Using brightness temperature data from passive microwave satellite imagery, this study examines tropical cyclones (TCs) with concentric eyewall (CE) in the western North Pacific between 1997 and 2011. The identified CEs are divided into two types according to the characteristics of the eyewall replacement cycle (ERC) in the microwave imagery: a CE with a typical ERC (T-ERC) and a CE without an ERC (N-ERC). It is indicated that 88 % T-ERCs reach peak intensity near (0.2 h after on average) CE formation, whereas 90 % N-ERCs reach peak intensity prior to (22.0 h on average) CE formation. In general, N-ERCs tend to occur when there are strong interactions between the environment and the CE, whereas T-ERCs occur in a relatively quiet environment. The three-dimensional conceptual models of the environmental configurations for both CE types are proposed. Specifically, N-ERCs are accompanied by stronger southwesterly and southeasterly inflows, active low-level trough, and stronger subtropical high (SH) and South Asia high (SAH), compared with T-ERCs. For N-ERCs, the stronger inflows may bring in a large amount of moisture, and the active low-level trough may result in a large vertical wind shear (VWS). The stronger SH and SAH may contribute to changes in the intensity and direction of the VWS for N-ERCs, and hence trigger the development of local convection in the outer eyewall. The asymmetries in the convection of the outer eyewall may weaken the ability to cut off the radial inflow to the inner eyewall. Consequently, N-ERCs fail to finish the ERC and weaken rapidly in intensity, even though the moisture remains sufficient after CE formation.
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.
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