Abstract: Based on the weather charts, satellite cloud images and the new generation weather radar data, a strong wind process on July 31, 2007 was analyzed. The results indicate that the process is downburst but not tornadoes, and it is produced by the merger process of the bow echo and supercell. The thunder shower and short-time rainstorm occur from the night on July 30 to wee hours on July 31, 2007 during the bow echo moving from southwest to northeast, which affects the six districts and counties of Tianjin. The maximum precipitation intensity reaches 37.6 mm·h^-1, while squall, hail and ground gale do not occur in six weather stations. By analyzing the radar data, it shows that the moving bow echo maintaining more than 5 hours merges with a pre-existing supercell in mountain area of Jixian and produces downburst. The weak echo region gap appears in the reflectivity factor chart. At the same time, the bow echo shape is changed and is disconnected above 2.4 ° elevation angle, whose echo intensity reaches 53 dBz. An obvious rear inflow jet occurs early and a mesoscale cyclone generates late companied with the middle and low divergence on the relative radial velocity chart. The strong anti-cyclone features occur in upper level. The height of the storm development is high, and the height of echo top is over 15 km. The nucleus of reflectivity factor drops 3 km rapidly with the occurrence of downburst.

Key words: Tornadoes, bow echo, supercell, downburst, Tornado, Bow echo, Supercell, Downburst, Tornado, Bow echo, Supercell, Downburst