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Huijun Zong and Liguang Wu

Abstract

Tropical cyclones (TCs) always develop from synoptic-scale disturbances. While early studies suggested that the presence of synoptic-scale disturbances may enhance large-scale conditions for TC formation, recent studies argued that TC-precursor disturbances can establish a rotation-dominant area, which can play a crucial role in organizing convective activity and converting convective heating to rotational energy for storm-scale intensification. To demonstrate the synoptic-scale influence of TC-precursor disturbances, 91 TC formation events within the monsoon trough over the western North Pacific during 2000–10 were examined by separating TC-precursor disturbances from the low-frequency background. The composite analysis shows that the synoptic disturbances indeed enhance the mid- and low-level relative vorticity and convergence, but contribute little to reducing vertical wind shear.

The dynamic composite that is conducted with respect to disturbance centers indicates that TC-precursor disturbances within the monsoon trough establish a rotation-dominant region with a radius of less than 550 km. The cyclonic rotation increases with time 72 h prior to TC formation and nearly all air particles keep recirculating in the core area with a radius of about 220 km. Analysis of a specific case suggests that vorticity increase occurs through the merger of mesoscale convective systems in the rotation-dominant area. The enhancing rotation in the core area may efficiently convert diabatic heating to kinetic energy for TC formation. Thus, it is suggested that the important role of TC-precursor disturbances in TC formation is the establishment of a limited, rotation-dominant area.

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Liguang Wu, Huijun Zong, and Jia Liang

Abstract

An observational analysis of observed sudden typhoon track changes is conducted with a focus on the underlying mechanism and the possible role of slowly varying low-frequency flows. Four typhoons that took a generally northwestward track prior to sharply turning northeastward in the vicinity of the East China Sea are investigated.

It is found that the sudden track changes occurred near the center of the Madden–Julian oscillation (MJO)-scale cyclonic circulation or at the bifurcation point of the steering flows at 700 hPa, and they were all associated with a well-developed quasi-biweekly oscillation (QBW)-scale gyre. Calculation of vorticity advection suggests that the peripheral ridging resulting from the interaction between the typhoons and the flows on the MJO and QBW scales can compress the typhoon circulation, leading to an area of high winds to the east or south of the typhoon center. The enhanced synoptic-scale winds shifted the typhoons northward and placed them in a northeastward orbit under the steering of the flows associated with the Pacific subtropical high. The sudden track change can be likened to the maneuvering of satellite orbit change in that the enhanced synoptic-scale winds act as a booster rocket to shift the typhoons northward to the southwesterly steering flows.

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Liguang Wu, Huijun Zong, and Jia Liang

Abstract

Large-scale monsoon gyres and the involved tropical cyclone formation over the western North Pacific have been documented in previous studies. The aim of this study is to understand how monsoon gyres affect tropical cyclone formation. An observational study is conducted on monsoon gyres during the period 2000–10, with a focus on their structures and the associated tropical cyclone formation.

A total of 37 monsoon gyres are identified in May–October during 2000–10, among which 31 monsoon gyres are accompanied with the formation of 42 tropical cyclones, accounting for 19.8% of the total tropical cyclone formation. Monsoon gyres are generally located on the poleward side of the composited monsoon trough with a peak occurrence in August–October. Extending about 1000 km outward from the center at lower levels, the cyclonic circulation of the composited monsoon gyre shrinks with height and is replaced with negative relative vorticity above 200 hPa. The maximum winds of the composited monsoon gyre appear 500–800 km away from the gyre center with a magnitude of 6–10 m s−1 at 850 hPa. In agreement with previous studies, the composited monsoon gyre shows enhanced southwesterly flow and convection on the south-southeastern side. Most of the tropical cyclones associated with monsoon gyres are found to form near the centers of monsoon gyres and the northeastern end of the enhanced southwesterly flows, accompanying relatively weak vertical wind shear.

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Liguang Wu, Zhongping Ni, Jingjing Duan, and Huijun Zong

Abstract

Tropical cyclones (TCs) over the western North Pacific (WNP) are usually embedded in the multitime-scale summer monsoon circulation and occasionally experience sudden track changes, which are currently a challenge in TC forecasting. A composite analysis of 15 sudden north-turning cases and 14 west-turning cases that occurred during the period 2000–10 was conducted with a focus on influences of low-frequency monsoon circulations. It is found that TCs in the two specific categories of track changes are embedded in a monsoon gyre of about 2500 km in diameter on the quasi-biweekly oscillation (QBW) time scale, which is also embedded in a larger-scale cyclonic gyre or monsoon trough on the Madden–Julian oscillation (MJO) time scale. The two types of track changes are closely associated with interaction between low-frequency and synoptic flows. Two different types of asymmetric flow patterns are identified on the synoptic time scale in the vicinity of these TCs. In the north-turning case, enhanced winds lie mainly on the southeast side of TCs due to strong ridging associated with interactions between low-frequency and synoptic flows. In the west-turning case, the westward extension of the subtropical high leads to ridging on the northwest side of TCs and the enhanced winds can largely offset the steering of enhanced southwesterly winds on the synoptic time scale. Thus the north-turning (west turning) sudden track changes are affected primarily by the synoptic-scale (low frequency) steering. This may be one of the reasons for the larger forecasting errors in the north-turning case than in the west-turning case.

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