西藏近35 年日照时数的变化特征及其影响因素

采用气候倾向率方法, 对西藏25 个站1971~2005 年逐月日照时数以及对日照有影响的 总云量、低云量、水汽压和降水量等资料进行了统计分析。结果表明: 近35 年西藏年日照时 数表现为极显著的减少趋势, 平均每10 年减少34.1 h。除冬季变化不大外, 其它各季均为减少趋势, 特别是近25 年, 夏、秋季日照时数减幅加大, 年日照时数减少明显。西藏20 世纪70 年代春、夏季日照充足, 秋、冬季日照偏少; 80 年代季日照时数均为正距平, 以秋季最为明显; 90 年代与80 年代截然相反, 季日照时数均偏少, 尤其是夏季。年日照时数异常偏多 年份均出现在20 世纪80 年代, 而异常偏少年份多发生在20 世纪90 年代。阿里地区年、季照时数的显著增加与总云量的显著减少、降水量减少有关, 其它大部分站点年、季日照时数显著下降与大气水汽压的增加关系密切。

Climatic Change of Sunshine Duration and Its Influencing Factors over Tibet during the Last 35 Years

Using the data of monthly sunshine duration,mean cloudiness,surface vapor pressure and precipitation at 25 meteorological stations over Tibet from 1971 to 2005,the trend variation of the sunshine duration are analyzed by using linear trend analysis. Main results are as follows: (1) In terms of linear trend,the annual sunshine duration increases with an average rate of (8.8-52.1) h/10a in Ngari district,Tingri,Amdo,Mshung and Chali counties,especially in Mshung. But the tendency of annual sunshine duration decreases mostly in other stations with a decreasing rate of (10.9-138.4) h/10a,and the maximum is in Nagqu. In summer,the sunshine duration has a decreasing trend in most parts of Tibet. In winter,the increasing trend of sunshine duration occurred in Ngari district,northern Chamdo district and most parts of Nagqu district. In most parts of Tibet,the annual and seasonal total cloudiness has shown a decreasing tendency during recent 35 years,and the low cloudiness trend reduced. The trend of annual and seasonal mean surface vapor pressure increased over Tibet,especially in summer.(2) On an average in Tibet,the annual sunshine duration has shown a significant decreasing tendency during recent 35 years with a decreasing rate of 34.1 h/10a. Except for no obvious trend in winter,the decreasing trend occurred in other seasons. Specifically,in recent 25 years,the reducing range of sunshine duration increased in summer and autumn,as a result,the annual sunshine duration reduced. (3) In the 1970s,the sunshine durations are more sufficient in spring and summer,while lower in autumn and winter. The sunshine duration of positive anomaly occurred in all seasons in the 1980s,especially in autumn. But to the contrary in the 1990s,the sunshine durations are lower in all seasons,especially in summer. (4) Also,the annual and summer sunshine durations are more anomalous years in the 1980s,whereas less anomalous years in the 1990s. In winter,more anomalous years occurred in 1987 and less anomalous years in 1995 and 2005. (5) Results indicate that the principal causes for the increase of sunshine duration are the obvious decrease of cloudiness and decrease of precipitation in Ngari district,and that the decrease of sunshine duration is mainly caused by the significant increase of surface vapor pressure in other parts of Tibet.