Classification characteristics of continuous extreme rainfall events in North China

Based on daily precipitation data provided by China Meteorological Administration for the period 1960—2015, 56 continuous extreme rainfall events in North China were filtered. Using the objective cluster analysis method based on the anomaly correlation coefficient and synoptic appraisal, these events were classified. Furthermore, the NCEP reanalysis data (2.5°×2.5°) was used to classify and synthesize the synoptic characteristics. Results show that the patterns of continuous extreme rainfall events in North China include warp-wise type, across-warp type, weakened landfalling tropical cyclone type and early summer type. The four types are generally related to the frontal dynamics process under different weather configurations. However, due to differences in the frontal structural features, the environmental atmospheric stratification state and the warm and humid transport channels associated with low-level jets, the spatial distributions of meteorological stations with high-frequency of daily precipitation ≥50 mm and the average cumulative precipitation show different patterns. (1) The intensity of the frontal zone corresponding to the continuous extreme rainfall event of across-warp type is significantly stronger than that corresponding to the event of warp-wise type. However, the stratification stability for the across-warp type is similar to the climate state of summer, while there is a weak-instability anomaly in the warp-wise type. As a result, the convective activity in the continuous extreme rainfall event of across-warp type is generally less violent than that of warp-wise type, and it exhibits a clearer frontal precipitation characteristics with continuous rainfall, that is, the frequency of daily precipitation ≥50 mm in the across-warp type is significantly lower than that in the warp-wise, while the across-warp type has more cumulative precipitation. (2) The water vapor transport channel, which was derived from the south side of the Western Pacific Subtropical High, only plays a leading role in the across-warp type. In the early summer type and weakened landfalling tropical cyclone type, the active Indian monsoon causes a strong low-level zonal southwesterly airflow anomaly to the south of 25°N, which becomes the main source of water vapor after passing through the Indo-China Peninsula or is "transferred" by tropical cyclone. In addition, the water vapor transport of the warp-wise type is also related to the westerly airflow anomaly originating from the south side of the Qinghai-Tibet Plateau. This factor might be the main reason why the summer precipitation in North China has a higher correlation with the Indian monsoon precipitation than that in other parts of eastern China. (3) Similar to the warp-wise type, the weakened landfalling tropical cyclone type is also dominated by meridional atmospheric circulation. However, as a result of more water vapor transport, greater vertical velocity and deeper unstable atmospheric state, the precipitation intensity of the tropical cyclone type is more concentrated. In addition, for the extreme rainfall events located at the northern ridge of weakened landfalling tropical cyclones, the invasion of weak cold air from higher altitudes is significantly effective.