Environmental and Internal Controls of Concentric Eyewalls Formation and Replacement: RAINEX and Beyond

There have been many theoretical and idealized modeling studies attempting to explain concentric eyewalls in intense hurricanes. It is no surprise that overwhelming conclusions from these studies attribute the internal dynamic processes as the cause for the formation of concentric eyewalls. However, they cannot explain why only some intense hurricanes develop concentric eyewalls and went trough eyewall replacement cycles and others do not. The lack of realistic representation of storm environment has limited these studies to address the key question related to formation of concentric eyewalls in hurricanes that are inseparable with their environment in reality. The Hurricane Rainband and Intensity Change Experiment (RAINEX) was designed to address the interactions of the hurricane's inner core, rainbands, and the storm environment. RAINEX employs three aircraft equipped with dual-Doppler radar and the GPS dropsondes. It involves the NRL P3 with the NCAR ELDORA dual-Doppler system plus the two dual-Doppler equipped NOAA WP-3D aircraft. RAINEX conducted multi-day and multi-aircraft missions into Hurricanes Katrina, Ophelia, and Rita with well-designed fly patterns that target important features in both inner core and outer rainband regions. The data collected in RAINEX provides a unique opportunity to examine the complex interaction between the hurricanes and their environment, especially in the rainband region, and how it led to different evolutions in terms of formation of secondary eyewall and eyewall replacement in Rita, but not in Katrina. High-resolution models have been able to predict the observed characteristics of Rita and Katrina, as well as other major hurricanes and typhoons with concentric eyewalls, including Hurricanes Floyd (1998), Isabel (2003), Frances (2004), Typhoon Choi-wan (2009) compared to those did not have concentric eyewalls. The results show that environmental conditions, such water vapor and initial wind distributions, are important contributors to the formation of concentric eyewalls. This talk will provide a review on recent researches related to this topic and the results from high-resolution coupled model simulations as well as observations from RAINEX, CBLAST, and the West Pacific.