Diagnostic analysis of summer rainless days in northwest China in recent 45a

Drought is main climate feature of northwest China,many specialists have analyzed drought features of northwest China with recipitation data of different season.The rainless day is a factor responding to drought and the summer is main rain season of northwest China.The paper calculated rainless days from the summer day-to-day precipitation data of northwest China during 1960-2004.With EOF,REOF,tendency analysis,Mor- let wavelet analysis methods,this paper analyzed the temporal and spatial abnormal distribution features of sum- mer rainless days.The results show that the south of Xinjiang is high number region of the spatial distribution of summer rainless days in northwest China,but the south of Qinghai is low number region;The consistent abnor- mal characteristics of the whole northwest China is the main spatial distribution pattern of summer rainless,the opposite change of eastern Qinghai plateau and the north of Xinjiang compared with the other regions of the northwest China is also an important main distribution pattern,this character explain northwest China is overal- ly controlled by the same weather system,but the summer rainless days also exist spatial difference because of terrain effect.The spatial distributionfeature of summer rainless days in northwest China can be divided into six main subareas of the east of Qinghai plateau area,the south of Xinjiang area,the north of Xinjiang area,the plateau of Qinghai area,the Hetao area and the east of northwest China area,every kind of subarea is identical to summer precipitation of northwest China;In recent 45 a,summer rainless days display a weak decent tenden- cy in the north of Xinjiang area,and a clear decent tendency in the south of Xinjiang area and the Qinghai plateau area,but summer rainless days display a ascent tendency in the Hetao area and the east of plateau area, and a remarkable ascent tendency in the east of northwest China;In the summer rainless days of all the six main spatial subareas of northwest China,the 11-13 a and 7-9 a periods are displayed very clear,in most areas the 3-5 a high frequency oscillation period is displayed in recent 45,although a more than 20 a low frequency oscil- lation period is also display in most areas,but because length of data is only 45 years,this period need more evi- dencies to prove its existance.