An interesting eyewall evolution occurred in Typhoon Zeb (1998) when it devastated Luzon. The eyewall of Zeb contracted before landfall and broke down and weakened after landfall; then a much larger new eyewall formed and strengthened as it left Luzon and reentered the ocean. The fifth-generation Pennsylvania State University-NCAR Mesoscale Model (MM5) with four nested domains was used to perform numerical experiments to understand the effects of terrain and land surface variation on the observed eyewall evolution. Results show that the presence of Luzon plays a critical role in the observed eyewall evolution. Quite different from the conventional concentric eyewall replacement, the eyewall replacement that occurred in Typhoon Zeb was triggered by the mesoscale landmass and terrain variation with a horizontal scale similar to the core of the typhoon. In Typhoon Zeb, the original eyewall contracted and broke down because of enhanced surface friction after landfall. The outer eyewall was triggered by convective rainbands near the western coastal region of Luzon and formed as a result of axisymmetrization well after the dissipation of the inner eyewall convection.Several sensitivity experiments were conducted to elucidate the roles of both condensation heating and planetary boundary layer processes in the evolution of the typhoon eyewall. It is found that although condensational heating is the key to the maintenance of the annular potential vorticity (PV) structure, surface friction plays dual roles. Although friction is a sink to PV and thus dissipates PV in the eyewall, it helps keep the PV annulus narrow by enhancing the stretching deformation in the lower troposphere when condensational heating is present. In the absence of condensational heating, however, surface friction enhances the inward PV mixing by boundary layer frictional inflow and thus destroys the PV annulus. FIG. 2. (a) The four nested domains in the three major experiments. (b) The model's terrain height (contour interval of 500 m) of Luzon within the third mesh in CTL. JANUARY 2009 W U E T A L .

[1] The interesting eyewall evolution of Typhoon Zeb before, during, and after its landfall at Luzon was documented from both the satellite observation and numerical simulation. It is proposed that the terrain plays a critical role in leading to such evolution: first the eyewall contraction just before landfall, a following breakdown, and then the eyewall reformation after the storm returned to the ocean. Further studies of the physical mechanisms responsible for such eyewall evolution shall improve both our understanding of and the skill in predicting the structure and intensity changes of tropical cyclones in general and landfalling ones in particular.

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