2024年7月山东龙卷群发事件的可能成因:Rossby波包活动与中小尺度条件OA

Tornadoes are well known as the extremely strong convective systems that usually caused severe disasters and huge economic losses.On 5 July 2024,Shandong experienced an outbreak event of tornadoes.By using the daily and hourly atmospheric dataset from ERA5 reanalysis,the possible causes of this tornado outbreak event were investigated by analyzing changes in the potential parameters of convective conditions and the propagation of Rossby wave packets in the upper troposphere.Results demonstrate that the tornado outbreak was triggered by a deepening extratropical cyclone originating from a southwest vortex that migrated eastward from Sichuan Basin.The strengthening of this extratropical cyclone was caused by the eastward propagation of upper tropospheric Rossby wave packet at a group velocity of 13.3° longitude per day.This wave packet originated from the Balkhash Lake region on July 2 and migrated eastward along the path of Balkhash Lake-Tianshan Mountains-Hetao Region-Shandong Province.As the wave packet reached Heze city on July 5,the outbreak of tornadoes occurred.This Rossby wave packet approaching Heze strengthened both the anticyclonic disturbancein the upper troposphere and the cyclonic circulation system in the lower troposphere,facilitating the formation and maintenance of deep convection.Moreover,the combined effects of the low-level jet stream and the mid-upper-level trough were also conducive to the development of supercells.Occurrences of tornadoes were intrinsically determined by meso-and micro-scale environmental conditions.The maximum Convective Available Potential Energy(CAPE)in the Heze area approached 2 500 J·kg-1 whereas the low-level vertical wind shear(SHR1)exceeded 10 m·s-1.Low-level storm relative helicity(SRH1),Significant Tornado Parameter(STP),Supercell Composite Parameter(SCP),and low-level energy-helicity index(EHI1)significantly surpassed their respective thresholds prior to the tornado occurrence,accurately indicating the strong potential for tornado outbreak in this Heze area.All results above are helpful to deepen the understanding of tornado genesis and provide usefulscientific clues to forecastingoccurrences of tornado events.