Under the background of global warming and climate extremes increasing, an increasing demand of high quality daily climatic observations is needed in climate change studies. Meanwhile, homogenized climatic series are becoming more and more important. Precipitation is an important climatic element, essential for studying water resources and the water cycle, and for quantifying climate change. However, long-term precipitation series in China are usually interrupted by various non-natural factors, such as changes to observation locations, the environment, instrumentation, observation practices, or the algorithms used for calculating any particular climate variable. The inhomogeneity caused by these factors could severely distort the true climate signals and introduce considerable bias to estimates of climate trends, variability, and extremes. So it is essential to develop a new homogenization method and establish a homogenized daily precipitation dataset. 

      Dr. LI Zhen, Prof. YAN Zhongwei, Dr. TU Kai in IAP, CAS and Prof. Wu Hongyi  in Beijing Meteorological Bureau developed a new homogenization method for daily precipitation series  based on a combination of the multiple analysis of series for homogenization (MASH) and empirical cumulative density function (ECDF) methods regarding climate extremes. After adjustment, the probability of extreme precipitation at Beijing towards to the higher (Fig. 1); while the decreasing trend of extreme event is more prominent than the estimate in the original series. Based on the homogenized daily precipitation dataset, the possible effect of urbanization on precipitation and extremes were quantified. For the urban-influenced sites, urbanization tends to slow down the magnitude of decrease in the precipitation and extreme amount series by approximately ？10.4% and ？6.0%, respectively; enhance the magnitude of decrease in precipitation frequency series by approximately 5.7%; reduce that of extremes by approximately ？8.9%; and promote the decreasing trends in the summer intensity series of both precipitation and extremes by approximately 6.8% and 51.5%, respectively. 

        It is worth noting that the homogenization of daily precipitation series is a challenging topic, as daily precipitation is not a continuous variable and has larger variability. Hence, there is greater uncertainty in the homogenization of precipitation than that of other climatic variables, such as temperature. Nevertheless, the additional adjustments based on the ECDF method provide an experimental approach to improving the homogeneity of daily precipitation series in terms of probability distribution, showing potential for further application in homogenizing daily climate series. 

Citation: Li, Z., Z. W. Yan, K. Tu, and H. Y. Wu, 2015: Changes and Possible Urbanization Effect of Precipitation and Extremes in Beijing Metropolitan Region during 1960-2012 based on Homogenized Observations. Adv. Atmos. Sci., 32(8), , doi: 10.1007/s00376-015-4257-x.  

Download: http://159.226.119.58/aas/EN/abstract/abstract2689.shtml 

Contact: Dr. Li Zhen, lizhen@tea.ac.cn