Concentric Eyewalls in RAINEX and TC Modeling

Concentric (or secondary) eyewalls and eyewall replacement cycles are key processes in tropical cyclone (TC) intensity change. For many years, our understanding of concentric eyewall formation had been limited, largely due to a lack of in situ observations and limited high-resolution modeling capabilities. The Hurricane Rainband and Intensity Change Experiment (RAINEX) in 2005, conceived to better understand TC intensity change that results from interactions between a TC's inner core and its rainbands, played a substantial role in narrowing this knowledge gap. By virtue of an innovative experiment plan and scientific serendipity, RAINEX proved to be a great success: after a high-resolution model had hinted at the formation of a secondary eyewall in Hurricane Rita, three aircraft sampled the storm simultaneously during its concentric eyewall stage - allowing for observations of an eyewall replacement cycle in detail never seen before. These data, together with subsequent modeling studies, fostered thorough investigations into the processes that lead to concentric eyewall formation and TC intensity change. The emerging findings have undoubtedly increased our understanding of concentric eyewalls, corroborated some theories, cast doubt on others, and sparked new hypotheses. Ultimately, the discoveries of RAINEX and the results from a multitude of modeling studies are helping to improve TC intensity predictions. In this talk, we will highlight the unique RAINEX observations of concentric eyewalls in Hurricane Rita and discuss recent insights into concentric eyewall formation from high-resolution modeling studies.