16C.4a
Comparison of secondary eyewall and principal rainband in Hurricane Rita (2005) as observed in RAINEX
Anthony C. Didlake Jr., University of Washington, Seattle, WA; and R. A. Houze Jr.
During the Hurricane Rainband and Intensity Chance Experiment (RAINEX), the National Center for Atmospheric Research (NCAR) Electra Doppler radar (ELDORA) observed the secondary eyewall and principal rainband of Hurricane Rita (2005) in exceptional detail. Observations of these prominent hurricane features offer a unique opportunity to compare the internal dynamics of these two features. This comparison leads to an understanding that the principal rainband and secondary eyewall are fundamentally different phenomena.
Previous studies have documented the convective-scale structure of the principal rainbands in Hurricanes Rita and Katrina. The principal rainband consists of intermittent convective cells embedded in a band of stratiform precipitation that spiral into the inner core of the storm. The individual cells are outwardly-leaning towers of intense reflectivity located along the inner edge of the rainband. The dominant vertical motions within the cells are an overturning updraft located along the inner edge of the reflectivity tower, a low-level downdraft embedded in the lower levels of the tower, and an inner-edge downdraft that creates a sharp inner boundary of the rainband. A strong mid-level tangential wind maximum occurs within the tower at 4-6 km altitude, and a weaker tangential wind maximum occurs in the lower levels (~2 km altitude) on the radially inward side of the rainband.
Rita's secondary eyewall adopted a different set of convective-scale dynamics as part of its transition into a mature eyewall. The secondary eyewall consisted of convective cells embedded in stratiform precipitation like the principal rainband, but the reflectivity towers of these cells had no preferred shape, orientation, or radial location. Updraft and downdraft cores did not occur at distinctly different altitudes or locations as in the principal rainband; but rather, they both occurred wherever the reflectivity tower appeared and they both peaked in the mid-levels (4-6 km). There was no inner-edge downdraft to form a sharp inner boundary of the secondary eyewall. Radial cross sections show three preferred locations of tangential wind maxima relative to individual convective towers. A low-level maximum at 2 km altitude, and a mid-level maximum at 5 km altitude repeatedly occurred within each reflectivity tower. A third tangential wind maximum occurred radially outward of the reflectivity tower near 2 km altitude. The low-level wind maximum within the reflectivity tower suggests that the convective elements in the secondary eyewall, unlike those in the principal rainband, were in the process of building the low-level tangential wind maximum. This would help the relatively new secondary eyewall to evolve into a more mature eyewall by building a connected low-level wind maximum around the storm.
To test this idea quantitatively, we used the convective-scale observations from the ELDORA data to examine the convective-scale mixing dynamics within the secondary eyewall. Perturbation fields, defined by removing the wavenumber-0 and wavenumber-1 structures, revealed a distinct pattern that existed in azimuthal cross sections involving regular structures of vertical velocity, tangential wind, and vertical vorticity. This pattern suggested that convective-scale perturbations were producing vertical fluxes of momentum and vertical vorticity within the secondary eyewall. Using the ELDORA velocity data, we calculated the contribution of these eddy fluxes to the overall momentum and vorticity. We separated the momentum and vorticity equations into mean and perturbation components and the results showed that vertical advection by perturbation velocities was increasing momentum and decreasing vertical vorticity in the average secondary eyewall below 5 km altitude. These effects may contribute to the strengthening of the secondary wind maximum that defines the secondary eyewall, helping it to evolve into a mature eyewall structure.
Session 16C, Tropical Cyclone Modeling: Structure and Intensity II
Friday, 14 May 2010, 10:15 AM-12:00 PM, Arizona Ballroom 10-12
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