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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, 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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