Tropical Cyclone Boundary-Layer Asymmetries in a Tilt-Following Framework

Previous observations and modeling studies showed that tropical cyclones (TCs) in a vertically sheared environment tend to develop an asymmetric boundary layer (BL). While the spatial association between the observed BL asymmetries and environmental shear has been demonstrated, the exact cause of these BL asymmetries and the phase relationship between them are less well understood. In this study, we examine the dynamical processes leading to the asymmetric kinematic structure of the TC BL in a sheared environment using idealized, convection-permitting model simulations.

Our results show that the emergence of the BL asymmetries is closely linked to the TC vortex tilt and rainband processes. Specifically, stratiform diabatic processes in the downtilt-left region result in deep-layer descending inflow in the mid-troposphere, which brings mid-tropospheric, low-θ_e air towards the BL and forms an asymmetric surface cold pool in the downtilt-left quadrant. This descending inflow also advects high absolute angular momentum inward, redistributing the vertical vorticity and causing a storm-scale tangential wind acceleration within the BL cold pool region. As the BL low-θ_e air advances inward, it becomes supergradient and decelerates radially, eventually becoming outflow downwind of the surface cold pool region.

As the tilted TC vortex and the accompanying rainband precess cyclonically over time, the above sequence of events and the resultant BL asymmetries also precess cyclonically, maintaining a quasi-stationary configuration relative to the vortex tilt. These results suggest that the primary organizing factor of the boundary layer asymmetries is the tilted vortex structure, and not strictly the environmental shear direction.