Boundary layer dynamical structure during secondary eyewall formation

Secondary eyewall formation (SEF) is widely recognized as an important research problem in the dynamics of mature tropical cyclones. While there is not a consensus on the phenomenon's fundamental physics, it has been shown that the development of the wind maxima in SEF occurs within the boundary layer and that it follows a chain of events initiated by a substantial radial expansion of the tangential wind field. Building on recent theoretical developments and on numerical investigations (based on both full-physics mesoscale and highly idealized integrations), it has been proposed that the boundary-layer dynamics of a maturing hurricane vortex is an important controlling element in SEF.

We analyze the radial and vertical structure of the forces (per unit mass) and accelerations in in the boundary layer at different stages in the life cycle of a secondary eyewall case. The case occurred in a mesoscale, convection permitting numerical simulation of a tropical cyclone, integrated from an initial weak mesoscale vortex in an idealized quiescent environment. The simulation exhibits a canonical eyewall replacement cycle and has been studied extensively in the literature. The tangential, radial and vertical momentum equations, expressed in cylindrical coordinates, are analyzed in a storm-relative coordinate system. We highlight the different character of different vertical boundary layer regions, from the bottom layers to the level where the secondary wind maxima emerges, emphasizing, in each identified layer, the role of friction and advective nonlinearities. Implications on the usefulness of the agradient force as an insightful diagnostic, the development of shock-like structures and the relative role of mean and eddies terms in the secondary eyewall evolution are discussed.