Initiation of a Supercell by Convectively Generated Gravity Waves in the Simulated Kaiyuan Tornadic Event of 3 July 2019

Abstract An EF4-rated supercell tornado occurred on 3 July 2019 in Kaiyuan, China, causing heavy casualties. A three-level nested-grid high-resolution numerical simulation is used to investigate the initiation of the tornadic supercell. Automatic weather station (AWS) data, FY-4A visible satellite data, and Doppler radar data are used to verify the model simulation. The most important aspects of the simulated presupercell mesoscale convective system (MCS) and the initiation of the supercell agree with observations. Detailed investigation of the model results reveals that the initial cells form first above a convective boundary layer (CBL) on the dry side of a surface dryline. Above the CBL is a moist layer in terms of relative humidity, and the layer is stable. Convectively generated gravity waves (GWs) emanating from the MCS and propagating southward along the stable layer above the CBL provide localized forcing for the actual triggering of initial cells at specific locations. The associated perturbation potential temperature and vertical velocity patterns confirm that the GWs trigger a series of cloud bands. The additional lifting by the updraft of a horizontal convective roll in the CBL underneath the GW updraft works together to promote faster growth of the initial cell that later becomes the supercell. Examination of the Scorer parameter profiles shows favorable conditions for vertical trapping of GWs along the waveguide in the stable layer, preventing the radiation of wave energy to the upper levels. Significance Statement This study reveals an atypical initiation mechanism of a tornadic supercell that occurred on 3 July 2019 in northern China. By analyzing high-resolution numerical simulation results and observations, the supercell is found to be initially triggered by convectively generated gravity waves (GWs) propagating along a moist and stable layer above the convective boundary layer. The GWs were excited by vigorous convection along the leading-edge gust front of a mesoscale convective system to the north. The vertical atmospheric structure acts to trap the gravity wave energy to the stable layer and supports the horizontal propagation of the GWs away from their sources. Our study indicates the elevated initiation mechanism of convection by convectively generated GWs, providing a better understanding of this type of convective initiation that is typically difficult to predict.