Tropical Cyclone Genesis And Development Under The Background Of Monsoon Over East Asian And Western North Pacific

The genesis and development of Tropical Cyclone(TC) is a great challenge in TC research and operational forecast. TCs forming in offshore area of China also form in a classic East Asia Monsoon Region. Their developments are significantly influenced by the complicated multi-scale interactions between the large-scale monsoon circulation and meso-scale initial vortex and convections. In our study, the TC genesis and development under the background of Monsoon over East Asia and western North Pacific are explored, using statistical analysis, dynamic diagnostics and numerical simulation. Our results are supposed to get better understanding on TC genesis and development over monsoon area, and may provide some clues for operational TC forecast.TC records of three best-track datasets from Chinese Meteorological Administration(CMA), Joint Typhoon Warning Center(JTWC) and Japan Meteorological Agency(JMA) over western North Pacific are compared firstly. Results show that there are large discrepancies about TC genesis and development. The differences of TC numbers mainly result from the number diversity of non-developing tropical cyclone(NTC, the maximum intensity is TD). Also, three datasets reflect similar features for developing tropical cyclone(DTC, the maximum intensity is ≥ TS), including geographic distribution, seasonal distribution and variations, but not for NTC. These uncertainties bring great challenges for the research of TC genesis.Then, statistical features of TC genesis over western North Pacific are studied based on CMA best-track data from 1983 to 2013. Results indicate that 34% TCs form in the East Asia Monsoon Region(west of 130oE), in which 75% are DTCs. But only 7% of DTC could intensify to severe typhoons and super typhoons. A large number of DTCs develop to tropical storm over northern area of South China Sea during May to September, while NTCs occur most frequently over East Asia Monsoon Region in May and October. Landfall is the main reason for NTC dissipation.The low-level circulations for TC genesis and development in East Asia Monsoon Region can be categorized into four patterns: Easterly Wave(EW), Monsoon Shearline(MS), Monsoon Convergence(MC) and Monsoon Invert Trough(MI). EW pattern only accounts for 14%, and the others, related to monsoon, account for 86%. Relatively, TCs genesis most frequently in MS pattern. Furthermore, the corresponding upper-level circulations are also various. When TCs develop in EW and MC pattern, there are active tropical upper tropospheric trough(TUTT) activity in middle Pacific. While the middle latitude trough influences TC genesis in MI pattern significantly. Under the different configurations of upper and low level circulations, TDs present various possibilities of developing to tropical storm. The highest possibility is in MC pattern, while the lowest one is MI pattern.The comparisons among different low-level circulation pattern demonstrate that the environmental factor, vortex enhancement and convection development present different features for NTC and DTC: 1) sea surface temperature(SST) and land impact are more important for TC development in EW pattern, while low-level vorticity in MC pattern and SST, vertical wind shear(VWS) in MI pattern; 2) During DTC enhancement, the vorticity vertical transfer is much stronger and last longer than that for NTC; 3) In MS and MI circulation pattern, deep convections are more concentrated for DTC than NTC, and most intense convections locate at southwest quadrant of TC circulation.However, there are common characteristics during DTC and NTC genesis: 1) Monsoon flow persistently provides enough water vapor for TC genesis to make more wet environment in middle level; 2) The low-level vorticity enhanced significantly, which relies on the effect of low level convergence; 3) The area of deep convections expand continuously, and the convection bursts onset at about 12 to 18 hours before TC genesis.Note that the MI pattern is a special monsoon circulation with the lowest TC developing probability. Typhoon Meranti is a typical DTC in this pattern, which formed in a monsoon trough stretching from southwest to northeast, inducing by another storm Malou in the northeast of Meranti. So the monsoon flow curved northward near Taiwan island with a large wind shear, which could be recognized as the initial disturbance for Meranti. This disturbance is proved to be critical by sensitivity numerical experiments. If there is no disturbance, there is no TC genesis. Besides, typhoon Malou is evidenced to be important for Meranti development. On the one hand, it strengthens the moisture transport by monsoon flow to humidify middle level environment. On the other hand, its cyclonic circulation could provide southward flow to the north of monsoon flow, to enhance the convergence at low level. Then the updrafts in convection strengthen, which leads to TC intensification. In addition, terrain also helps the vortex intensification after its formation.Meanwhile, Meranti is also influenced by an upper-level cold low. Numerical study indicates that the cold low is not a key factor for TC genesis but could favor its development. It could help the establishment of multi outflow channels in TC upper atmosphere. In addition, there are positive potential vorticity(PV) wave trains between cold low and TC circulation. A series of positive PV split from cold low and are transported to middle level, then merge into TC vortex at about 700 h Pa. Both of them are beneficial for TC intensification.In addition, Meranti experienced strong VWS during its genesis. Then it presented a remarkable asymmetric inner core structure. That is, the deep convections occurred in downshear(southwest) to help establishing TC secondary circulation with low-level inflow and upper-level outflow. Before deep convection bursts, there are active monsoon convections in the south of initial disturbance. At the north edge of convections, there is compensating subsidences. And the air becomes cold and dry by evaporation and descends into boundary layer. Then it is transported northward by monsoon flow, and confronts high entropy air to trigger deep convections. Meanwhile, an enhancement of humidity occurs in the middle-level mesoscale vortex. After deep convections bursts, the vortices in the middle and low level approach each other and intensify simultaneously. Then Meranti completes its genesis finally.Based on the above analysis results, a logistic regression model for TD development forecast is attempted to build. Results show that, using six environmental factors, including divergence at 200 h Pa, relative humidity at low level, SST, vorticity at 500 h Pa and 850 h Pa, and land impact, the model could be most effective. The predicting accuracy could reach about 80% for whether TD can develop. But the developing cases are overestimated.