Distribution,Trends And Causes Of Clouds,Precipitation And Extreme Disastrous Weather Events On The Tibetan Plateau

The Tibetan Plateau(TP)is one of the highly sensitive and vulnerable to global climate change and is experiencing an approximately double global average warming.Under a warming climate,changes in clouds and precipitation,and extreme disastrous weather events(thunderstorm,hail,gale and heavy precipitation)might profoundly affect the regional water cycle,ecological environment and disaster prevention.However,the deep investigations in distribution,trends and causes of clouds and precipitation,and extreme disastrous weather events over the TP and its subregions are very limited.Given the large spatial variations of influential weather and climate systems over the TP,this paper divides the whole plateau into the subregions of SE,SC,NE,NW,and SW.Considering the only one station in the NW subregion and three stations in the SW subregion,this study primarily investigated the temporospatial distribution and trends of clouds and precipitation,and extreme disastrous weather events in the subregions of SE,SC and NE and the whole TP,and associated possible mechanisms with meteorological station data and ERA-interim(the Interim ECMWF Re-analysis)data from 1979 to 2016.The results show that the high total cloud amount in northern TP(NE and NW)mainly occurs from March to May,while that in southern TP(SE,SC and SW)primarily happens from June to August.The low cloud amount and precipitation events on the TP mainly occur from May to September in the SE and SC subregions.The high number of snow cover days and amount of snow depth mainly appear from December to February.The spatial distribution of snow cover is closely related to the high mountain ranges on the TP.The total cloud amount in southern TP(SE and SC)in cold seasons tends to decrease significantly,while the low cloud amount on the whole TP tends to increase significantly,especially in the NE subregion in warm seasons.Although the TP is getting significant warming and an overall wetting,the changes in precipitation in warm seasons have obvious differences in spatial distributions.In the NE subregion,the total number of precipitation days(PN)has a weak increasing trend,while the total precipitation amount(PA)has a significant increasing trend,indicating that the precipitation intensity(PI)has a significant strengthening trend.In the SE subregion,the total PN has a significant decreasing trend while the total PA has a weak decreasing trend,indicating the PI also has a significant strengthening trend.Therefore,under a warming climate,the PI in both NE and SE has a significant increasing trend,suggesting that the secondary geological disasters such as debris flows and landslides caused by heavy precipitation events might have an increasing trend in these subregions in warm seasons.Both the total number of snow cover days(SN)and mean snow depth(SD)on the TP tend to decrease,which might cause a potential impact on glaciers,lakes,water cycles and ecosystems on the TP.The investigations in temporospatial distributions and trends of extreme disastrous weather events indicate that thunderstorm,hail and heavy precipitation events primarily occur in warm seasons.The SE subregion has the most frequent thunderstorm and heavy precipitation events,while the SC subregion has frequent hail events.Gale events are closely associated with the high-altitude mountains in SC and SW subregions,and strongly influenced by the prevalent westerlies in the spring season.Thunderstorm,hail and gale events show significant decreasing trends,while heavy precipitation events have an insignificant increasing trend over the whole TP.In terms of different subregions,the SE and SC subregions have much more obvious significant decreasing trends in thunderstorm,hail and gale events,while the NE subregion has a significant increasing trend in heavy precipitation events.The possible underlying mechanisms responsible for changes in clouds and precipitation,and extreme disastrous weather events over the TP have been further elucidated in the paper.The results indicate that the changes in clouds and precipitation are closely associated with anomalous atmospheric thermodynamics,dynamics and synoptic circulations,as well as water vapor transport on the TP.On the one hand,in cold seasons,the significant expanding of the Hadley cell causes the westerlies to shift northward and the subtropical high to shift toward the southern TP,which increases the atmospheric stability and significantly reduces the middle and high clouds,and snow events on the TP.In warm seasons,the high-level South Asian high tends to strengthen and shift northward,and significantly reduce precipitation in the SE subregion but increase precipitation in the NE subregion.On the other hand,the strengthening South Asian monsoon may bring more water vapor from the Indian Ocean into the low levels of the TP,and enhances low clouds and precipitation.In addition,the enhanced easterly winds in the south flank of the strengthening anticyclone located between the southern Lake Baikal and the north of the TP bring more warm moist air from the northern Pacific Ocean into the TP,which is favorable for the increase of clouds and precipitation in the NE subregion.The anomalies of atmospheric thermodynamics,dynamics,synoptic circulations and water vapor transport on the TP in warm seasons are found to be closely related to the northwestern Atlantic Sea Surface Temperature(SST)anomaly associated with the North Atlantic Oscillation(NAO).A strong wave train triggered by the northwestern Atlantic SST anomaly propagates from the northern Atlantic to East Asia through Europe,and induces a more upper-level warming over the TP and an anomalous anticyclonic circulation near the Lake Baikal,resulting in more stable atmosphere and blocking effect,which forces the mid-latitude westerlies and associated cold air to shift poleward.On the one hand,the weakened cold air advection over the TP decreases the baroclinic instability and convection initiation,and finally causes the significant decreasing trends in thunderstorm,hail and local gale events.On the other hand,the enhanced easterly winds in the southern flank of the anticyclonic circulation can significantly increase the water vapor flux from the eastern boundary of the TP,coupled with the notable increase in low cloud amount in the NE region,which is conductive to the significant increase in heavy precipitation events in the NE subregion.