Rainbands and secondary eye wall formation as observed in RAINEX

It has been observed in major hurricanes that the primary eyewall often contracts as the storm intensifies and a secondary eye wall may form in the outer rainband region. As the outer eyewall develops and contracts, it replaces the inner eyewall and causes the storm to weaken. As was observed in Hurricane Frances and Ivan in 2004, these cycles result in significant intensity changes in intense hurricanes. The mechanisms that trigger these cycles are not well understood. thus the reasons that hurricanes such as Frances undergo these cycles while hurricanes such as Isabel remains unknown. Previous studies have attempted to explain the mechanisms. However, the early observational studies were limited by a lack of simultaneous observations in the inner-core and rainband regions. Many modeling studies use idealized models that do not have moist physics to capture the formation of the secondary eye wall. The, the generation of potential vorticity (PV) in the rainbands and its role in the development of the outer eye wall remains unexplained.

The Hurricane Rainband and Intensity Experiment (RAINEX) was designed specifically to address this problem. During the RAINEX field program from August-September 2005, coordinated multi-aircraft missions have collected valuable airborne dropsonde and radar data in two major hurricanes, Katrina and Rita. Although both storms underwent rapid intensification in the Gulf of Mexico, there was a significant difference in terms of the formation of the secondary eye wall and eye wall replacement cycle. During the observational period of August 27-28 for Katrina, maintained and intense, primary eye wall with no eye wall replacement cycles, whereas Rita developed a secondary eye wall and underwent a complete eye wall replacement cycle from September 21-23. In this study, we composite the GPS dropsonde data collected in the rainband regions of Katrina and Rita. Preliminary analyses indicates that the outer rainband region of Rita was more moist than in Katrina. The radar reflectivity data shows ore intense rainbands in Rita than Katrina. The formation of a circular ring of convection in the outer rainband region precedes the secondary wind maximum prior to the eye wall replacement cycle. The observations are currently compared with high-resolution model simulations of Hurricanes Katrina and Rita in which the observed features in rainbands and the formation of the secondary eye wall were explicitly resolved. Further analysis on both the RAINEX observations and modeling results will be presented at the conference.