Extended Abstract (148K)12A.2
The eyewall replacement and intensity changes in Hurricane Bonnie (1998)
Tong Zhu, NOAA/NESDIS, Camp Springs, MD; and D. L. Zhang
Because of their significance in hurricane intensity changes, the eyewall replacement processes have attracted considerable attention since the publication of Willoughby et al. (1982). In this presentation, a series of 5-day explicit simulations of Hurricane Bonnie (1998) is performed, using the fifth-generation Pennsylvania State University-National Center of Atmospheric Research Mesoscale Model (MM5) with the finest grid length of 4 km, in order to examine (a) the predictability of hurricane intensity and intensity changes, and (b) the mechanisms whereby the intensity changes occur.
It is found that a control simulation and four sensitivity experiments with varying cloud microphysics processes all captured the eyewall replacement cycles, and their developments appear to be closely related to hurricane intensity changes. The model simulations show that the eyewall evolves from a partial to double and a near-concentric eyewall during the eyewall replacement period. Specifically, one outer spiral rainband at R = 120 km begins to propagate into the northwest-to-western quadrant as the large-scale descending inflow weakens. Similarly, more rainbands develops in the outer regions and propagate cyclonically into the northwestern quadrant. Meanwhile, the inner eyewall weakens with time. Subsequently, the newly formed eyewall shrinks in width and radius, so do the outer rainbands. The outer rainbands also intensify in terms of upward motion, local tangential winds and surface pressure gradients as they move cyclonically inward. A secondary wind maximum starts to emerge at R = 150 km, and intensify with time as its radius of maximum wind shrinks. These developments are consistent with the moderate deepening of the storm during the period. By the end of replacement cycle, the inner eyewall becomes disintegrated with a few weak convective cells, and the inner RMW loses its characteristics starting from the surface. It takes less than 6 h to complete this eyewall replacement cycle. It is shown that the eyewall replacement scenarios are more or less determined by the large-scale sheared environment, but their associated inner-core structural changes, timing and location differ markedly, depending on the hurricane intensity.
Session 12A, Tropical cyclone intensity change I: Inner core processes
Thursday, 6 May 2004, 8:00 AM-9:30 AM, Le Jardin Room
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