Hurricane Katrina's eyewall replacement cycle over the northern Gulf and accompanying double eyewalls at landfall: A key to the storm's huge size and devastating impact over a three-state coastal region

Hurricane Katrina made landfall on the Gulf Coast with the 3rd lowest central pressure (920 hPa) of any landfalling hurricane in United States history, but the pressure had been much lower before landfall. During the period of rapid strengthening over the central Gulf, Katrina contained one very intense eyewall. As the storm moved closer to the northern Gulf coast, the structure of Katrina changed dramatically when the storm began an eyewall replacement cycle. During this cycle, a developing outer eyewall began to encircle the inner eyewall and Katrina became a concentric two-eyewall storm. Katrina was one of five storms in 2005 to display a multiple eyewall configuration (Hawkins et al., 2006). The development of an outer eyewall greatly aided the dramatic increase in the size of Katrina as it approached the coast and allowed hurricane force winds to expand to a distance greater than 160 kilometers from the center. As Katrina went through the eyewall replacement cycle, the peak category 5 winds in the inner eyewall began to decrease as winds in the outer eyewall strengthened; during this structural transition, Katrina's winds officially slipped to a category 3 hurricane at landfall. Powell et al. (2007) show that even though the storm weakened to a category 3 at landfall, the great horizontal expansion of the wind field allowed Katrina's winds near landfall to maintain nearly the same kinetic energy of its earlier category 5 strength.

Numerous platforms, including microwave satellite, Doppler radar, GPS dropsonde, and airborne Stepped Frequency Microwave Radiometer (SFMR) measurements all indicate the existence of a double eyewall structure in Katrina as the storm made landfall on the northern Gulf Coast. The low brightness temperature bands detected by microwave satellite were shown to correlate with abrupt wind increases measured by anemometers at a shipyard in Pascagoula MS and at a C-MAN on Dauphin Island AL, further supporting the hypothesis of an outer eyewall over or near these locations. The microwave satellite imagery shows potential for identifying regions in the hurricane containing strong sustained winds and gusts. The MIMIC system, developed by CIMSS (Wimmers and Velden, 2007) is particularly helpful in identifying eyewall replacement cycles in hurricanes such as Katrina. While making landfall on the Mississippi/Louisiana coast, dropsonde measurements indicate low-level wind maxima >67 m/s in the stronger inner eyewall and >63 m/s in the expansive outer eyewall. Using mean eyewall profiles from dropsondes (Franklin et al., 2000, 2003), 10 m one-minute sustained winds for marine exposure likely reached mid category 2 intensity in the outer eyewall, agreeing well with nearby SFMR measurements there. The data strongly suggest that much of extreme southeast Louisiana and virtually the entire Mississippi coast were impacted by at least one of Katrina's eyewalls, with some locations experiencing the passage of two eyewalls.

Many hurricane wind field reconstructions, such as H*Wind and those used in the IPET study, do not display this double eyewall structure in Katrina. Lack of double eyewalls in Hurricane Katrina simulations would likely have major ramifications on location and arrival time of eyewall-like damaging winds and possibly have impacts on storm surge characteristics. Very strong winds associated with Katrina's outer eyewall likely reached coastal locations hours before the center of the storm made landfall, catching many emergency personnel and last minute evacuees by surprise at the early arrival of severe eyewall-like winds on the coast. Also, many Katrina survivors who experienced the storm well to the east of the center along the eastern Mississippi coast were surprised by the ferocity of the storm so far from the center. Record storm surge levels were recorded as far east as Mobile Bay, Alabama (over 150 km to the east of Katrina's Louisiana/Mississippi border landfall), likely aided by Katrina's nearby outer eyewall late on the morning of 29 August 2005.