An observational study on the genesis of concentric eyewall hurricanes

A number of observational analyses (Willoughby et al.,1982; Willoughby,1990; Black and Willoughby,1992) showed that during the development of some intense hurricanes, spiral ranibands form a partial or complete ring of heavy precipitation around the eyewall, and that the ring usually contains a well-defined wind maxima. This pattern of inner and outer convective rings is generally referred to as concentric eyewalls. Hurricanes with concentric eyewalls often undergo characteristic intensity change. As the outer eyewall contracts and intensifies, the intensity of hurricane stops increasing and starts to weaken,i.e., a rise of the minimum central pressure, a decrease of the maximum tangential wind, and an increase of the eyewall's radius. Some time later, the outer eyewall replaces the inner one and becomes a new primary eyewall. After the eyewall succession, hurricanes may resume intensification if they do not make landfall shortly. For example, Hurricane Andrew (1992) almost regained its original peak intensity after an eyewall replacement cycle. It was speculated that the destructiveness of Hurricane Andrew was related to the eyewall replacement (Willoughby and Black,1996). Understanding the initial formation and following development of outer eyewall should help us gain insight on hurricane intensity change associated with concentric eyewall cycles.

Owing to the lack of direct measurements of upper-level atmospheric conditions, we choose to use reanalysis data from National Centers for Environmental Prediction (NCEP) and National Center for Atmospheric Research (NCAR) and European Center for Mdeium-Range Weather Forecasts (ECMWF). A somewhat lengthy evaluation suggests that both datasets are marginally suitable for case studies of tropical cyclones. Our major findings are as follows: (1) a causal relationship does not always exist between environmental forcing and genesis of a secondary eyewall. Some cases, for example, Hurricane Allen of 1980, Hurricane Elena of 1985 and Hurricane Opal of 1995, show a good and clear relationship between their eyewall replacement cycles and their external forcings. Some cases, for example, Hurricane Gilbert of 1998 and Hurricane Andrew of 1992, show some degree of causal relationship. Some cases, for example, Hurricane Emily of 1993 and Hurricane Gabrielle of 1989, show a weak or close to no causal relationship. (2)With the results of Hurricane Frederic of 1979 and results from concentric eyewall hurricanes, we can conclude that the interaction between a tropical cyclone and its upper-level synoptic environment is neither sufficient nor necessary for the genesis and development of concentric eyewall cycles in reality. (3)The maps of isentropic potential vorticity (PV) only provide qualitative information on the occurrence of the interaction. The strength of the interaction should be determined quantitatively by the eddy PV fluxes which should be calculated in a storm-moving coordinate system.

Our conclusion is only in partial agreement with what Molinari and his colleagues (1992, 1995) have found in their observational studies of Hurricane Allen (1980) and Hurricane (1985). The discrepancy leads us to hypothesis two mechanisms of the genesis. One is the interaction between a hurricane and the ocean underneath. The other is the tilting of high PV inner core with the storm and followed up projection of cyclonic vorticity down to the ocean surface.