Statistical Study on the Plateau Low Vortex Activities

The activity characteristics of the Plateau Low Vortex(PLV) in summer(from May to September) are investigatedby using the daily synopticcharts of 500 hPa at 08:00 and 20:00(BST) from 1980 to 2004.Some significant results areobtained as follows: The occurrence frequency of PLVs in summer has evident interdecadal,interannualand intraseasonal variation characteristics,and shows a decreasing tendency in the 1990s comparing with that in the 1980s.July is an active period of PLVs.The four sources of PLV forming in the Tibetan Plateau are between Shenzha and Gaize,northeastern Naqu,northeastern Dege,and Songpan.PLVs moving out of the Tibetan Plateau also have four sources: Northeastern Naqu,Qumalai,Dege and Maqin.Some PLVs can survive over 36 h in the Plateau and move eastward including three routes(northeast,southeast and east) that PLVs moving toward northeast are in the majority,but the routes out of the Plateau are different from those in the Plateau that most of PLVs are moving toward east first,then toward northeast and southeast.There are two main routes when PLVs are moving out of the Plateau,one is northeast to Hexi,Ningxia and Loess Plateau,the other is southeast to Sichuan Basin,and PLVs moving to Loess Plateauare the most.PLVs move out Plateau also has evident interannualand intraseasonal variation characteristics.Most of PLVs will weaken and vanish during 12 h after moving out of the Plateau,some can persist for 60 h,and only a few for 100 h,even 192 h.They would influence the precipitation in the vast area of east China,and maybe Korea peninsular and Japan.Warm PLVs are nearly two timesmore than baroclinic PLVs in the beginning state,while after moving out of the Plateau,the characteristics of PLVs change a lot during 12 h and most are baroclinic vortices.The occurring sources,removing routes and properties of PLVshave changed since the middle of 1980s compared with 1960s~1970s.