Research On The Organizational Mode Of Convective Storms In Southern China And The Mechanism Of The Occurrence And Development Of Mesoscale Vortice

Under the influence of the East Asian summer monsoon,the convective activities over southern China are more active than other regions.Its unique large-scale environmental conditions make the prediction of convective storms in this region a challenge.Using Doppler radar data,surface observational data and mesoscale numerical model,this paper investigates the characteristics of organizational modes of convective storms and associated severe convective weather,environmental conditions,the impact of vertical wind shear on organizational structure and embedded mesoscale vortices,as well as the initiation and developing mechanisms of mesoscale vortices in two types of typical convective storms(bow echoes and supercells)and causes of severe convective wind gusts in spring and summer over southern China.The major conclusions are summarized as follows:(1)The storm morphologies over southern China are classify into 10 categories,possessing three types of cellular storms,which are isolated cells(IC),clusters of cells(CC),and broken lines(BL);six types of linear systems,which are squall lines with no stratiform rain(NS),squall lines with trailing stratiform rain(TS),squall lines with leading stratiform rain(LS),squall lines with parallel stratiform rain(PS),bow echoes(BE),as well as embedded lines(EL);and nonlinear systems(NL).Cellular storms,linear systems,and nonlinear systems account for 36.2%,34.2% and 29.6% of all samples,respectively.TS mode is most common(14.6%)among the linear systems,followed by EL mode(11.2%).The duration of TS(5.2h)and BE(5.0h)modes is relatively longer than that of other modes.The diurnal cycle of frequency for these convective storms exhibits a primary peak during the early afternoon to evening(1400–2000 local standard time,LST = UTC + 8 h.)and several secondary peaks during the night to the morning.The dominant modes of convective storms frequently evolve into the other modes in their initial,mature and decaying stages.TS and BE systems developed more frequently in convective storms that begin as NS.Comparatively,EL systems are prone to developing in convective storms that begin as NL.The severe weather observations produced by linear systems are most prevalent,with BE and TS modes ranking the first and second highest probability in spring.(2)Using the k-means clustering algorithm,the environmental conditions of convective storms are classified into two types: the "high-vertical-wind-shear–low-convective-availablepotential-energy" and the "low-vertical-wind-shear–high-convective-available-potentialenergy".Most severe convective storms possess high wind shear indices,among which,TS,BE and BL modes are triggered and developed in unstable moist conditions that featured with strong low-and middle-level wind shear in spring,indicating that dynamic conditions are one of the important factors for triggering convection.The idealized simulation of changing vertical wind shear shows that by increasing(decreasing)the vertical wind shear across the entire layer,the intensity of convection enhances(weakens)and the convective storms are prone to organizing into linear systems(decentralized).(3)The formation of the simulated bow echo is closely related to the rear inflow which develops in response to the formation of midlevel horizontal perturbation pressure gradients to the rear of the convection.The wind gusts produced by bookend vortices that located on either end of the bow echo contribute 40%～60% to the rear inflow.Based on the three-dimensional vorticity equation it is found that the horizontal vorticity are mainly generated within three source regions:(i)the horizontal buoyancy gradient region at the leading edge of the surface cold pool,(ii)the horizontal buoyancy gradient region in the middle layer,and(iii)the ambient front-to-rear ascending flow.The vortex lines in these regions rise from the north of the bow echo and then sink in the south,forming a cyclonic vortex in the north and an anticyclonic vortex in the south,respectively.The cold pool produces an important impact on the initiation,development and evolution of mesoscale vortices.When the mesoscale vortices are far away from the cold pool,their intensity decreases,while the mesoscale vortices are close to the cold pool,they are strengthened at the low level and develop upward.Mesoscale vortices are enhanced rapidly due to the development of the convective-scale low-and middle-level convergence associated with the cold rear-to-front descending and warm front-to-rear ascending flows,and the cold air descending from the downdrafts plays an important role in mesovortex genesis.(4)The relationship between horizontal vorticity and vertical motion is also examined in supercell thunderstorms.The updrafts(downdrafts)occur when the horizontal vorticity vectors rotate counterclockwise(clockwise).Mesoscale vortices are initially generated by the tilting of horizontal vorticity within the front-to-rear updrafts,and then positive vorticity is rapidly intensified by the stretching of vertical vorticity within the mesoscale vortices.Apparent differences exist between the bow echoes with system tilting upshear and supercells with updrafts leaning downshear on accounting for the surface gusts: the superposition of rotational winds generated by low-level mesoscale vortices within the bow echoes and local enhanced descending motion of the rear inflow leads to the surface gusts,while the rapid collapse of supercell storms results in the surface gusts with the weakening of mesoscale vortices.