Analysis on Variation of Sunshine Duration in Xi′an in Recent 60 Years and Its Influencing Factor
Using the long term record(about 60 years) of meteorological parameters of Xi′an and east of Guanzhong region in Shaanxi Province and nine years of MODIS data,the causes of decreasing trend for sunshine duration during rapid economic developing period have been analyzed.The result shows that there is an obviously decrease of sunshine duration in Xi′an from 1981 to 2001.The decreasing trends are also found in Weinan and Huaxian,east of Xi′an.And there is the good correlation of percentage reduction of sunshine duration at two different stations.But the decrease of sunshine duration is not found in Hua mountain.The cause of the reduction is at height below 2 km.And there is a high value center of AOD on the east side of Guanzhong region in Shaanxi Province,while after eliminating of fog day,total cloud cover and low cloud cover,there are no obvious increase of fog day and cloud cover based on the increase of aerosol caused by the development of the cities.So the main cause of sunshine duration over east of Guanzhong in Shaanxi Province is not the indirect effect of aerosol,but most probably direct radiation.That means in the special terrain and prevailing wind,anthropogenic aerosol caused by the development of cities in Guanzhong region of Shaanxi Province accumulated in east part of Guanzhong,thus there is a high aerosol concentration layer below 2 km above the surface layer.The existence of high concentration aerosol layer could reduce the amount of solar radiation reaching the earth surface and decrease the sunshine duration.The increasing sunshine duration from 2002 to 2008 probably has some relation with anti-pollution measures and variation of prevailing wind,which need further study.