Regimes of rainfall preceding regional rainfall events over the plain of Beijing City

Abstract

Four regimes of rainfall that preceded strong regional rainfall events (RREs) over the plain area of Beijing City (BJP) were studied using hourly station-based rainfall observations during the warm season (May–October) of the past 8 years. The RREs were outlined by the maximum rainfall at all stations over the BJP each hour. The strong RREs were classified into four groups based on the rainfall that occurred in the 3 hr before the start of the events, which originated from areas northwest, southeast, northeast and southwest of the BJP. The four groups of RREs exhibited different characteristics. The number of RREs from the northwestern mountains was the highest, that is, approximately twice (four times) that from the northeastern mountains and the southwestern plain (southeastern coastal areas). The amounts, intensities and durations of the RREs from the southwestern plain were two to fourfold higher than those from the three other directions. The mean rainfall centres of the RREs from the northeast and southeast were located outside the BJP. Correspondingly, the spatial coverages and homogeneities of the RREs in the BJP from the northeast and southeast were poorer than those from the two other directions. All four groups of RREs ended to the southeast of the BJP and occurred mainly from the late afternoon to the early morning. Detailed rainfall classification based on the origination and feature analyses of all rainfall processes will enrich our knowledge of the rainfall evolutions of the BJP. In addition, combined with a study of the circulation of the rainfall processes, comprehensive understanding of fine-scale rainfall features will provide further information for rainfall forecasting. In the future, our analyses will be applied to the corresponding atmospheric circulations of those RREs with different origins to identify the key factors that lead to different rainfall distributions.