Characteristics of Summer Precipitation in Beijing in the Context of Urbanization

Based on the precipitation data from Beijing meteorological observatory, daily precipitation data of 20 operational meteorological stations, and hourly precipitation data from stations located in urban areas, by means of linear trend estimate, Mann-kendall test method,the Spatial and temporal characteristics and it's influence factor are analyzed and evaluated, such as rainfall amount, frequency, and duration in Beijing. Some new results are discovered as follows: The results reveal a downward trend of precipitation. The obviously decreasing trend of rainfall amount is detected after 1999, when Beijing is in continual drought period. An obvious rainfall center in summer located to the north, west, northwest, which is closely related to terrain. Other obvious rainfall centers located to urban areas and its northeast, which are closely related to city effects. The spatial distribution that little reduction trend of rainfall amount and days is detected, which located in the urban areas and its west, and northeast in small area, that is probably related to the greatest temperature gradient caused by urban heat island, where convective weather frequently occurred. Urban areas and its southwest often have heavy rain and rainstorm, northwest of mountainous region in Beijing often have moderate rain and light rain in summer.

The summer accumulated rainfall amount in urban areas concentrates in the evening to midnight (16-23pm LST), the greatest accumulated rainfall amount appears at 16-20pm, and the second at 21-22pm. The rainfall events appears the most frequent at 18-22pm, and the second in the morning about 4-6am. The rainfall events often appears from the evening till after midnight (17pm-02am), consisting of short duration rainfall events, i.e., less than 7 hours. During 16-18pm, it is primarily the rainfall events less than 3 hours, which is mainly convective weather with the greatest intensity, and having great contribution to accumulated rainfall amount. During 17-21pm, and after midnight(3-4 am)there is high value of accumulated rainfall, and intensity less than at 16-18pm. The accumulated rainfall of short duration is obviously greater than long duration, where rainfall events lasting less than 7 hours have the major contribution to accumulated rainfall amount.

In the urban areas, the precipitation shows a tendency of short duration time and high intensity. Accumulated rainfall amount of short duration events increases gradually, while that of long duration events decreases significantly.