On September 28, the Landsat 8 satellite passed directly over Ian’s eye as the storm approached southwest Florida. The natural-color image above was acquired by the Operational Land Imager (OLI) at 11:57 a.m. local time (15:57 Universal Time), which was three hours before the storm made landfall in Caya Costa.
An eyewall is an organized band or ring of cumulonimbus clouds that surround the eye, or light-wind center of a tropical cyclone. Another name for eyewall is wall cloud.
The circular zone of fair weather at the storm’s center is known as the eye of a hurricane. It is surrounded by a towering ring of incredibly powerful thunderstorms called an eyewall, the part of the hurricane with the strongest winds. The swirling clouds along the edges of the eyewall are mesovortices. These are small-scale rotational features found in hurricanes with exceptionally strong winds.
According to the National Hurricane Center, when Ian’s eyewall crashed into Florida, its maximum sustained winds were 150 miles (240 kilometers) per hour. That is the equivalent of a category 4 storm on the Saffir-Simpson wind scale. It is also fast enough to tear the roofs off homes and snap power lines.
“Those breathtaking low-level cloud swirls in Ian’s eye might provide clues into some important processes that affect a hurricane’s intensity,” said Justin Whitaker. He is a researcher with NASA’s Short-term Prediction Research and Transition Center (SPoRT). The SPoRT team, based at Marshall Space Flight Center, focuses on improving weather forecasts using NASA data. “At SPoRT, we’re studying how these inner-core asymmetries can affect a hurricane’s structure, its potential to intensify, and whether lightning will occur within the storm’s eyewall.”
The animation above represents the evolution of Ian’s wind field between September 25 and September 29, 2022. The strongest winds appear in bright yellow; more moderate winds are shades of orange and bright purple. Atmospheric data was analyzed through NASA’s Goddard Earth Observing System (GEOS) model, a data assimilation model that scientists use to analyze global weather phenomena. The GEOS model ingests wind data from more than 30 sources, including ships, radiosondes, buoys, dropsondes, satellites and aircraft. The model output is spaced on a 0.25 to 0.3 degree grid, so it does not necessarily capture the gust peaks measured by individual instruments at the surface.
As Hurricane Ian passed Cuba and passed through the Gulf of Mexico on September 27, the eye was about 20 kilometers wide. As the storm turned northeast, satellites observed a second, larger eye forming around it and finally enveloping the original eye. This is a process called the eyewall replacement cycle. Eyewall replacement cycles are common in severe hurricanes, usually causing the wind field to spread out over a larger area.
“An eye wall replacement cycle occurs when a hurricane develops concentric eye walls and the inner eye wall collapses,” explained Charles Helms. He is an atmospheric scientist at NASA’s Goddard Space Flight Center. “As a result, the eye of a hurricane gets very large, and these cycles are often associated with a temporary pause in intensification. There’s still a lot we don’t understand about these cycles, and that remains an active subject of research in the tropical community.
In Ian’s case, the eyewall replacement cycle caused the eye to expand to a width of 34 miles (55 kilometers), causing the eyewall to expand as well. Like some hurricane experts noted, that meant the full extent of Charley’s hurricane-force winds could have fit in Ian’s eye. (Hurricane Charley was a compact Category 4 storm that hit the same part of Florida in 2004.)
“While Hurricane Charley was similar in intensity to Hurricane Ian and caused extensive damage in the immediate area as well as central Florida, Charley was relatively small and moving quickly,” Helms said. “Hurricane Ian was considerably larger than Hurricane Charley and moved much slower. This means the structures were subjected to high winds and storm surges for much longer during Ian than during Charley. .
By the time OLI acquired the image, Ian’s eye had narrowed to 26 miles (42 kilometers) as the storm passed through another period of strengthening, dropping to just below Category 5 before touching down earth. The intensity of the winds and the large size of the wind field helped the storm push what turned out to be a catastrophic storm surge through coastal communities including Cape Coral and Fort Myers. According to new reports, the winds and floods have destroyed or damaged scores of homes and knocked out power to millions of people.
NASA Earth Observatory image and video by Joshua Stevens, using US Geological Survey Landsat data, NASA GSFC Global Modeling and Assimilation Office GEOS-5 data, and roads from OpenStreetMap.