Meso-β-scale environment for the stationary band complex of vertically sheared tropical cyclones

The stationary band complex (SBC) is a distinct, quasi-stationary convective asymmetry outside the eyewall of a tropical cyclone (TC) and plays an important role for TC intensity. Seminal work by Willoughby et al. related the SBC to the kinematic boundary between the TC and its environment and proposed a formation mechanism for the SBC. These results have been derived from observational data that are limited in radial and azimuthal coverage. The current study revisits these results using data from an idealized numerical experiment, which are not subject to such limitations. Consistent with Willoughby et al., the SBC in the idealized experiment is located near the boundary between the TC's 'moist envelope' and the lower theta(e) environment. In contrast, however, the SBC is not associated with the kinematic boundary between the TC and the environmental flow. Furthermore, the proposed formation mechanism does not operate in the idealized experiment. I propose an alternative formation mechanism that links the SBC to two fundamental consequences of environmental vertical wind shear: (i) the distortion of the 'moist envelope' and (ii) the tilt of the TC vortex. In the idealized experiment, the respective regions of high-theta(e) air and a positive vorticity anomaly overlap at low levels in the semicircle to the right of the shear vector well outside the eyewall. A simple slab boundary-layer model indicates that frictional convergence associated with the vorticity asymmetry is a viable, dynamical forcing mechanism for deep convection. While ascent out of the boundary layer occurs over a radially broad region, the large radial shear of the TC's swirling winds promotes the organization of the ensuing convection into a band-like pattern. I emphasize that this formation mechanism considers only the meso-beta-scale envelope of the SBC. As shown previously by others, convective-scale processes contribute further to the organization of the SBC within this envelope.