Predictability of an Extreme Rainfall Event in North China

In this paper, the predictability of an extreme rainfall event in North China is discussed by diagnostic big data analysis method. The data of the event (July 18th-20th, 2016) is from the deterministic forecast and ensemble forecast of the European Centre for Medium-Range Weather Forecasts (ECMWF). The results show that, the rainfall event is characterized by evident variations of the precipitation magnitude and rainfall belt location from July 18th to 19th. The rain belt moved almost 10 degree northward, bringing significant heavy rainfall to North China, thus it caused a big challenge for forecast operation. Further analyses reveal that in this event, the predictability of the rain belt location was determined by the strength of blocking high system over the Bohai Sea (120-130 degrees E). The blocking high system above the Bohai Sea was favorable for the low vortex over North China to move northward slowly. Meanwhile, the eastward-moving vortex and the blocking high strengthened the southwest wind in the low levels, increased the precipitation magnitude, and drove the rain belt to move further northward in North China. Furthermore, the comparison between the deterministic forecasts and ensemble forecasts with different leading times demonstrates that the maximum forecast leading time for the rain belt location in the deterministic model is 96 h, while that in the ensemble model is 120 h.