| With the research on global climate change ceaselessly thorough,the climate change caused by human activities has attracted much attention.In recent years,in the context of global climate change,extreme weather events have increased significantly,especially the frequency of extreme precipitation has increased,it made the risk of flood disaster in some river basins has increased.Exploring the possible influence of human activities on catchment scale precipitation changes can help understanding the contribution of human activities to the climate change at the regional scale,providing guidance for the water conservancy department to prevent and mitigate disasters in key basins,and providing scientific suggestions for the decision makers to formulate greenhouse gas emission reduction plans,which has great research significance and practical value.In this paper,two sets of precipitation observation data CPC-Global,CN05.1 and the CAM5.1-1degree model in the C20C+ D&A Project were used.By calculating the relative root mean square error and the significance test,the abilities of this model in simulating the average and extreme precipitation of three typical river basins(Yangtze River basin,Huang-Huai-Hai River basin and Mekong River basin)at different latitude zones were evaluated.On the basis of the evaluation,by comparing the difference between two sets of historical scenarios simulations All-Hist(a factual scenario including effects of all forcing factors)and Nat-Hist(a counterfactual scenario only consider the effects of natural forcing factors),their changes relative to the reference period under Nat-Hist were calculated to analyse the possible impact of human activities to the time and space distribution characteristics on average precipitation and extreme precipitation over the three different typical watersheds.Depended on the aboving conclusions and some research results about the corresponding relationship between atmospheric circulation and monsoon index with the change trend of precipitation,taking the Yangtze River basin as an example,the dynamic causes of precipitation changes possibly caused by human activities were preliminarily analyzed.The specific conclusions are as follows:(1)The interannual changes and spatial distributions of mean precipitation and extreme precipitation simulated by the two scenarios of CAM5.1-1degree both have significant difference,it indicated that human activities have indeed made contribution to both the changes of mean precipitation and extreme precipitation on the catchment scale.The multi-run average simulation results(MRA)of All-Hist scenario can well reproduce the spatial and temporal changes of the annual average precipitation and the three extreme precipitation indexes(RX5day,R90 p,R99p),which representing the extreme precipitation,over three basins relative to the reference period,shows that the All-Hist scenario considering the effects of human activitie is closer to the observation fact.(2)The MRA of CAM5.1-1degree All-Hist scenario has a better simulation ability on the spatial and temporal variation characteristics of average precipitation than that of extreme precipitation.Moreover,the model has various degrees of underestimation on the spatial and temporal variation of average precipitation and extreme precipitation over different basins.The simulation results of the time change characteristics of mean precipitation show that the simulation ability of the tropical Mekong River basin is better than that of the subtropical Yangtze River basin and the temperate Huang-Huai-Hai River basin,only in the Mekong River basin,MRA can accurately simulate the extreme inter-annual fluctuations.While for the extreme precipitation,although the MRA can roughly reproduce a similar interannual fluctuation as observation,but it significantly underestimates the variation range of extreme precipitation index;In terms of the simulation ability of mean precipitation spatial distribution change,MRA can accurately simulate the position of extreme value center where obviously change occur,especially the position where extreme value centers show obviously reduce.However,for the extreme precipitation,except for the temperate Huang-Huai-Hai River basin shows good simulation ability,other river basins still have large room to improve.(3)In terms of the influence of human activities on catchment scale precipitation changes,the research shows that under the influence of human activities,the time variation of average precipitation in three typical river basins at different latitude zones are mainly reduced.The average precipitation of the subtropical Yangtze River basin showed the most obvious decreasing trend under the influence of human activities.The decreasing change of the average precipitation over the temperate Huang-Huai-Hai River basin lasted for the whole research period,while the decreasing change of the tropical Mekong River basin was relatively weak.In addition,the season with the greatest contribution to the average precipitation reduction in the temperate Huang-Huai-Hai River basin is summer,and the period with the greatest contribution is 1960?1969,while the season with the greatest contribution in the subtropical Yangtze River basin and tropical Mekong River basin is winter or dry season with less precipitation,and the period is 2000?2009;Under the influence of human activities,the extreme precipitation in the three basins showed an increasing trend,which gradually weakened or even decreased with time.Among the three basins,the extreme precipitation in tropical Mekong River basin has the most significant change under the influence of human activities.Among the three extreme precipitation indexes,the increasing trend of R90 p was most significantly weakened.The period when human activities have the most significant influence on extreme precipitation was 2000?2009.(4)The results also indicate that there are significant spatial differences in the spatial distribution of average precipitation and extreme precipitation changes over the three watersheds affected by human activities.Compared to the Nat-Hist scenario that only consider natural forcing factors,the All-Hist scenario which including the influence of human activity,both average precipitation and extreme precipitation,the space showed reducing trend was enlarged,the space showed increasing trend was narrowed,and the degree of decrease in the area showed decreasing trend under both scenarios was intensified.In conclusion,from the change of spatial distribution characteristics,the influence of human activities on average precipitation and extreme precipitation in three river basins from different latitude zones was mainly reduced.The most significant affected areas by human activities in these three river basins are: the upper reaches and the southern part of the Yangtze River basin,the Liujiaxia area of the Yellow River,the southern part of the Huaihe River,the northern part of the Haihe River,and the 20°N~25°N area of the Mekong River Basin.In addition,comparing the temporal and spatial changes of different degrees of extreme precipitation under the impacts of human activities,it is found that the stronger the extreme degree of precipitation is,the more obvious the temporal and spatial changes characteristics will occur when considering the impact of human activities,that is,the stronger the influence of human activities is.(5)To further explore the mechanism of basin scale precipitation change resulting from anthropogenic forcing,we take summer rainfall over the Yangtze River basin as the research object,analyze that under the impact of human activties how those main circulation factors which associated with Yangtze's summer precipitation change.The results show that under the influence of human activities,westerly jet in wind field at 200 hPa was enhanced,the water vapor transport to the Yangtze River basin in wind field at 850 hpa was reduced,and the west Pacific subtropical high(WPSH)was enhanced and its location in 500 hpa geopotential height field was northward and eastward shifted,all these changes had been mentioned in existing researches that corresponded to the reduction of Yangtze River basin's summer precipitation.These might be the atmospheric dynamics reason why the anthropogenic forcing caused precipitation change. |
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