Meso-β-scale environment for the formation of a stationary band complex in idealized numerical experiments

The stationary band complex (SBC) is a distinct, quasi-stationary convective asymmetry outside of the eyewall of tropical cyclones. The SBC has long been hypothesized to play an important role for changes in storm intensity. Convective-scale processes within the SBC have received considerable attention following a major field campaign in 2005. In the current work, we focus on meso-β-scale (20 – 200 km) processes that may lead to favorable conditions for the formation of a SBC.

Results of seminal work by Willoughby et al. (1984) indicate that SBCs form in storms that are embedded in environmental flow with vertical wind shear. To reduce the inherent complexity, we here examine SBC formation in previously-published, convection-permitting idealized numerical experiments of tropical cyclones in vertical shear. Consistent with a result of Willoughby et al., we find that the SBC is located near the thermodynamic boundary between the TC's “moist envelope” – high-θ_e air that encompasses the TC's inner core – and the lower-θ_e environment.

Our re-examination of the SBC formation suggests that the previously-proposed formation mechanism based on vorticity conservation needs to be revised. As one step towards a more complete understanding of SBC formation, we here propose a mechanism for the generation of a favorable, meso-β-scale environment in which the SBC then forms. This mechanism comprises two fundamental effects of vertical wind shear on the tropical cyclone: vortex tilt and the distortion of the moist envelope. For tropical cyclones that are subject to approximately uni-directional vertical shear, one can expect an azimuthal wavenumber-1 pattern of high-vorticity and high-θ_e values to the right of the shear vector extending to radii of 150 – 200 km. This region outside of the eyewall is thus favorable for the initiation of deep convection. The radial shear of the swirling winds then promotes the organization of the convection into a banded structure. A simple boundary layer model and trajectory analysis is employed to examine further the forcing for ascent in the SBC.