Research On Moisture Sources For Precipitation Over Drylands Of China

Drylands cover nearly half of Chinese land area.The perennial lack of precipitation has brought about some problems such as drought and land desertification.This study is devoted to exploring the formation mechanism of extreme precipitation events in summer and winter over Chinese drylands,the long-term trends of precipitation over Chinese drylands and the causes for them from the perspective of moisture source during 1979-2018.This study divides Chinese drylands into five sub-regions,including northern Xinjiang,southern Xinjiang,central Tibetan Plateau,central Chinese dryland and northeastern China.The precursor signals of summertime extreme precipitation events over the five sub-regions can be traced back to the-6th,-4th,-4th,-4th,and-4th days during the extreme precipitation event.The precursor signals of wintertime extreme precipitation events appear earlier on the-12 th,-10 th,-12 th,-12 th and-6th days,respectively.The circulation anomalies locate in the upstream continue to disperse wave fluxes to the downstream,which makes circulation anomalies in the downstream continue to strengthen while their positions continue to move eastward.A Rossby wave train structure of circulation anomalies which is favorable for triggering extreme precipitation over the five sub-regions forms at the onset of the extreme precipitation event.The wave train structures in summer can be divided into two types: a northwest‐southeast‐oriented linear wave train structure and an inclined L-type wave train structure.In winter,except for the inclined inverted L-type wave train structure which is favorable for triggering extreme precipitation over northern Xinjiang,the rest wave train structures all belong to the first type of the wave train structures mentioned above.All sub-regions experience a temperature variation of warming first and then cooling.Therefore,the water vapor transported to each sub-region through large-scale convergence is first accumulated and stored in the atmosphere,and then released to form precipitation when cools.The moisture contribution for the occurrence of extreme precipitation in summer and winter mainly comes from the water vapor transported by the large-scale convergence.The key moisture source regions for summertime extreme precipitation over the five subregions are central Asia,local,southern Asia,southern China and southern China,respectively.The key moisture source regions for wintertime extreme precipitation are central Asia,southern Asia,southern China and other drylands except local regions.This paper investigated the causes for “drying in the east and wetting in the west” in the long-term variation of summertime precipitation and wintertime precipitation over Chinese drylands from the perspective of Lagrangian method.The result shows that local evapotranspiration and the water vapor transported by the East Asian summer monsoon play an important role.It is revealed that the water vapor transported by the East Asian summer monsoon from the south to Chinese drylands tends to migrate westward due to the existence of an anticyclonic anomaly over Mongolia.This anticyclonic anomaly over Mongolia was accompanied by the weakening of southwesterly winds in northeastern China and the strengthening of easterly winds in northern China.The Mongolian anticyclonic anomaly has a eastward moving trend,with the greatest impact on northeastern China during 1999-2008,causing a rapid expansion of the boundary of Chinese drylands during this period.The important factors affecting the water vapor supply for wintertime precipitation are local evapotranspiration,westerly winds and East Asian winter monsoon.The long-term variation of moisture source for the wintertime precipitation over northeastern China,where the dryland boundary changed significantly,is mainly that the water vapor transported by the East Asian winter monsoon through the Mongolian Plateau increased first and then gradually decreased.The main cause is that there exists a cyclonic anomaly over West Siberia that enhanced the water vapor transportation by the westerly winds and the East Asian winter monsoon to northeastern China.Then the phase of the cyclonic anomaly changed into an anticyclonic anomaly and gradually intensified,inhibiting the water vapor transport process.