Collaborative Research: Observational and Modeling Study of Hurricane Rainbands and Intensity Changes

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The Principal Investigators will perform a coordinated modeling and observational study of hurricane intensity changes in relation to inner-core vortex dynamics, formation of secondary wind maxima in the outer rainband region, eyewall replacement cycles, and dynamic/thermodynamic feedbacks of outer rainbands. The two main objectives of this study are: 1) To examine structures (both dynamic and thermodynamic) of hurricane outer rainband and inner core regions simultaneously. 2) To investigate the interactions of the rainbands and primary hurricane vortex circulation and the role of these interactions in hurricane intensity changes. Rainbands are major sources of asymmetric heating outside of the hurricane inner core. This asymmetric heating leads to localized potential vorticity (PV) features, which may in turn be "axisymmetrized" by the radial shear of the tangential wind. The eyewall dynamics are then restructured and the overall storm intensity changes. Thus, the detailed evolution of the structure of the rainbands is integrally involved in storm evolution. The internal structure of the rainbands will be examined by subdividing the bands into convective and stratiform regions where the vertical distributions of heating and thus PV generation patterns are different. By simultaneously examining the eyewall and rainband structures, the PV features of the rainbands can be related to storm intensity changes. The PV features generated in the convective and stratiform regions of rainbands will be examined in relation to the formation of secondary wind maxima in the rainbands. As the secondary wind maxima take shape, they will be examined for how they may intensify and axisymmetrize and eventually take over as the main eyewall. To address objective 1), the Principal Investigators will use aircraft equipped with Doppler radar and dropsondes. First, data collected by the NOAA P3s during the 1998-2003 hurricane seasons will be analyzed. Priority will be given to Fabian (2003) and Isabel (2003), which exhibited well-defined eyewalls, rainbands, and eyewall replacement cycles. Although airborne Doppler radar data were collected in these storms, the flight tracks were not dedicated to the objectives of the proposed study. The analysis of these storms will, therefore, serve as the foundation for the design of a multi-aircraft flight program dedicated to meet the specific objectives of this research. This flight program will be called the Rainband and Intensity Change Experiment (RAINEX), and will involve the two Doppler equipped NOAA WP-3D aircraft plus the NRL P3 with the National Center for Atmospheric Research ELDORA Doppler system. RAINEX will operate within the context of the National Oceanic and Atmospheric Administration (NOAA) 2005 hurricane field program over the Atlantic and will be coordinated with NOAA scientists.To address objective 2), the Principal Investigators will use the University of Miami's vortex following, multi-nested, non-hydrostatic mesoscale model (MM5) coupled with a wave model and an ocean model to carry out 3-7 day long integrations showing the evolution and fine structures of the hurricane inner-core and rain-bands at 1-2 km grid resolution. Consistency of the model simulations with the airborne radar and dropsonde data will be sought. Near real-time simulations will be done for cases investigated in the 2005 field program. This study will contribute to the societal goal of improving predictions of hurricanes and associated extreme wind and flash flood events that have a devastating impact on life and property in the U. S. coastal regions.