The Role of the Boundary Layer Dynamics in Secondary Eyewall Formation

Mature Tropical Cyclones (TCs) often experience secondary eyewall formation (SEF). In light of different boundary layer (BL) dynamical pathways to SEF proposed in the literature, this study aims to explore the role of BL dynamics in SEF. Previous studies suggested that the unbalanced responses in the BL can serve as an important mechanism for SEF. Other studies also showed that the local gradient of vorticity in an environment of low absolute vorticity can induce frictionally forced updraft and the consequent positive feedback can serve as the key for SEF.

Adopting a nonlinear diagnostic BL model, this study attempts to understand how flow in the BL and lower troposphere responses to the vortex structure aloft (mostly in gradient wind balance) and the differences of BL responses at the vortex’s different stages before SEF. Results show that the applied BL model can well capture the major flow characteristics prior to SEF that was identified in previous studies. Next, by fitting the prescribed vortex structure, we obtain the idealized gradient wind profiles that can represent the TC’s primary structure. The idealized profiles are used as the BL model’s upper boundary condition for the idealized control run. Results show that 6 hours before SEF, without any gradient wind perturbation, the BL model can diagnose larger values of supergradient wind and vertical motion. Furthermore, sensitivity experiments are conducted by adding a bump in the prescribed gradient wind profile at different radius. It is shown that the frictional updraft appears stronger while the added gradient wind bump is located at some specific radii. Moreover, to further examine the BL response to the evolving TC structure aloft, this study presents a modified nonlinear diagnostic BL model in which the upper boundary condition is updated during the integration. Results show that the modified BL model can capture a range of radii with persistent secondary upward motion maximum while the upward motions at outer radii dissipate rapidly.