The Reason Of The Meso-β Scale Severe Convection In MCS Under Two Scales Of Vortexes

A MCS process that occurred in Shandong region on August 3,2015,was investigated and compared with latent heat sensitivity experiments,by the WRF mesoscale numerical model based on NCEP/NCAR reanalysis and doppler radar observation data.The study revealed the reason for the occurrence and development of the mesoscale vortex in this process and the evolution reason and vorticity structure of meso-β scale severe convections in two scales of low vortexes.The results 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 background for the lower mesoscale vortex and convection occurrence.The low-level mesoscale vortex development caused the MCS formation,along with turning,merging,and bowing of meso-β scale severe convections and intensification of the rainstorm.(2)By analyzing vorticity budget of the mesoscale vortex,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 advection term,the former having a greater effect,and the positive vorticity of the middle layer is mainly affected by the vertical advection 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 of 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 of cold and warm air.The increase of the perturbation kinetic energy promotes the development of convections.The environment not only provides energy for the severe convections,but also influences the evolution of the strong convection.(4)By analyzing the evolution of meso-β scale convections in MCS,under the background of the synoptic scale vortex,the low mesoscale vortex shear caused emergence of several meso-β scale convective belts and small troughs,and meso-β scale convections developed,along with small grooves turning to be vertical,when fields in troughs were changed from the west-southwest wind to the northwest-southwest wind.The strengthening of the south and north wind on the convection is the key to be vertical.It can be seen from the analysis of vorticity equation,that the low-level heeling term contributed the most to vertical vorticity.Horizontal vorticity converted to positive vorticity in the front of severe convective zone,because of vertical velocity influence,and before convections were combined,positive vorticity can be extended to another trough,leading to the mergence and strengthening of positive vorticity areas of two troughs,the mergence of two troughs and convections,the deepening of the trough,and the strengthening of the south and north wind respectively.The divergence term contributed the most to negative vorticity that was mainly in the back of severe convective zone.The increase of negative vorticity zone and the strengthening of anticyclonic disturbance because of the divergence term caused the severe convection to be bowed.By analyzing the vorticity structure of meso-β scale convections,that the horizontal vorticity vector in the front of the convection zone rotates counterclockwise and this region corresponds to a strong upward motion,leads to the increase of the vertical velocity shear in the horizontal direction,and then the positive contribution of the heeling term increases.Because of the heeling term,the horizontal vorticity transforms to the vertical vorticity,and then the lower layer vertical vorticity is transported upward through vertical advection,facilitating the intensification of convections.(5)Latent heat being closed before the convection became strong,lead to the change of the three-dimensional wind field,the weakening of the trough,the scatter of the wind disturbance and the positive vertical vorticity zone being mainly corresponding to the subsidence motion zone,therefore the vertical vorticity cannot be transported to the upper level,and convections disappear rapidly.