Essential Dynamics of the Vertical Wind Shear Affecting the Secondary Eyewall Formation in Tropical Cyclones

Abstract

This study investigated the effects of vertical wind shear (VWS) with varying magnitudes on secondary eyewall formation (SEF). It turns out that weak-to-moderate VWS advances the timing of SEF. Strong VWS, however, is unfavorable for SEF in our idealized simulations. VWS affecting SEF mainly lies on its influence on the outer rainbands (ORBs). Under weak-to-moderate VWS, ORBs develop more quickly in the downshear side and have distinct stratiform features in the upshear-left quadrant. The asymmetric inflow associated with the stratiform cooling descends into the boundary layer, reinforcing radial convergence at the radially inward side of ORBs. The radial convergence enhances the low-level convection, resulting in strengthened boundary layer inflow and accelerated low-level tangential wind jet. A budget analysis reveals that tangential advection extends a tangential wind jet farther downwind, forming supergradient winds above the boundary layer in the upshear-right quadrant. As the ORBs propagate into the upshear-right quadrant, the pre-existing supergradient winds enhances the low-level convection, facilitating the closing of the secondary convective ring. The evolution in the upshear side exhibit quadrant-dependent interactions between ORBs and boundary layer. Following that, azimuthal-mean tangential wind acceleration becomes visible, forming the secondary tangential wind maximum. Under strong VWS, the storm is weakened and the boundary layer in the upshear-left quadrant is invaded by low-entropy air, resulting in decreased conditional instability and low-level thermal buoyancy. The decreased stratiform precipitation due to weakened convective activity in the upshear-left quadrant prevents the upshear propagation of ORBs and thus is unfavorable for SEF.