Abstract
The problem of tropical cyclogenesis in a triple‐scale coexisting system which includes a subtropical high ridge, a preexisting cyclonic circulation with the initial maximum tangential wind Vbmax of 5 m s−1, and single‐, double‐, or four‐cluster anomalies is examined in this paper, using a shallow water primitive equation (SWPE) model. This work validates the recent results concerning the upscale cascade mechanism suggested for tropical cyclogenesis in the double‐scale coexisting system containing the basic state vortex and anomalies, and it is shown that, for the double‐cluster anomaly, the mean azimuthal maximum tangential wind of the final vortex is increased and the radius of the maximum tangential wind (RMW) is contracted in the case of 1.388 ≥ Kut ≥ 0.0 in which Kut = ∣usmax∣/Vbmax with usmax being the maximum of the easterlies south of the initial subtropical ridge, which demonstrates an amplification of the vortex within this range of parameter. For the single‐cluster anomaly of large amplitude the anomaly will become a master vortex, and for the four‐cluster one the spin‐up of the initial basic state vortex will be decreased. Compared with the case of still environmental currents, introducing the subtropical ridge will have the following impact on axisymmetrization in horizontal shearing currents: (1) The interaction of a single‐cluster anomaly with the basic state vortex will occur in the environmental currents with cyclonic vorticity when the anomaly is located initially in the SW/SE quadrant of the basic state vortex; the interaction between the anomaly and the basic state vortex makes the spin‐up double that in the case of still environmental currents. (2) Although axisymmetrization of vortex configuration may occur, it is mainly limited to the inner region of the vortex with a distinct new nonaxisymmetry in its outer region, and there exist a series of cells of smaller scales in the northeastern quadrant of the outer region. (3) The total kinetic energy for the inner region of the vortex in the mean increases but fluctuates with time, unlike the still environmental currents case in which it increases monotonically.
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