Flying in Circles: The Ongoing Contributions of RAINEX to the Understanding of Tropical Cyclone Rainbands

Led by Robert A. Houze, Jr. from the University of Washington, Shuyi Chen from the University of Miami, and collaborations and partnerships with numerous universities, government agencies, and institutions, the Hurricane Rainband and Intensity Change Experiment (RAINEX) accomplished its goal of obtaining a comprehensive observational dataset from tropical cyclone rainbands during the 2005 Atlantic hurricane season. Through multiple flights into Hurricanes Katrina and Rita, coordinated P-3 circumnavigation flight patterns with the high-resolution Electra Doppler Radar (ELDORA) and the NOAA P-3 tail radar allowed for the most comprehensive mapping of mature hurricane rainbands of its time, providing a unique opportunity to verify and extend existing understanding of hurricane rainband structure.

Dual-Doppler analysis resulting from this campaign characterized the convective upwind, stratiform downwind precipitation distribution and the three-dimensional wind structure along the length of the band, which confirmed the previously hypothesized updraft/downdraft structure of cells within the convectively active portions of the principle rainband. These analyses expanded upon hypothesized mechanisms for how these updrafts can, in aggregate, lead to the formation of secondary tangential wind maxima within the rainband region. Combining the dual-Doppler analysis with dropsondes collected during the campaign led to an expansion in understanding for how two types of convective downdrafts could provide a pathway for low θ_e air into the boundary layer. The rainband convective cells exhibited consistent variations in their circulations with respect to distance from the storm center.

These studies also examined the downwind stratiform regions of the rainband, which revealed a marked distinction between convectively-active stratiform regions of the rainband and homogeneous "stratiform" precipitation that likely resulted from radial particle advection from the eyewall. The convectively-active stratiform found in the downwind portions of the principle band were found to have vertical mass fluxes and mesoscale rear descending inflows similar to a mesoscale convective system. In contrast, other regions of homogeneous precipitation were found to lack the vertical wind structure associated with convectively-active stratiform. This finding led to increasing questions as to the usefulness of convective/stratiform separation algorithms within tropical cyclones. 

The ideas and hypotheses formed from this research provided the foundation for a statistical analysis of the global spaceborne radar dataset, confirming that the precipitation features found in these and previous studies are common in tropical cyclones worldwide. Indeed, the legacy of RAINEX is defined by the expansion of ideas generated by this dataset into other key areas of tropical cyclone research. Further dual-Doppler analysis of the inner rainband regions provided significant insight into how rainbands may contribute to secondary eyewall formation. Grounded by the observations from this campaign, modeling studies are contributing further insight into these hypotheses, with the advantage of being able to directly examine how rainbands impact vortex evolution. In addition to filling a large gap in the tropical cyclone observational dataset, RAINEX continues to provide a wealth of data to mine to further understanding of how these previously neglected features contribute to tropical cyclone intensity change.