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Double (Concentric) Eyewalls in Hurricane Katrina at Landfall: A Key to the Storm’s Huge Size and Devastating Impact over a Three-State Coastal Region

Keith G. Blackwell University of South Alabama/Coastal Weather Research Center (kblackwell@usouthal.edu )

Pat Fitzpatrick Stennis Space Center/Mississippi State University (fitz@HPC.MsState.Edu)

Christopher Velden and Tony Wimmers NOAA/Cooperative Institute for Meteorological Satellite Studies (CIMSS) (chris.velden@ssec.wisc.edu; wimmers@ssec.wisc.edu)

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 100 statute miles 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. Calculations by Powell (2006) show that even though the storm weakened to a category 3, the great horizontal expansion of the storm allowed Katrina’s winds near landfall to maintain or even exceed the 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. While making landfall on the Mississippi/Louisiana coast, dropsonde measurements indicate low-level wind maxima >150 mph in the stronger inner eyewall and >140 mph 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- to upper-category 2 intensity in the outer eyewall, agreeing well with nearby SFMR measurements there. The data strongly suggest that virtually the entire Mississippi coast was impacted by at least one of Katrina’s eyewalls, with some locations experiencing two eyewalls. Many hurricane wind field reconstructions, such as H*Wind, do not display this double eyewall structure in Katrina.

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