Secondary eyewall structure in Hurricane Rita: Results from RAINEX

Although hurricane track forecasting has improved in recent years, intensity forecasting has not been as consistently reliable. The difficulty in properly forecasting the intensity a hurricane will attain is complicated by the number of intensity changes a hurricane can undergo during its lifetime. One of the most difficult intensity change types to forecast is the rapid intensification cycle, which is responsible for producing most of the major hurricanes. Occasionally associated with rapid intensification is the eyewall replacement cycle, which occurs when the principal rainbands that generally spiral radially outward from the eye propagate inward to form a secondary concentric eyewall around the original eyewall. This secondary eyewall eventually dominates the moist static energy and chokes off the moist radial inflow to the inner eyewall. The inner eyewall then collapses, and the outer eyewall replaces the old inner eyewall during which storm intensity changes rapidly. Because of the relationship of the eyewall replacement cycle to rapid intensity change, it is important to better the understanding of the processes that lead to secondary eyewall formation.

During the time the hurricane rainbands are in their normal, radially-outward spiraling configuration, they are characterized as having extensive stratiform rain areas, which are embedded with deep convective cores. However, some of the rainbands take on a more constant radius of curvature, begin to slope outward like an eyewall, and form inward propagating secondary eyewalls. During this process, the incipient eyewalls take on a more two-dimensional structure of azimuthally continuous solid convection. This study seeks to document the process of transition from rainband to eyewall structure by analyzing airborne Doppler radar data measurements obtained in Hurricane Rita during the 2005 Hurricane Rainband and Intensity Experiment (RAINEX) and examining output from high-resolution numerical model simulations of this storm.

On 22 September 2005, RAINEX flew three aircraft equipped with Doppler radar (the two NOAA P3s and the NRL P3, which was equipped with the NCAR/ELDORA radar system) through Hurricane Rita when concentric eyewalls were present. Dropsonde observations were obtained at 5-10 min intervals during these flights. One NOAA P3 aircraft intensively sampled the two eyewalls with repeated radial penetrations, while a second NOAA P3 flew broad “figure-four” patterns to map the precipitation structure of Rita. The NRL P3 flew repeated circumnavigations around and between the primary and secondary eyewalls and observed them with the ELDORA radar. The data obtained during this flight provided an unprecedented view of a mature hurricane undergoing an eyewall replacement cycle.

The radar data collected during this mission have been ingested into the NCAR ZEBRA visualization software, which allows the user to not only view the horizontal structure of the radar variables but also allows the overlay of the radar data with other observations and for user-defined vertical cross-sections to be displayed. Output from high-resolution three-dimensional MM5 models run for this day of Rita by the University of Miami will also be ingested into ZEBRA to allow the structure of the modeled hurricane to be compared to the airborne Doppler radar observations of the storm. Zebra will be used specifically to analyze the evolution of the horizontal and vertical structure of the precipitation and kinematic characteristics during the eyewall replacement cycle as the rainbands transition to an eyewall structure. Additionally, the difference in structure and circulation between the inner and outer eyewalls of Hurricane Rita will be examined. Multiple Doppler radar synthesis of the data from the three aircraft and analysis of the intensive dropsonde data will be performed to determine the change in kinematic structure during the transition from rainband to eyewall structure. The change from cellular upward motions in rainbands to the two-dimensional outward sloping circulation of the eyewall bands observed by Doppler radar will be compared to numerical model output. The three-dimensional distribution of vorticity in the rainbands and eyewalls will be calculated to analyze how the vorticity field becomes axisymmetrized during the rainband-to-eyewall transition.