Variation of Summer Precipitation in Tianjin Region and Its Urbanization Effect
天津夏季降水演变规律及其城市效应

The precipitation data of 25 rain gauges in Tianjin from 1956 to 2005 were selected to analyze the spatial and temporal variation using accumulated variance,correlation analysis and Mann-Kendall method.Furthermore,the spatial and temporal characteristics of the urbanization effect on summer precipitation were revealed by the spatial analysis tool of GIS.The result showed that:more than 70% of the rainfall in a whole year concentrates in summer,and the summer precipitation exhibited a decreasing trend during the past 50 years.The summer precipitation slopes of the 25 sites were below zero,and 60% of them reached the significant level of.The weighted average of summer precipitation showed the slope from 1956 to 2005 was 24.7 mm/10a,the slope was 7.6 mm/10a from 1956 to 1988,and it was -28.6 mm/10a from 1989 to 2005.So it was concluded the decline of summer precipitation mainly happened in recent 17 years.After an advanced study,it was found that the down trend of summer precipitation differed between northern and southern,between urban and suburban areas,which showed rain island effect.The effect turned more significant after the early 1970s,and had a strong relation with amount of the rainfall.The rain island effect was more apparent in high rainfall years than that of low rainfall years.