Impact Of Monsoon Trough On Tropical Cyclogenesis And Sudden Track Change

Tropical cyclone (TC) is one of the costliest natural disasters in China. An average of 7-8 TCs made landfall on China mainland. Tropical cyclogenesis and sudden track changes are the priority and key issues in the operational prediction and scientific research of TCs. This study using several kinds of the observational data and high resolution numerical simulations, investigates the tropical cyclogenesis events and sudden track changes over the western North Pacific during August to September 2004. The main conclusions are as follows:(1) TC always occurs in a synoptic-scale tropical cyclogenesis precursor. An observational analysis of tropical cyclogenesis events over the western North Pacific during 11 August to 10 September 2004 is conducted to demonstrate the role of synoptic-scale disturbances in establishing a limited area of low-deformation vorticity for tropical cyclogenesis. The OW concentrated areas are within the tropical cyclogenesis precursors with a radius of about 400-500 km and can be found as early as 72 hours prior to the formation of the tropical depression. When the TCs reached the tropical storm intensity, the concentrated OW is confined to an area of 200-300 radius and the storm centers are coincident with the centers of the maximum OW. This study indicates that the tropical cyclogenesis occurs in the low-deformation 18-72 hours prior to the formation of tropical depressions, suggesting the importance of low-deformation vorticity in pre-existent synoptic-scale disturbances. Although the Rossby radius of deformation is reduced in TC genesis precedes, the reduction does not sufficiently make the effective conversion of convective heating into the kinetic energy within the low-deformation area.(2) Tropical cyclogenesis precursors in the monsoon trough from 11 August to 10 September 2004 were chose. Through calculating the kinetic energy budget, we analyzed the development of the tropical cyclogenesis precursors. Aside from Chaba, others all formed in the cyclonic circulation of the northwestward-propagating synoptic-scale wave trains. The cyclonic circulation can couple with the northeast-southwest-titled anticlockwise circulation of the westward-propagating MRG wave. Kinetic energy budegt analysis reveals that kinetic energy of tropical cyclogenesis precursors increased before 12-72 hours prior to the formation of the tropical depression. Local convergence of the eddy geopotential flux was the dominant term. When the precursor first appeared at the far eastern end of the monsoon trough, energy accumulation was also a main factor for disturbance development. As the disturbance moved northwestward entering the monsoon shear region, barotropic energy conversion by the meridional gradient of the basic zonal flow was indeed another important energy source. For the case of Sarika, it formed during the MT westward retreat phase. The nolinear term played another important role for this disturbance development.(3) Extended numerical experiments in this study were conducted with the Weather Research and Forecast (WRF) model to demonstrate additional low-frequency influence on TC activity, which results from the role of the low-frequency circulation in the development of the synoptic-scale wave train. The cyclonic circulation of the wave train provides the low-level synoptic-scale disturbances that are necessary for TC formation. The observed TC formation events during 14 August to 10 September 2004 was first successfully simulated with the initial and lateral conditions derived from the National Centers for Environmental Prediction (NCEP) Final (FNL) Operational Global Analysis. Then the 27-day extended experiment was repeated only with the initial and lateral conditions derived from the FNL low-frequency (longer than 20 days) background. It is found that the development of the observed synoptic-scale wave train can be well simulated with TCs forming in the cyclonic circulations of the wave train although the wave length of the simulated wave train is substantially reduced due to the absence of higher-frequency influences. Sensitivity experiments indicate that the development of the wave train is sensitive to the monsoon trough structure. This study suggests that the low-frequency background has an important influence on the development of synoptic-scale wave trains.(4) Typhoons Aere and Meari that formed in the western North Pacific in 2004 tracked northwestward in a similar manner until their tracks underwent changes over the East China Sea, where Aere moved southwestward and Meari take a sudden northeastward recurving track. This study shows that the interaction between low-frequency circulation and typhoons are mainly responsible for the track changes. As Aere was located to the south of the subtropical high (SH), it interacted with the SH through the advection of the low-frequency vorticity by synoptic-scale flows, enhancing negative vorticity on the northwestern side of the storm and thus the northeastward steering flow. Meanwhile, the Rossby wave energy dispersion strengthened the southwestward steering flows on the southeastern side of Aere. As a result, the synoptic-scale steering became weak, and Aere moved southwestward due mainly to the steering of the low-frequency circulation. On the other hand, Typhoon Meari gradually approached the western side of the SH, the synoptic-scale winds on the southeastern side of the typhoon was strengthened, which made Meari took a sudden northeastward track with no enhanced northeastward steering flows like the Aere case.