that is uncertain.
The loop of microwave satellite imagery above shows the stronger convection within the core of Florence. Note the inner ring of convection near the center of the storm fading as a larger ring of convection takes over farther from the center. With time the inner convection fades out, the eye clears, and a new much larger eye takes over.
Since the eye of Florence is now quite large, and the core of convection around the eye is also larger in area than before, Florence will have an opportunity to reach a higher intensity than was seen yesterday. Conservation of angular momentum will lead to steadily increasing winds simply by the eye contracting over time, and with favorable upper-level winds and plenty of warm water, convection in the storm should remain very strong, helping to drop surface pressures. Further, storms with larger eyes tend to be more resistant to wind shear and other disruptions. As eyewalls shrink to very small diameters, any disruption to the inner circulation can cause rapid destruction of the entire eyewall leading to quick decreases in wind intensity. Larger eyewalls will still eventually collapse under persistent strong shear, but they often hang on for longer and weaken more slowly.
The set of images below show the visible satellite imagery views and microwave views of Wilma during an eyewall replacement cycle in 2005. Note the tiny "pinhole" eye at the start of the cycle, and the much larger eye after the cycle was complete. While maximum winds are always lower after an eyewall cycle completes, the larger size of the eye means that the intense winds are spread out over a larger area. So, if the storm makes landfall with a larger eye, more of the coast will see strong winds, and the strongest winds will last longer since it will take longer for the eyewall structure to pass over.