“日光城”拉萨日照时数的变化特征

Changes in Sunlight Duration in Lhasa,the "City of Sunlight"

利用拉萨1970年～2004年的日照时数、水汽压、降水、云量等资料，采用气候统计方法讨论了35年来拉萨日照时数气候变化特征及变化原因，主要得出以下结论：拉萨年日照时数达2980h；一年中10月份的日照时数最长，长达281h，8月份日照时数最短，为219h。4个季节中，秋季日照最丰富，长达777h，其次是春季、冬季，分别为769h、736h，夏季最少为698h。1970年～2004年以来拉萨的年日照时数呈减少趋势，平均每10年减少近58h，20世纪90年代末期以来日照时数更是显著减少；4个季节中夏季日照时数的减少幅度最大，减幅为27h／10年。六气水汽压、降水是影响拉萨日照时数变化的主要因子，尤其是20世纪90年代以来，它们的显著增加对该时期日照时数的减少有着重要贡献。

Lhasa, the capital of Tibet Autonomous Region, is known as the ＂City of Sunlight＂. Because of the rich sunlight radiation, thermal, and water resources, Lhasa is one of the best regions for development of agriculture and livestock production. Many scholars have studied climate change in terms of temperature and precipitation on the Qinghai-Tibet Plateau, but few have studied the changes in solar energy resources in this region, which includes Lhasa. Until now, there is limited understanding of how Lhasa＇ s solar energy resources have changed and the main impact factors. These problems are relevant in the context of global climate change. Sunlight duration, as an index of solar energy resources, is an important climatic factor. In this article, we analyze the climatic features of sunlight duration in Lhasa, using data on monthly sunlight duration, mean cloudiness, surface vapor pressure and precipitation from 1970 - 2004. Results show that the average annual duration of sunlight in Lhasa is 2980h. The month of October has the most sunlight, with 281h, and August has the lowest number of sunlight, with 219h. Of the four seasons, autumn has the most sunlight, with 777h, followed by spring （769h）, winter （736h）, and then summer （698h）. From 1970 to 2004, the annual sunlight duration decreased at an average rate of 58h/10a, and this trend is the most obvious beginning in the late 1990s. Except for a somewhat weaker trend of decreasing sunlight hours in winter, the other three seasons exhibit an obvious decline. The summer months had the most rapid decline, of 27h/10a. Results indicate that the main factors influencing sunlight duration in Lhasa are surface vapor pressure and precipitation. The significant increase in surface vapor pressure and precipitation led to the decrease in sunlight duration over the study period. These two factors are especially prominent from the late 1990s through 2004.