Analysis of the evolution reason of the mesoscale vortex and its influence on MCS under the background of the northeast cold vortex

An event of mesoscale vortex with MCS occurred in Shandong region on 3 August 2015 was investigated and analyzed using the WRF mesoscale numerical model based on NCEP/NCAR analysis data and Doppler radar observation data. The study revealed the reason for the occurrence and development of the mesoscale vortex and its effect on the mesoscale convection system under the environment of the northeast cold vortex. The results are as follows. (1) The upper-level jet stream' s strong divergence in front of the trough of the synoptic-scale northeast cold vortex and the low-level instability energy release provided favorable condition for the lower mesoscale vortex and convection occurrence. The generation of the low-level mesoscale vortex promoted the occurrence of the MCS. (2) When the mesoscale vortex forms and develops, the positive vorticity of the lower layer is mainly affected by the horizontal divergence term and the vertical transport term, with the former having a greater effect. The positive vorticity of the middle layer is mainly affected by the vertical transport term. Due to positive contributions of the two terms, the positive vorticity increases, and the vortex develops.When the mesoscale vortex weakens, the horizontal divergence term in the middle and lower layers weakens and the vertical transport term becomes negative, leading to the decrease of positive vorticity generation and the increase of negative vorticity generation, and therefore the vortex decreases. (3) When the MCS develops, the increase of perturbation effective potential energy mainly comes from the latent heat release and the conversion from zonal mean effective potential energy to disturbance effective potential energy, and then the perturbation effective potential energy is transformed into the perturbation kinetic energy through the vertical motion. The increase of the perturbation kinetic energy promotes the development of convections. The mesoscale vortex not only provides energy for the severe convection, but also influences the evolution of the strong convection.