Abstract: The torrential downpour happened in southwest of Hebei Province in 3-5, August, 1996 was analyzed. It was caused by three periphery mesoscale convectional cloud clusters, which connected with Typhoon No. 9608, and had no direct impact from the westerly trough. They happened in high unstable energy and weak baroclinit condition. They were triggered by unsteady ageostrophic gravitational wave in the low-level jet from southeast of the outskirts of the typhoon. A perpendicular circulation cell about 1000 km in horizontal scale had developed between torrential rain area and the westerly trough to the northeast. Q-vector frontogenesis function analysis indicated that such circulation cell generated energy transforming system between torrential rain area and the distant westerly to the northeast. The analysis of water vapor imagery showed that the upper troposphere cyclone divergence above the torrential rain area was strengthened due to the movement of the high latitude short wave trough toward southeast. The lower torrential rain cloud cluster had redeveloped and had a magnificent reinforcing effect on the heavy rainfall process.