Characteristics And Formation Mechanism Of Summer Extreme Precipitation In The Qaidam Basin

The Qaidam Basin(QB),as the largest alpine arid inland basin in China,lies in the northeastern Tibetan Plateau and the northwest China.It is not only a sensitive area for climate change but also a fragile zone for ecological environment.The frequency of precipitation and extreme precipitation events here occurs rarely compared with eastern China,however,understanding the local characteristics and physical processes of extreme precipitation in the QB is full of meaning to the natural ecosystem and economic construction in the arid region.This paper focuses on the study of the temporal-spatial distribution characteristics of summer extreme precipitation in the QB and corresponding characteristics of atmospheric circulation and water vapor transport during 1981?2017.And quantitatively analyzes the changes of key weather systems and water vapor transport at the four boundaries of the basin during extreme precipitation.In addition,the physical mechanisms of the atmospheric apparent heat source affecting extreme precipitation in the QB is investigated.Finally,the impact of the synoptic-scale transient eddy activity on the formation and evolution of blocking circulation is investigated through two types of typical blocking high events associated with summer extreme precipitation in the QB,and the possible relationship between the synoptic-scale transient eddy activity and the summer extreme precipitation in the QB is discussed further.Several conclusions are demonstrated:(1)In the summer of 1981?2017,the extreme precipitation thresholds and extreme precipitation days in the eastern part of the QB are greater than of in the western part of the QB,which is consistent with the summer precipitation.And the trend of both summer precipitation and extreme threshold threshold are upward during this period.The extreme precipitation occurs rarely here which appears only 1?2 days on average in whole summer,but it contributes a lot to summer precipitation,up to about 12%.(2)During the period of extreme precipitation in the QB,in the middle layer,there is a weak ridge over the Caspian Sea and the Aral Sea,and a trough in the northwest of the QB in the mid-high latitudes.The zonal circulation prevails over Europe,and the meridional circulation prevails in the upstream regions of the QB.In low latitudes,there is a stable trough in the Bay of Bengal,and the South Asian summer monsoon and plateau monsoon are significant and have enhanced.The South Asian High(SAH)has enhanced and its center located over the Iranian plateau.The subtropical westerly jet is divided into two parts,the east and west branches,while its intensity decrease slightly,hence it affects extreme precipitation in the QB by pattern rather than intensity.At this time,the QB lies on the right-hand of the entrance zone of the east branch of westerly jet and on the southern flank of the SAH in upper level,corresponding to the divergence zone,which provides a facilitate dynamic environment for summer precipitation in the QB.As a result,the convective is active here.(3)There are two water vapor channels corresponding to the atmospheric circulation associated with extreme precipitation in the QB.One is the north branch water vapor transport channel.The water vapor in Eurasia is transported eastward and southward to QB by the north branch of the westerlies through the north side of the Tibetan plateau.The other is the southern branch water vapor transport channel.A little water vapor comes from Eurasia and is transported by the southern branch of the westerlies.The remains is mainly from the Arabian Sea and the Bay of Bengal and then carried by the South Asian summer monsoon eastward and northward to the QB under the relay role of the plateau monsoon.The increased water vapor content during the period extreme precipitation in the QB most attributes to the enhanced water vapor inflows from western and southern boundaries,which increase by 27.37kg.s-1 and 44.31kg.s-1,respectively,and the decreased water vapor outflow across the eastern boundary which decreases by 53.91kg.s-1,compared with the climatic state.(4)The atmospheric apparent heat source over the QB can significantly affect the water vapor transport and convection activity associated with extreme precipitation by triggering an abnormal cyclonic circulation over the Tibetan plateau,which makes water vapor over the surrounding areas of the Tibetan Plateau to converges into the QB and enhances the local convective activity.(5)When there exists typical meridional circulation in mid-high latitudes,the synoptic-scale transient eddies have an impact on the extreme precipitation in the QB by affecting the blocking circulation.Before blocking high onset,there are poleward transient momentum fluxes throughout troposphere,and equatorward transient heat fluxes in mid-low level but poleward transient heat fluxes in upper level in the blocking region,which shows the synoptic-scale transient eddies tend to develop the blocking.On onset day,the transient momentum and heat fluxes transport enhance,the effect of the synoptic-scale eddies enhance,maintaining the blocking high.Subsequently,as the[u*'v*']and[v*'T*']transport decrease,the impact of transient eddies on blocking circulation decrease,which favors the collapse of blocking high.In addition,transient eddies barotropically develop the blocking circulation before the blocking high onset and maintain the blocking circulation after the blocking high onset via barotropical energy conversion.While this barotropic energy interaction is more obvious in the Urals blocking than that in Baikal blocking.Finally,The synoptic-scale transient eddies transport water vapor originating from high latitudes southward in the eastern part of the blocking region or the upstream region of the blocking high during the period of blocking,then there is convergence of v*'q*',which provides water vapor for the extreme precipitation in the QB.The influence of synoptic scale transient eddies on water vapor transport is closely related to the blocking circulation.The blocking is more obvious,the northerly airflow in front of the high pressure ridge or behind of the trough lying upstream region of blocking is more stronger,and there is more southward water vapor transported by transient eddies,as well as the precipitation in the QB.