Analysis on the Characteristics and Causes of Severe Convective Weather in Mesoscale vortex rain belt

Affected by rainstorm, thunderstorm and tornadoes in the mesoscale vortex rain belts, the west of Huanghuai river basin in china suffered great damage and casualties(two deaths) on July 17th and 18th 2010 ( referred to as process 7.17 and 7.18). In process 7.17, the rainstorm was located in the Huaihe River basin and Nanyang basin where maximum daily precipitation occured in Tongbai station, up to 199mm. Maximum precipitation intensity was up to 103.3mm/h.The location of rainstorm was west by north in process 7.18 than 7.17. The maximum precipitation occurred in Nanzhao station (180mm/d), and the maximum rainfall intensity was up to 63.5mm/h in Zhengzhou station. The great destructive tornadoes occurred between 0500～0600UTC on 17th in Suixian, Xiayi and Yucheng and at 2130UTC on 18th in Changge county respectively. Based on the conventional meteorological observations, the regional automatic stations, Doppler radar and NCEP reanalysis data,the processes were studied. Mesoscale vortex rain bands was generated in a favorable circulation background. In the processes northern boundary of the ‎West Pacific Subtropical High(WPSH) was located near the Huaihe river basin and typhoon "Conson" and " Chanthu" landed in the South China coast. The warm-moist air flow between typhoon periphery and periphery of WPSH added together to form southwest jet which is helpful to transport warm-moist air flow toward north.The mesoscale vortex formed and developed in the low level shear beside the north of WPSH because of the joint action of cold and warm air. During the strengthening period of the vortex, mesocyclone on the ground formed and moved to the northeast under the guidance of the high-altitude southwest airflow.The main regions of severe convective weather occurred in the convergence zone of southwest inflow in the southeast of cyclone and east or northeast inflow in the north of cyclone. According to doppler radar products, the severe convective weather formed and developed in the mesoscale vortex rain belts. In the southeast part of vortex rain belts, there was mesoscale-β narrow-band echo in the convergence zone of southwest jet. At 0100～0300UTC and 0700～1000UTC on 17th, the reflectivity of the rainbelts was over 50dBz and some mesocyclones appeared which resulted in rainstorm. At 0135UTC, SSR products show that a strong γ-scale vortex appeared and led to the heaviest rain and stronger thunderstorm in Tongbai country. Cold air inflow area from the northeast of vortex rain belts was also the regions of severe convective weather characterized by rainstorm. At 0800～1000UTC on 17th, significant feature of the mesocyclone and TVS were observed in Shangqiu county when the maximum wind shear reached 20～50E-3/s. In the process 7.18, a similar structure of rain bands appeared and characteristics of the mesocyclone also existed. The difference was that mesoscale cyclones were identified more frequently when tornado occurred in the southeast of rain belts at 2130UTC on 18th. In short, according to the mesoscale cyclones' activity, doppler radar products can make tornado warning before half an hour earlier. The mesosacle vortex rain belts were simulated using mesoscale numerical model(WRF) . The results show that the formation and development of mesoscale vortexes belts are impacted by terrain. The coupled dynamic structure between high-level divergence and low-level convergence near the center of vortex are in favor of the development of vertical ascending motion, which also increases the convergence and lifting of water vapor. In the process 7.17, the vortex formed in high energy tongue and developed in the south of energy front.Convective instability and conditional symmetric instability co-existed, convective instability was the main features of the atmosphere, rainfall intensity was relatively large in this process. In the process 7.18, vortex formed in the south of energy front, developed in the centre of energy front. Conditional symmetric instability was the main features of the atmosphere, rainfall intensity was weaker. In addition, analysis on vorticity suggests that the peak values of local vorticity transfers from top to bottom and vertical transport is the major contribution of local vorticity increasement.