Helical convective vortices in the hurricane boundary layer simulated with the University of Wisconsin Nonhydrostatic Modeling System (UW-NMS)

This study examines coherent boundary layer structures observed in an idealized hurricane simulated with the University of Wisconsin Nonhydrostatic Modeling System (UW-NMS), which are referred to as helical convective vortices (HCVs). Bouyant forcing likely plays a key role in HCV formation and evolution, since the HCVs are found just above a superadiabatic layer in the lowest part of the HBL, with a neutral layer at the level of the HCV circulations. The wind shear profile also appears crucial in HCV formation. There is intense speed shear as well as directional shear in the HBL from frictional forcing causing winds to decrease and turn to the left as one moves down through the hurricane below the tangential wind maximum. Indeed, the HCVs are found to be aligned with the shear vector, as one would expect from directional shear inflection point instability. The directionally sheared nature of the HBL also results in HCVs being helical in nature.

HCVs may be an important part of the eyewall dynamics puzzle, since it appears that plumes of vorticity in the eyewall associated with HCVs being turned upwards act as building blocks for the high vorticity annulus in the eyewall. The helical circulations associated with these plumes, rooted in HBL HCVs, may resist entrainment and transport nearly adiabatic contents to the outflow layer. This is similar to the processes associated with vortical hot towers (VHTs), as described in previous studies, though it seems more appropriate to refer to the structures in the eyewall emanating from HCVs as helical convective plumes (HCPs). Additionally, since HCVs transport lower angular momentum from the HBL into the eyewall, they may be an important mechanism by which the secondary circulation of the hurricane injects lower angular momentum into the eyewall.