The Boundary Layer Dynamics of Tropical Cyclone Rainbands

Spiral bands are ubiquitous features in tropical cyclones and significantly affect boundary layer thermodynamics, yet knowledge of their boundary layer dynamics is lacking. Prompted by recent work that has shown that relatively weak, balanced axisymmetric vorticity perturbations outside of the RMW in tropical cyclones can produce remarkably strong frictional convergence, and the observation that most secondary eyewalls form by the “wrapping up” of a spiral rainband, we explore the effect of asymmetric vorticity perturbations, specifically those that mimic spiral bands. The mass field corresponding to a modified Rankine vortex with added spiral vorticity band is constructed using the nonlinear balance equation, and supplied to a diagnostic three-dimensional boundary layer model. The resulting flow has strong low-level confluence and convergence along the vorticity band, associated with a marked updraft, and extending some distance downwind. There is a marked along-band wind maximum in the upper part of the boundary layer, similar to observations, which is stronger than the local frictionless flow. There is also a marked gradient in the inflow layer depth across the band and an increase in the surface wind factor (i.e., the ratio of surface wind speed to nonlinear-balanced wind speed) near the band. All of these features are similar to those seen with axisymmetric vorticity perturbations, and it appears that the boundary layer dynamics near a spiral vorticity band form a continuum with the flow near an axisymmetric vorticity ring. We examine the sensitivity of the flow to band parameters, including the length, width, location, crossing angle and amplitude. The possible contribution of boundary layer dynamics to the formation of the principal rainband formation will be discussed.