Secondary Eyewall Formation and Concentric Eyewall Replacement in Association with Increased Low-Level Inner-Core Diabatic Cooling

Abstract

Abstract The role of increased diabatic cooling in secondary eyewall formation (SEF) and eyewall replacement cycle (ERC) is examined using idealized numerical simulation. The experiment with the low-level inner-core diabatic cooling increased by 30% features the low-entropy air and downward motion in the inner-core region whereas the convergence and active convective updrafts are in the outer-core region. In collaboration with the favorable ambient dynamical conditions and boundary layer dynamical processes, the concentric convective ring is initiated with the aid of the outward expansion of strong wind field, and then contracts inward to replace the inner eyewall. Subsequently, the deep-tropospheric radial outflows driven by the large outward-directed agradient force related to the massive strong tangential wind generate a largely outward-tilted eyewall, eventually forming a large-eyed storm. The sensitivity to the strength and radial location of diabatic cooling shows that neither the 20% increase nor 10-km radially inward shift of the low-level cooling produces a pronounced SEF and ERC because of the lack of an evident moat region. In contrast, both the 40% increase and 10-km radially outward shift of cooling lead to the active outer rainbands occurring at a larger radius. In the former case, because of the deep-layer radial outflow above the boundary layer, the largely outward-tilted concentric eyewall shrinks slowly, directly creating a large-eyed structure. In the latter case, the formation of concentric eyewall is delayed because of the low inertial stability at a large radius, but experiences an expeditious ontraction because of the strong radial inflow.