Temporal and Spatial Distribution Characteristics of Days with above Heavy Rainfall over Northwestern China

Heavy rainfall has both advantages and disadvantages to northwestern China since it may form flood and mud-rock flow disasters;meanwhile,it can be beneficial to relieving agricultural drought and reservoir impoundment electricity generation.Studying heavy rainfall is therefore of great importance to improve our understanding of regional climate resources and their exploitation over northwestern China.With Mann-Kendall,trend analysis,and wavelet analysis,spatial distribution characteristics and the evolution of days with above heavy rainfall across northwestern China for 47 years were comprehensively analyzed based on daily data during the period 1961-2007 from 138 stations.Results show that days with above heavy rainfall decreased from the southeast to the northwest.The Qilianshan and Tianshan areas showed relatively high values.As for seasonal distribution,days with above heavy rainfall were least in winter and most in summer,with autumn being larger than spring.Days with above heavy rainfall for most places over this region exhibited slightly increasing trends,especially over the Tianshan areas.For the eastern part of the region,there existed slightly decreasing trends in the days with above heavy rainfall.In general,the days with above heavy rainfall had a 22-year interannual cycle.There was a remarkable mutation in 1967 over the Qilian mountainous areas.By sequence analysis,it was found that since the mid-1990s,days with above heavy rainfall in winter and summer increased to a certain degree,decreased in autumn,and remained fairly invariant in spring.Northwestern areas exhibited a single peak of days with above heavy rainfall as a whole,within the peak occurring in July and the majority of the days being concentrated between May and September,which accounted for 92% of that of a year.The maximum daily rainfall ranged between 12.8 mm and 203.3 mm over this region,decreasing from the southeast to the northwest,peaking around Foping in southwestern Shaanxi,Lintao in Gansu,Telingha and Tianshan's Baluntai in northern Qinghai.The maximum annual rainstorm intensity generally increased,except for the Tianshan areas.In the context of global warming,extreme weather and climatic events tend to frequently occur.There were differences in days with above heavy rainfall between the east and the west of northwestern regions,with the east decreasing and the west increasing slightly.The decreases in the east might be related to the weakening East Asian monsoon.The generally increasing days with above heavy rainfall might also be correlated with global warming.