| 摘 要： | In this paper, an unusual rainfall in Beijing that occurred on 4 September(‘9.4') 2015 is studied to clarify the reasons for such a strong rainfall in autumn. It was indicated that various factors, including stationary westerlies disturbance(i.e. low in the west and high in the east), forward-titling trough,warm shear line, unstable stratification and convective available potential energy release, low level jet as well as a series of mesoscale convective systems produced the strong rainfall. Ordinarily, this situation is uncommon in autumn.
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