近50年拉萨日照时数的变化特征

Characteristics of Sunshine Duration Variation in Lhasa in 1953-2005

采用气候倾向率方法对拉萨1953～2005年的年、季节日照时数变化趋势分析表明,近50年来拉萨年日照时数和各季节日照时数都存在不同程度的减少趋势,年日照时数以每10年27.6h倾向率减少,且这种减少的趋势在增大,主要表现在夏、秋季。20世纪50～80年代年日照时数均为正距平,90年代以负距平为其主要的年际变化特征;90年代夏季日照明显偏少,60年代冬季日照充足。年日照时数90年代末出现异常偏少年,夏季日照时数仅在2000年出现了异常偏少现象,而冬季日照时数多异常年份,主要发生在50年代。年平均总云量减少趋势明显,低云量下降趋势显著。

Using the data of monthly sunshine duration from 1953 to 2005 in Lhasa of Tibet, the interannual and interdecadal variations of the annual and seasonal sunshine duration are analyzed by means of the linear trend analysis method. The results show that the decreasing trends of the annual and seasonal sunshine duration were different during recent 50 years. The annual decreasing rate was 27.6 h/10a, and the trend is still getting stronger, especially in summer and autumn. The anomalies of annual sunshine duration were positive from the 1950s to 1980s, and became negative in the 1990s. The sunshine duration was obviously less in summer of the 1990s, abundant in winter of the 1960s. The annual sunshine duration was less than normal at the end of the 1990s, extremely less in 1998 and 2000; while there were more anomalous years in winter mainly in the 1950s. The results show that there was a decreasing trend in total annual mean cloudiness and low cloudiness.