Study On Explosive Developed Cyclone Transformed From Typhoon LUPIT Over The Northwestern Pacific

The structure, evolutionary process and physical mechanism of an explosive extratropical cyclone transformed from supper typhoon LUPIT over the Northwestern Pacific from 25 to 30 October 2009 are investigated by using reanalysis data, FNL (Final Analyses) from NCEP (National Centers for Environmental Prediction) and MTSAT-1R (Multi-functional Transport Satellites-1R) infrared channel albedo data from JMA (Japan Meteorological Agency). The tropical cyclone initiated over the east of the Philippines of Northwestern Pacific at 18 UTC 25 October 2009, moving from northwest to northeast. The moment that the tropical cyclone was transformed into extratropical cyclone was at 06 UTC 26 October 2009, then explosively developed and disappeared at 12 UTC 30 October 2009. The whole process lasted about 102 hours after extratropical transition. The maximum deepening rate was 2.4 hPa hour-1 and the lowest central pressure was 955 hPa.It was found that factors contributed to the explosive development of extratropical cyclone transformed from tropical cyclone as 200 hPa divergence,500 hPa low vortex strengthen, water vapor transport at lower level of troposphere, potential vorticity distribution and baroclinic frontal systems are important at different stage to cyclonic development:1. Jet stream of 200 hPa intensifies cyclone mainly during the development stage and mature stage. During development stage, cyclone was strengthened by strong divergence because its location underneath the south of the jet axis. At mature stage, the cyclone was influenced by the upward vertical movement at beneath the left exit region of jet stream. Both stages are closely related with upper divergence but the reasons causing the divergence are different.2. The trough at level of 500 hPa intensified cyclone mainly during the initial stage and the development stage:(1) During the initial stage, cold advection west of the trough axis decreased with height favor the trough being deepened, positive vorticity advection east of the trough increases depressurized ground cyclone.(2) During the development stage, except the cold advection, a short wave-length trough moved from upstream area and merged with cyclone, closed vortex was formed in the trough. Above the location of cyclone, positive vorticity advection increased with time, prompted the cyclone’s rapid decompression. In the later stage of development, the center position of cyclone was closed to low vortex center gradually, and vorticity advection at lower troposphere decreased with time. The vertical distribution of vorticity advection increased with height reinforced the vertical upward movement, rapidly strengthened the explosive cyclone development.3. PV acted on mainly during the initial stage and development stage. Due to positive anomaly of PV and PV axis tilted to west with height, ground cyclonic circulation would be strengthened. On the other hand, because of downlink of high PV, it also made the cyclone strengthening. In the mature stage, the vertical distribution of PV axis was beneficial to maintain cyclone’s intensity.4. The frontal system influenced on the cyclone at the initial stage and development stage. In addition to the baroclinic instability of frontal system itself was advantageous to the cyclone development, frontal uplift made convective instability energy release which prompted cyclone strengthen.5. The northward component of wind at lower-level of troposphere provided sufficient moisture into the cyclone. Latent heat release caused by moisture condensation played a significant role to the cyclone explosive development.The simulation result with WRF (Weather Research Forecast)-3.4.1 model was initiated from 06 UTC 26 to 12 UTC 28 October 2009 with 30-km resolution reproduced the cyclone reasonably well. WRF modeling results showed latent heat influenced the circulation indirectly to contribute to the explosive development of cyclone:plenty of water vapor condition and relatively deep convection system made the position of latent heat release occurred in the upper troposphere. Due to the cyclone locating in the upper trough, southwest airflow might convey heat to the northeast. When the heat accumulation to a certain extent, geopotential height which above the heating center of the upper atmosphere will increase and low-level geopotential height will decrease. It will make vorticity advection which above cyclonic location increase with height, and then prompt rapid depressurization of cyclone. This is a positive feedback.Although this positive feedback mechanism and the CISK (Conditional Instability of the Second Kind) mechanism are similar, both of them develop by convective condensation, latent heat release and synoptic-scale system promote each other, the main difference is that strong environmental wind shear, the downstream direction transmission of heat and latent heat effect on the circulation at the downstream of middle-level trough, instead of cyclone in the ground from the CISK.