Interaction between Eyewall and Outer Rainbands During ERC

Observations and many previous numerical simulations show that the inner eyewall weakens as the outer concentric eyewall forms and eventually disappears with the intensi-fication of the outer eyewall. In fact, as indicated by Figs. 3.7 and 3.11, the inner eyewall starts to weaken long before the secondary tangential wind maximum forms. Although

intensely studied, there are still debates on what causes the demise of the primary eyewall.

Willoughby et al. (1982;1990) argued that the outer eyewall could induce substantial sub-sidence over the inner eyewall region to inhibit the eyewall convection and eventually lead to the demise of the inner eyewall. This argument is questioned by Rozoff et al. (2008) who derived an analytical solution of transverse circulation equation of a balanced vortex model and showed that the subsidence induced by the outer eyewall is primarily within the eye and the moat. The simulation and Sawyer-Eliassen diagnoses performed in this study allow us to revisit this issue using the realistically simulated numerical data of a TC vortex by a 3D full physics model.

The decomposition of total heating into the eyewall and outer rainband components performed previously provides a useful way to examine the interaction between the eyewall and outer rainband convection during ERC. Figure3.21 shows the subsidence induced by the outer rainband heating overlapped with the WRF simulated azimuthal-mean vertical velocity at different time during the ERC. The outer rainband induced sub-sidence contributes significantly to the moat and the strongest subsub-sidence is at the outer edge of rainband updraft, which is typical for the convection induced subsidence. How-ever, the figure clearly shows that the outer rainband induced subsidence is non-negligible right over the inner eyewall updraft at all time. As indicated by Figs. 3.11a, 3.11b, and 3.11c, the inner eyewall starts to weaken well before the outer concentric eyewall forms. Apparently, the rainband induced subsidence is one of reasons to cause such weakening of inner eyewall. This mechanism is enhanced with the increase of sub-sidence as the outer rainbands evolve into a closed ring of convection. This result

pro-vides the convincing evidence to support Willoughby et al. (1982;1990)’s argument about the role of downdraft induced by the outer eyewall in causing the demise of the inner eyewall. But it should be emphasized here that the weakening of inner eyewall starts well before the formation of the outer eyewall. The suddenly enhanced outer rainband convec-tion can induce sufficiently strong subsidence to affect the evoluconvec-tion of inner eyewall.

Figure3.22 is a similar plot to Figure 3.21 but for the subsidence induced by the in-ner eyewall heating. Similar to the subsidence induced by the outer rainband heating, the strongest subsidence occurs at the outer edge of the eyewall updraft. It is the leading cause for the formation and development of moat. The eyewall heating also induces fairly strong subsidence over the outer rainband but its influence decreases with the weakening of inner eyewall as the outer eyewall develops. An immediate question is why the outer rainband induced subsidence can inhibit the development of inner eyewall whereas the inner eyewall induced subsidence does not appear to have a significant impact on the de-velopment of the outer rainbands. The answer may lie in the radial inflow induced by the heating and momentum forcing. Figure 3.23 shows the radial flow induced by the eyewall and outer rainband heating and momentum forcing, respectively. Both eyewall and outer rainband forcings induce a similar structure of radial flow with outflow concen-trated within the eyewall and rainband updraft and inflow extending radially outward from the outer edge of outflow. Before the 72^nd h when outer rainband convection is weak, the eyewall induced radial inflow extends well beyond the “SEF region”. The moist air carried by the radial inflow provides fuel for the development of eyewall. How-ever, the outflow induced by the outer rainband forcing provides a mechanism to cancel

the inflow induced by the eyewall. The cancellation increases as the outer rainband con-vection enhances and the eyewall weakens possibly due to the reduced inflow. By the time the outer eyewall forms (78^th h), its induced radial outflow extends throughout the vertical column, which works as a barrier to cut off the radial inflow into the inner eyewall, and indeed as the figures indicated the radial inflow induced by the inner eyewall at this time is really confined between the inner and outer eyewall. The cutting off fuel to the inner eyewall causes it to rapidly decay and eventually disappear. Again, the importance of enhanced rainband convection to ERC is clearly demonstrated from the perspective of transverse circulation. In short, the analyses here along with the evidence provided suggest a mechanism for the demise of the inner eyewall that involves the inter-action between the transverse circulations induced by the eyewall and outer rainband convection. In particular, the outflow induced by the outer rainbands plays an important role in confining the inflow induced by the eyewall. The cutting off fuel into the inner eyewall appears to be the leading cause for the rapid decay of the inner eyewall.