| Using the WRF mesoscale numerical model,NCEP/NCAR reanalysis data and the observation data of Doppler radar,this study analyzed the formation mechanism and structure of a multi-mesoscale-bow-rainband precipitation process in Central China on May 5,2018 and found the following results:(1)The rainbands occurred in the southwest airflow on the south side of the shear line.Before the appearance of the multi-mesoscale-bow-rainband,on large scale the cyclonicrotating horizontal vorticity vector in the high and low level and convective effective potential energy provided a favorable background for precipitation which was conducive to the upward movement.The mesoscale-bow-rainband was initially formed under the conditions of convective instability and low-level airflow convergence.The downward motion caused by local heavy rainfall led to the occurrence of middle and low level gales.The anticyclonic circulation on the south side of low-level gale center merged with southwest airflow in the background field,forming a short-wave trough where the tail of rainband emerged.The head of the bow-rainband was generated in the cyclonic wind shear on the north side,and the faster moving of the gale center made the middle part of the rainband bulged forward.The short wave trough of airflow field occurred below 700 h Pa,the bidirectional propagation of the trough to the front and back sides was an important trigger factor for the formation of multi-mesoscalebow-rainband.(2)Through the analysis of the thermal and dynamic structure of the bow rainbands,this study found that the vertical upward motion was strong near the warm air column inside the rainband during the convection development period.Besides,the equivalent potential temperature on both sides of the rainband presented a "high-low-high" distribution from the lower to the upper troposphere,and the internal temperature of the vertical convection cloud tower was higher than the external environment temperature.The study on the distribution of water condensate particles in the bow-rainband showed that the distribution of cloud water particles has a good synchronization with the development process of the warm air column,and the local warm and wet center of the heating column formed in the middle and upper layers of the rainband may be related to the formation of graupel particles.(3)The horizontal vorticity of the low layer near the mesoscale-bow-rainband has different structural characteristics in different stages.In the strong period of the bow-rainband,the horizontal vorticity vector is cyclonic bending at the front and head,and anticyclonic bending at the tail.In the weakening period,the cyclonic bending at the head becomes anticyclonic,and the strength of the rainband weakens.In the dissipation period,the cyclonic bending at the front still exists while the anticyclone at the tail basically disappears.In the upper layer,the horizontal vorticity vector is cyclonic in the development and strong period of the rainband,anticyclonic in the weakening period.(4)Through the comparison between the two terrain sensitivity tests(SEN1,SEN2)and the original control test(CTL),it is found that the mountain terrain in the southwest of Sichuan Basin is the important reason for the convergence of the low-level wind field,which is the trigger factor of this convection.In the process of occurrence and development of multimesoscale-bow-rainband,the mountain terrain in the east of Sichuan basin made the vertical movement near the rainbands more intense,and also promoted the wind shear and the propagation of short wave trough,which contributed to the development and maintenance of rainbands. |
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