Abstract: This study is focused on a strong convective event that included three convective storms occurred in Beijing-Tianjin-Hebei region on 16 July 2014. Convective-scale dynamical and thermo-dynamical mechanisms are presented to clarify dynamical effects of outflow boundaries on localized initiation and rapid enhancement of convective storms over the complex terrain area in the region mentioned above using observations from radars, radiosondes, and auto weather stations and high-resolution numerical results simulated by a rapid-refresh 4D variational assimilation (RR4DVar) of multi-radar observations and a 3D cloud-scale numerical model. The results from radiosonde observations and high-resolution simulations indicate that meso-scale environmental conditions including a strong thermal instability, a low-level southerly warm and moist flow (jet), and a low-level vertical shear were favorable for the initiation and development of convective storms in the region from the daytime to nighttime on 16 July. The localized initiation and rapid intensification of a multi-cell storm over Tianjin area was first triggered by an eastward-moving near-surface shear line at about 1200 BJT (Beijing time) during the period of the strong convective event. Meanwhile, a northwestward-moving strong outflow boundary was generated by the strong convective storm. At about 1800 BJT, a new storm cell that formed over the northwestern mountains of the region in the early afternoon reached the mountains to the northwest of Beijing with southward outflow. Due to the topographic forcing, strong low-level convergence and updraft occurred over the mountain slope due to the interaction between the storm outflow that accelerated down the slope and the low-level warm and moist southerly flow that moved up along the mountain slope. Meanwhile, dynamical instability developed and superposed on the slope because of the collision of the southward-moving and northwestward-moving outflow boundaries. Under the influence of the factors mentioned above, the storm cell rapidly evolved into a strong supercell storm while it went down the slope. Characteristics of circulation over the key area were similar to that in the so called "triple point" area for storm initiation and intensification in complex terrain area in this region, where two outflow boundaries collided near the storm and interacted with the low-level southerly jet. At about 2200 BJT when the outflow boundary generated by the supercell reached the southern plain, a new key area with properties similar to the "triple point" properties formed due to interaction between the boundary and the convergence area formed by low-level southerly jet and easterly moist flow, which resulted in strong updrafts of warm and moist air along the boundary in the convergence area. As a result of the boundary triggering, convective cells continuously initiated and intensified along the boundary and its adjacent area, and finally formed a multi-cell storm line (squall line) that oriented approximately along west-east direction at about 2300 BJT.