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Xuyang Ge, Ziyu Yan, Melinda Peng, Mingyu Bi, and Tim Li

Abstract

The impact of different vertical structures of a nearby monsoon gyre (MG) on a tropical cyclone (TC) track is investigated using idealized numerical simulations. In the experiment with a relatively deeper MG, the TC experiences a sharp northward turn at a critical point when its zonal westward-moving speed slows down to zero. At the same time, the total vorticity tendency for the TC wavenumber-1 component nearly vanishes as the vorticity advection by the MG cancels the vorticity advection by the TC. At this point, the TC motion is dominated by the beta effect, as in a no-mean-flow environment, and takes a sharp northward turn. In contrast, the TC does not exhibit a sharp northward turn with a shallower MG nearby. In the case with a deeper MG, a greater relative vorticity gradient of the MG promotes a quicker attraction between the TC and MG through the vorticity segregation process. In addition, a larger outer size of the TC also favors a faster westward propagation from its initial position, thus having more potential to collocate with the MG. Once the coalescence is in place, the Rossby wave energy dispersion associated with the TC and MG together is enhanced and rapidly strengthens the southwesterly flow on the eastern flank of both systems. The steering flow from both the beta gyre and the Rossby wave dispersion leads the TC to take a sharp northward track when the total vorticity tendency is at its minimum. This study indicates the importance of good representations of the TC structure and its nearby environmental flows in order to accurately predict TC motions.

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Ziyu Yan, Xuyang Ge, Zhuo Wang, Chun-Chieh Wu, and Melinda Peng

Abstract

Typhoon Jongdari (2018) had an unusual looping path before making landfall in Japan, which posed a forecasting challenge for operational numerical models. The impacts of an upper-tropospheric cold low (UTCL) on the track and intensity of Jongdari are investigated using numerical simulations. The storm track and intensity are well simulated in the control experiment using the GFS analysis as the initial and boundary conditions. In the sensitivity experiment (RCL), the UTCL is removed from the initial condition fields using the piecewise potential vorticity inversion (PPVI), and both the track and intensity of Jongdari change substantially. The diagnosis of potential vorticity tendency suggests that horizontal advection is the primary contributor for storm motion. Flow decomposition using the PPVI further demonstrates that the steering flow is strongly affected by the UTCL, and the looping path of Jongdari results from the Fujiwhara interaction between the typhoon and UTCL.

Jongdari first intensifies and then weakens in the control experiment, consistent with the observation. In contrast, it undergoes a gradual intensification in the RCL experiment. The UTCL contributes to the intensification of Jongdari at the early stage by enhancing the eddy flux convergence of angular momentum and reducing inertial stability, and it contributes to the storm weakening via enhanced vertical wind shear at the later stage when moving closer to Jongdari. Different sea surface temperatures and other environmental conditions along the different storm tracks also contribute to the intensity differences between the control and the RCL experiments, indicating the indirect impacts of the UTCL on the typhoon intensity.

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