Downwind Development in a Stationary Band Complex Leading to the Secondary Eyewall Formation in the Simulated Typhoon Soudelor (2015)

Abstract

Abstract Previous studies have demonstrated the importance of the downwind development of a rainband in secondary eyewall formation (SEF) in tropical cyclones. However, the details of the transition from rainbands to a secondary eyewall are not well understood. This study examined the convection onset in the early stage of SEF in the numerically simulated Typhoon Soudelor (2015). Results show that the convection onset in the SEF region was associated with the organization of a stationary band complex (SBC), which resulted from the outward propagation of inner rainbands and downwind propagation of secondary rainbands, with convection enhanced in the downwind sector of the SBC. The outward propagation of the inner rainbands involved the mesoscale pressure perturbations-induced unbalanced boundary layer dynamics, which were responsible for the radial outflow above the boundary layer inflow and the related secondary circulation on the inward side of the rainband. In the downwind propagation, secondary rainbands evolved into stratiform precipitation in the downshear-left quadrant, where mesoscale descending inflow continuously occurred and transported low equivalent potential temperature (θe) and dry air downward and downwind, leading to a decrease in low-level θe on the outward side of the rainband. A balanced state of the cold pool dynamics was finally reached in the downwind sector of the SBC between the supergradient-induced vertical wind shear and the low-θe-induced cold pool, resulting in the convection enhancement. Our results strongly suggest that both the unbalanced dynamics and rainband processes were essential in the early stage of SEF. Significance Statement The purpose of this study is to better understand how spiral rainbands evolve into a stationary rainband complex (SBC), and the physical processes responsible for the convection enhancement in the downwind sector of the SBC. This is important because it is this downwind convective enhancement that leads to the secondary eyewall formation (SEF). Our results provide the details of the transition from rainbands to a secondary eyewall and suggest that both the unbalanced boundary layer dynamics and rainband processes are essential in the early stage of SEF.