Paper on the unusual eyewall replacement cycles in Hurricane Irma published in Monthly Weather Review - NOAA's Atlantic Oceanographic and Meteorological Laboratory

Summary: Accurate forecasts of hurricane strength are necessary to protect people in the path of a storm. The strongest winds in a hurricane are found near the center of the storm, in a ring of dangerous weather called the eyewall. As a hurricane grows older, it is common for the eyewall to eventually weaken and get replaced by a newer, larger eyewall. This process is referred to as an eyewall replacement cycle (ERC). Such ERCs are important, since the newer, larger eyewall can bring dangerous weather over a larger region than the older, smaller eyewall would have. Additionally, hurricanes often experience temporary changes in strength (both up and down) during an ERC. What causes the newer, larger eyewall to form during an ERC is not well understood.

This study analyzed radar observations collected from NOAA’s P3 aircraft, which repeatedly flew through the eyewall of Hurricane Irma (2017) as the storm went through back-to-back eyewall replacement cycles over a 24–36-hour period (see figure). It has been typically thought that during an ERC, the hurricane loses strength as the original eyewall weakens while the newer, larger eyewall forms. Once the newer eyewall becomes dominant, the hurricane may then grow stronger. However, Hurricane Irma did not follow this pattern, and instead, the storm experienced continued strengthening, especially during the second of the two ERCs. Ultimately, Irma reached category-5 strength on the Saffir-Simpson Hurricane Wind Scale.

Wind speed (m s^-1) observed in Hurricane Irma as measured by NOAA’s P3 aircraft. In this figure, the distance from the center of the storm (km) increases from the left side to the right side of the figure, while time increases from the bottom to the top of the figure. Here, time is labeled in the format of month-day-hour. The black, dashed lines show the location of the strongest wind. The two eyewall replacement cycle (ERC) events analyzed in this study occur at the times labeled “ERC I” and “ERC II”. Each ERC is linked to an outward jump in the location of the strongest winds, as the newer, larger eyewall forms.

Important Conclusions:

The processes that cause the newer, larger eyewall to form during an ERC are not well understood. This study found different factors contributed to the formation of a new eyewall in each of the two ERCs examined. These results demonstrate that there is not one, single process that explains the formation of the newer, larger eyewall in an ERC, and instead, multiple factors need to be considered.
Each ERC in Hurricane Irma happened over a period of less than 12 hours. This is much quicker than the average period of 36 hours. During both ERCs in Irma, the size of the region of most dangerous weather quickly expanded, increasing the threat of the storm (see Figure). These results highlight the need to accurately forecast the timing of ERCs, as such events can place a larger region at risk of experiencing the most dangerous weather.

You can find the paper at https://journals.ametsoc.org/doi/abs/10.1175/MWR-D-19-0185.1.