Changes In Summer Precipitation And The Impact Of Urban Heat Island On Hourly Precipitation Extremes In Beijing Area

Based on hourly precipitation observations during 1977-2013 in Beijing area,hourly precipitation was classified into three categories including light, moderate and heavy precipitation. The climatic characteristics and the changing characteristics of summer precipitation in the mountainous, suburban and urban at scales in Beijing were analyzed and studied. The analysis of climatic characteristics shows that the contribution of light precipitation in mountainous areas is higher than that in suburbs and urban areas, but the moderate precipitation and heavy rainfall are significantly less. The annual contribution of moderate precipitation and heavy rainfall in urban areas is similar with suburbs. The precipitation in Beijing is dominated by short duration precipitation, the duration of heavy rainfall events is longer, and the,duration of light precipitation events is shorter. In the aspect of diurnal variation,light precipitation shows two peaks in early morning and evening while moderate precipitation and heavy rainfall show single peak in the evening. The peak time of the light precipitation at early morning in the suburbs are slightly ahead of the urban and mountainous areas while the peak time of the light precipitation at evening in urban and suburban areas are slightly delayed as compared to mountain areas. The peak time of moderate precipitation in urban areas is at 22:00, which is lagging behind suburbs and mountains. Heavy rainfall peak in urban and suburban areas occur at 21:00 but 17 o'clock for mountain area. From the perspective of summer precipitation change, the results revealed that both light and moderate precipitation types decreased significantly during the research period but heavy precipitation showed no pronounced trend in the Beijing area. The contribution of heavy rainfall to the total precipitation in summer in the urban area became greater as compared to the suburban area since 2004, as opposed to the less contribution from light rainfall.Although the duration characteristics of summer precipitation showed no noticeably trend, the asymmetry of rainfall is obvious, especially for the heavy precipitation.The index of asymmetry in heavy precipitation process in the urban area exhibited a significant decreasing trend. There are obvious differences in diurnal variation of different intensity precipitations at peak time. Light precipitation at peak-time in the early morning decreased significantly in the whole Beijing area. As of the year 2004,a shift can be found as light precipitation occurred less frequently in the urban area but heavy precipitation occurred more frequently at evening peak-time as compared with the suburban area.Analysis of the Impact of urban heat island effects on hourly precipitation extremes showed that the urban heat island index has significantly increased with the rapid urbanization and it is higher at night-time than day-time, which leaded hourly precipitation extremes to fall during night-time in urban area. The amplified urban heat inland effect, on one hand, facilitated formation of more thermal-low and resultant ascending branch; on the other hand, it also favored to establish unstable stratification in the lower level. That explains the preference of hourly precipitation extremes on urban areas during nighttime.