Hurricane Frances (2004) represented an unusual event that produced three consecutive overlapping eyewall replacement cycles (ERCs). Their evolution followed some aspects of the typical ERC. The strong primary eyewalls contracted and outward-sloping secondary eyewalls formed near 3 times the radius of maximum winds. Over time these secondary eyewalls shifted inward, became more upright, and replaced the primary eyewalls. In other aspects, however, the ERCs in Hurricane Frances differed from previously described composites. The outer eyewall wind maxima became stronger than the inner in only 12 h, versus 25 h for average ERCs. More than 15 m s(-1) outflow peaked in the upper troposphere during each ERC. Relative vorticity maxima peaked at the surface but extended to mid- and upper levels. Mean 200-hPa zonal velocity was often from the east, whereas ERC environments typically have zonal flow from the west. These easterlies were produced by an intense upper anticyclone slightly displaced from the center and present throughout the period of multiple ERCs. Inertial stability was low at almost all azimuths at 175 hPa near the 500-km radius during the period of interest. It is hypothesized that the reduced resistance to outflow associated with low inertial stability aloft induced deep upward motion and rapid intensification of the secondary eyewalls. The annular hurricane index of Knaff et al. showed that Hurricane Frances met all the criteria for annular hurricanes, which make up only 4 of all storms. It is argued that the annular hurricane directly resulted from the repeated ERCs following Wang's reasoning.
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