Climatic characteristics of the Jiulong low vortex in recent 30 years based on the ERA-interim reanalysis data

Based on the ERA-interim reanalysis data, the weather processes that are associated with the Jiulong low vortex (JLV) have been counted and analyzed during the period of 1 January 1986 to 31 December 2015. The result suggests that T_1-2JLVs with a lifespan of 1-2 hours and T_3-4JLVs with a lifespan of 3-4 hours usually occur at 100°-101.5°E, 27°-28.5°N; T_5-6JLVs with a life span of 5-6 hours usually occur at 102°-103.5°E, 29°-30.3°N, T_≥7JLVs with a life span of more than 7 hours usually occur at 101.5°-103.5°E, 28°-29.5°N, and vortexes with even longer life spans often occur at the southwestern gap of Sichuan Basin. The occurrence frequency of Jiulong low vortex increased during the past 30 years, but decreased in recent years. Annually, the frequency increases first and then gradually decreases with month, i.e., it increases from January to May and decreases from May to December with the maximum in May and the minimum in September. T_1-2JLVs occur most frequently in March and least frequently in September, T_3-4JLVs occur most frequently in April and least frequently in December, T_5-6JLVs and T_≥7JLVs occur most frequently in June, T_5-6JLVs never occur from January to April, and T_≥7 JLVs occur least frequently in December. Summer is not the highest frequency season for Jiulong low vortex occurrence, but it is the easiest time for long life span Jiulong low vortexes to form and move out of the genesis place. The Jiulong low vortexes with life spans more than 24 hours (T_5-6JLV、T_≥7JLV) occur more frequently in the night than in the daytime, whereas the nighttime occurrence of vortexes with life spans less than 24 hours (T_1-2JLV, T_3-4JLV) are not obvious. The frequency of Jiulong low vortexes that can move out of their genesis field first increases and then decreases gradually with month, i.e. the frequency increases from January to June and decreases from June to December with the maximum in June. T_≥7JLVs mainly move forward following the east path at the first several months, and then follow the southeast path and the northeast path after July. T_5-6JLVs move forward only following the east and northeast paths. The Jiulong vortexes with the life span of less than 24 hours may move out of the genesis place because they occur near the edge of the statistical area.