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Trends of Daily Extreme Air Temperature in the Wei River Basin in the Future

LIU Zhaofei, XU Zongxue ( Key Laboratory of Water and Sediment Sciences, Ministry of Education; College of Water Sciences, Beijing Normal University, Beijing 100875, China)  
Global climate change can have significant impact on local and regional hydrological regime, which will in turn affect the ecological, social and economical systems. However, the study on impact of climate change upon hydrologic regime has not been investigated. The main reason is that Global Circulation Models, which are widely used to simulate the climate scenarios in the future, do not provide daily and local data that are reliable enough for hydrological modeling. At present, this problem is usually resolved with downscaling methods. A Statistical Downscaling Model (SDSM) is applied to the case study in the Wei River basin. In this paper, daily maximum and minimum air temperature (Tmax/Tmin) are selected as the predictands. The selection of appropriate downscaling predictors is based on the relationship between meso-scale atmospheric variables and daily Tmax/Tmin series in the basin, which are obtained from the NCEP re-analysis data set and observed data respectively. Data series is divided into two groups with time ranges of 1961-1990 and 1991-2000, in order to calibrate and validate the model respectively. Then, the outputs form A2 and B2 scenarios simulated by using HadCM3 are downscaled to station level. Finally, the future trends of daily Tmax and Tmin, in three periods, which are the 2020s (2010-2039), the 2050s (2040-2069), and the 2080s (2070-2099) in the Wei River basin, are analyzed according to the baseline of 1961-1990. The results show that some meso-scale circulation variables are closed related to Tmax/Tmin in the basin, including surface mean temperature, near surface specific humidity, 500 hPa and 850 hPa geopotential heights and there is an obvious increasing trend in daily Tmax and Tmin in the future. Both daily Tmax and Tmin show a greater increasing range in A2 scenario that in B2 scenario, while daily Tmax shows a greater increasing range than Tmin in both scenarios. The spatial distributions of daily Tmax and Tmin are similar over the periods of the 2020s, 2050s and 2080s. The daily increments of Tmax and Tmin in A2 and B2 scenarios become smaller from the northwest to the southeast of the basin. Since the air temperature is important for runoff generation, the warmer temperature in the future could have great impacts on the runoff and the sand in the basin. The results of this paper can be used as reference for the generation of climate change scenarios in the impact assessment of climate change, it is also indicated that the responses of hydrological regime to climate change need further to be investigated.
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