Vortex Alignment Processes in Vertically Sheared Tropical Storms

A number of recent radar-observed tropical depressions and tropical storms in sustained moderate vertical wind shear share a seemingly similar pathway to future intensification: A shallow surface-concentrated vortex intensifies without much vertical development while a distinct mid-level vortex persists within the downshear convective region. A more rapid intensification may occur if the highly asymmetric vortex configuration evolves into one dominated by a single vertically-coherent vortex extending through the lower to middle troposphere. This transition to the vertically-coherent vortex is generally difficult to observe in its entirety due to limitations of 12-hourly airborne radar sampling, although ongoing attempts to view the evolution in a multi-case composite framework may provide further insight into the mechanisms involved.

While some recent studies have emphasized vortex redevelopment (i.e., the formation of an entirely new, dominant surface circulation) as a pathway to the vertically-coherent vortex, it is not yet clear that this is the primary mechanism responsible for the transition from tilted to aligned vortex structure in most cases. One possibility is that the vortex tilt evolves according to the vortex-Rossby wave damping process previously applied to vertically-developed, hurricane-like vortices in vertical wind shear. A simple linear theory has proven useful in understanding that problem. In light of the pronounced asymmetry in cloudiness characteristic of the cases considered here, approximations made in the linearization of the thermodynamic equation in particular are first assessed. Then the simple linear theory is evaluated at the post-genesis tropical depression and tropical storm stage by comparing the theoretical tilt predictions with those from sheared tropical storm simulations in CM1. Further comment on the relationship between vortex tilt and intensification is also made.