Double Southwest Vortex Analysis And Numerical Simulation

The mesoscale southwest vortex(SWV)in the southwestern China is regarded as the second most important type of meteorological event in China.Applying the National Centers for Environmental Prediction Final Operational Global data,the CMORPH precipitation data,Japan MTSAT-2 satellite data,the weather diagnosis methods and the Weather Research and Forecasting Advanced Research model(WRF),this paper studies the binary SWV events over the Sichuan Basin(SCB)during 4-5 August 2011 and 16-17 July 2010,reveals the three-dimensional structural characteristics of the binary SWV,and performs diagnostic analyses and numerical simulation to its thermodynamic and kinetic mechanisms.The results are as follows:(1)A clear definition of the binary SWV is given.The so-called binary SWV is a SWV in the southwest China on the Eurasian weather map,but on a smaller area of the weather map,the SWV is composed of two smaller mesoscale vortices;The two smaller mesoscale vortices must be generated by the same mesoscale SWV that precede them;The central locations of two smaller mesoscale vortices must be located within southwest China(26–33°N,100–108°E).(2)During 4-5 August 2011,the binary SWV can be observed over the SCB.Within the binary SWV,the horizontal spatial scales of the two vortices are not the same.A lager mesoscale vortex C1 is located near the Bazhong and another small mesoscale vortex C2 is located near Liangshan.On the satellite image,two mesoscale convective cloud clusters corresponding to the centers of the binary SWV further confirm the existence of the binary SWV.At the same time,under the influence of the water vapor from the Bay of Bengal and the South China Sea,an obvious northeast-southwest rainband occurs between the vortex C1 and C2 during the typical period of the binary SWV.(3)The main circulation situation of the binary SWV during 4-5 August 2011 is that the binary SWV is generated on the right side of the high-level jet and in front of the 500 hPa long wave trough.At the same time,under the combined influence of the West Pacific subtropical high,the tropical cyclone and the southwesterly airflow,the binary SWV slowly moves eastward.The diagnosis of vorticity budget shows that the horizontal convergence(divergence)term and the vertical advection term play a major role in the occurrence and development of the vortex C1,the horizontal convergence(divergence)term and the tilting term play a major role in the occurrence and development of the vortex C2.In addition,The WRF model is well able to simulate the binary SWV event and its associated precipitation.The simulation results show that the binary SWV maintains for 20 hours.(4)The binary SWV phenomenon is not an isolated case.By analyzing another typical binary SWV event over the SCB during 16-17 July 2010,it is find that a larger scale mesoscale vortex is located near Jiulong,and another small mesoscale vortex is located near Langzhong.In the circulation situation,on 16-17 July 2010,the binary SWV is located on the right side of the upper level jet and in front of the 500 hPa long wave trough.The West Pacific subtropical high and the tropical cyclone are favorable to the occurrence and development of the binary SWV.(5)The WRF model is also well able to simulate the binary SWV event during 16-17 July 2010 from the generation and evolution to eastward movement(dissipation).The event can be divided into four stages.During the gestation period,one vortex C5 was observed over the SCB at 1600 UTC 16 July.At the generation stage,the vortex C5 split into vortex C6 and vortex C7 at 1700 UTC 16 July,the binary SWV is shallow system.At the development stage,vortex C6 and vortex C7 further continued to evolve over the SCB at 1800 UTC 16 July to 2200 UTC 16 July,respectively.When the phenomena that the SWV have two cores are presented,this event enters into the mature stage.The nearly saturated relative humidity around the centers of the vortex C6 and C7,the binary SWV have warm and moist structures,and there are two obvious upward motion centers over the binary SWV cores.At the decayed stage,vortex C6 and vortex C7 dissipated at 0300 UTC 17 July.The life span of the binary SWV is about 10 hours from generation to dissipation.(6)The sensitivity test of the binary SWV over the SCB on16-17 July 2010 shows that the unique Tibetan Plateau(TP),Yungui Plateau(YGP)and the latent heat release play an important role in the binary SWV.The location and orientation of the binary SWV is mainly decided by the unique plateau topography,and the latent heat release has an important feedback on the generation and maintenance of the binary SWV.