Tropical cyclone eyewall cycle impact on intensity and wind field structure

Intense tropical cyclones frequently contain multiple eyewalls as viewed by passive microwave satellite sensors. Storm intensity, the radius of maximum winds and the corresponding surface wind field evolve as the eyewall replacement cycle unfolds. Unfortunately, our ability to monitor these three vital characteristics has been woefully lacking since the majority of Category 5 systems inhabit tropical basins other than the relatively reconnaissance aircraft rich Atlantic. However, the 2005 season provided an unusual opportunity to map three Cat 5 storms (Emily, Katrina, and Rita) with multiple aircraft platforms and sensors and correlate with satellite eyewall configurations.

Eyewall cycles have the following pattern; a) a single small intense eyewall that contracts till, b) a secondary eyewall forms at a larger radii, engulfing the inner eyewall, c) outer eyewall cuts off circulation to the inner eyewall that decays and eventually collapses, and d) outer eyewall becomes the primary inner structure and it begins contraction again if environmental conditions permit. The eyewall replacement cycle occurs in approximately 75% of western Pacific storms reaching 120 kts or higher (averages 6/year), but operational aircraft flights ceased in 1987. In contrast, the Atlantic basin typically has only two 120 kt or stronger storms per year and ~60% of them have multiple eyewalls.

The United States provides routine storm monitoring in the Atlantic, Gulf of Mexico and Caribbean Sea with sophisticated recon aircraft for all systems that may impact the US mainland and much of the Caribbean and Mexico. The aircraft provide accurate measurements of eyewall diameter and the radius of maximum winds in addition to surface wind observations via a passive microwave sensor and dropsonde values at the lowest reporting levels. Thus, 2005 aircraft data in Emily, Katrina and Rita will help gauge the pulse of each storm as they undergo eyewall cycle structural changes.

Warning agencies are now aware of eyewall cycles and are just beginning to gain confidence that when a storm starts a cycle that a temporary intensity decrease has begun. Documenting storm characteristics as outlined here will greatly assist in bridging a gap in our understanding of storm values when aircraft data are not readily available and aid forecast agencies throughout the world.