利用1961 -2017年青藏高原110个气象台站逐日降水观测资料及美国环境预报中心（NCEP）逐日再分析资料， 通过气候平均候雨量稳定通过阈值法和有序样本最优分割法相结合划分青藏高原雨季起讫期， 分析起讫期时空演变特征， 重点探讨了高原雨季开始/结束早晚的大气环流异常特征。结果表明： （1）高原雨季开始期呈明显的提前趋势， 结束日期呈微弱推迟趋势， 进入21世纪后， 高原雨季开始期明显提前、 结束期逐渐推迟， 因而雨季持续期延长； （2）高原夏季风的建立为高原低层气流提供了辐合上升条件， 随后东亚夏季风爆发， 孟加拉湾西南暖湿气流进入高原提供了利于降水的水汽条件； （3）南亚高压从中南半岛建立开始， 其西北向的移动与高原雨季开始的演变具有很好的一致性， 40候之后向东南缩小减弱， 与雨季结束时间的演变具有很好的一致性； （4）雨季开始前对流上升运动最先在高原南部中低层开始， 随着雨季推进， 高原自南向北整层均为不断增强的对流上升运动， 雨季结束后， 无明显上升运动； （5）雨季开始/结束偏早、 偏晚的合成差值表明： 南亚高压偏南偏弱、 副高位置偏西偏强、 高原夏季风偏强有利于高原雨季开始偏早、 结束偏迟， 反之雨季开始偏迟、 结束偏早。

The article based on the Qinghai-Xizang Plateau 110 stations daily precipitation data and NCEP daily reanalysis data from May to September from1961 to 2017， and using the methods of the climatic mean rainfall which through the threshold stably and ordered samples segmentation to divide the Qinghai-Xizang Plateau rainy season onset and offset date and then analyzed the characteristics of the time and spatial evolution about Qinghai-Xizang Plateau rainy season onset and offset， finally the anomalous characteristics of atmospheric circulation at the Qinghai-Xizang Plateau rainy season onset and offset are discussed especially.The results shows that： （1） Qinghai-Xizang Plateau rainy season onset date in the Qinghai-Xizang Plateau shows an advanced trend， but the offset date shows an slightly delay trend， thus the Plateau rainy season duration become longer； （2） The establish of the Qinghai-Xizang Plateau summer monsoon is helpful to low level airflow convergence， after with the beginning of East Asian summer monsoon， the Bay of Bengal westsouthern wind provide water for the Qinghai-Xizang Plateau which is helpful to the plateau precipitation； （3） The South Asia High eatablished in Indochina， and its westnorthern movement is consistent with the evolution of the Qinghai-Xizang Plateau rainy season onset， and its movemnet of southeast shrinking and weakening is also consistent with the evolution of the plateau rainy season offset； （4） Before the onset of the Qinghai-Xizang Plateau rainy season， the convective upwelling first starts at the middle-low levels in the southern part of the plateau， with the advancement of the plateau rainy season， the convective upwelling is increasing over the whole layer of the plateau from the south to north continuelly； （5）The onset or end of plateau rainy season composite difference show that South Asian high more weaker and souther or the subtropical high more wester and higher or the plateau summer monsoon stronger is beneficial to the beginning and ending of the plateau rainy season onset earlier or offset later； Contrawise， the plateau rainy season onset later or offset earlier.