Intensity and structure variations associated with eyewall replacement cycles

The formation of a hurricane secondary eyewall around an intense, preexisting eyewall often results in the weakening of the original eyewall and a broadening of the wind field at distant radii. In time, the outer eyewall becomes the dominant convective feature and upon contraction the hurricane returns to a single eyewall state. This process is referred to as an eyewall replacement cycle (ERC). Hurricanes can undergo multiple ERCs during their lifetime and they can be especially hazardous when an outer eyewall forms shortly before landfall. A broadened wind field results in gales affecting a larger portion of the coast and impacting it sooner, reducing the evacuation time. Efforts have been made to predict when secondary eyewalls will form and case studies have shown that changes in intensity and structure vary from storm to storm. This work sets out to establish representative intensity and strength changes associated with ERCs to aid in wind radii and intensity forecasts.

Flight level data from US Air Force C-130 and NOAA P-3 aircraft are utilized to examine inner-core structure and intensity changes associated with concentric eyewalls and ERCs. Radial legs of storm-relative tangential wind speed are composited for several concentric eyewall cases. Hurricanes with sufficient data will be handled on an individual basis to determine characteristic intensity changes associated with the formation and contraction of an outer eyewall.