横断山脉中西部降水的季节演变特征

Characteristics of the seasonal evolution of precipitation over the central western part of the Hengduan Mountain

利用台站观测逐日降水资料,对横断山脉中西部地区的降水季节演变特征进行了分析,发现该地区降水具有独特的季节变化特征:雨季开始早,从第10候前后降水开始迅速增加,至第19候前后就达到第1个峰值;雨季时间长,从第10候前后雨季开始,至第60候雨季结束,雨季持续长达8个月;多峰值特征明显,雨季先后经历3个降水峰值,分别在第19、35、55候前后。通过再分析资料对这一地区风场的季节变化进行分析发现,这些降水的季节演变特征与这一地区独特地形下风场季节演变密不可分。雨季开始早与第10候起低层西风、南风迅速加强,特别是西风加强有关;第2、3个降水峰值则与西南风,特别是经向南风增强有关。对3个降水峰值时刻的环流背景进行了分析,第1个降水峰值发生时,位势高度东西方向水平差异小,降水主要受西风系统影响,与西风系统的季节变化密切相关;而第2、3个降水峰值分别发生在西太平洋副热带高压西伸、东退进程中,位势高度场东高西低,降水主要受西南风控制,并伴有南风辐合,与南风的季节变化相关。

Using the station rain gauge data, the seasonal evolution of precipitation over the central western part of the Hengduan Mountain has been analyzed.Characteristics of the seasonal evolution over this region arc unique.Firstly, the rainy season starts early; rainfalls over this region greatly increase from Pentad 10 and get the first maximum at Pentad 19.Secondly, the rainy season lasts long, which covers the period from Pentad 10 to Pentad 60,for about eight months.Thirdly, the rainy scason has three peaks, which occurs at Pentad 19,35 and 55.Icy investigating the rcanalysis data, the seasonal rainfall evolution is found to be closely linked to the wind fields.The first peak of the rainy season can be attributed to the enhanced westerly and southerly,especially the westerly.The second and third peaks of the rainy season arc linked to the southwesterly, especially its south wind components.When the rainfall gets the first peak,the gcopotential height at 500 hPa is characterized by the little tonal difference.The variation of rainfall system is greatly influenced by the westerly.The second（third） peak is associated with the western extension （eastern retreat） of subtropical high,when the gcopotential height at 500 hPa shows negative tonal gradient.In these two periods, the middle troposphere over this region is controlled by the southwesterly and the rainfall systerns arc greatly influenced by the southerly.