Abstract:Based on the observational and NCEP reanalysis data, the frontogenesis and instabilities in two continuous autumn torrential rain days in Henan Province in September 2011 are diagnosed. The results show that there existed a northeast southwest frontal zone in north central Henan Province in the two torrential rain days. The frontal zone showed the quasi stationary feature. The similarities between the frontogenetical functions of two torrential rain days are that, before the heavy rain, frontogenesis was demonstrated in most parts of the frontal zone; in the occurrence of heavy rain, frontogenesis got strengthened in high and low levels but weakened in middle level of 600-500 hPa and frontolysis even appeared at about 400 hPa, which was caused by the strengthening of vertical motion. When the heavy rain got weak, frontogenesis in high level and frontolysis area both descended in height, corresponding to the weakening of vertical motion. The differences are that the middle tropospheric frontal zone was stiff during the night of the second torrential rain day, frontogenesis centers were getting near warm zone, which was the result of strengthening of convective rainfall and enhancement of precipitation efficiency and condensation latent heat release. The roles of terms of frontogenetical function were different. The contribution of deformation process to forntogenesis was the greatest; the role of tilting term was mainly frontolysis. The strong frontolysis center at 400 hPa during heavy rain is the distinct manifestation of contribution of tilting term. The two torrential rain days are both of inertial stability. The first torrential rain day was convective stable but symmetric instable, so the heavy rain band was the result of symmetric instability. On the second torrential rain day, convective instability and symmetric instability coexisted at about 700 hPa in frontal zone, so the heavy rain band was caused by convective symmetric instabilities and thunderstorms during the night were of the characteristics of elevated thunderstorms. Furthermore, on the first torrential rain day, there was consistent slanted updraft in the warm side of frontal zone; frontogenesis center, symmetric instability center and upward motion center appeared together, showing that frontogenesis provided a favorable condition for the release of symmetric instability. On the second torrential rain day, there was an obvious mixture of slanted convection and vertical convection; the starting point of vertical updraft was just the convective symmetric instabilities area. Vertical updraft was much stronger than the slanted upward flow on the first torrential rain day, indicating that gravitational convection is dominant.