中国西北地区云的分布及其变化趋势

利用1983年7月—2001年9月ISCCP D2云的月平均资料,针对西北地区15种不同类型云的分布特征进行了分析,给出了中、低云量之和以及高云量在3个气候子区的多年变化趋势,初步探讨了其形成机制。结果表明:水层云、冰层云、水雨层云、冰雨层云和深对流云的光学厚度和云水路径值最大;水层云主要出现在天山山区、北疆地区和陕西南部,冰层云主要出现在北疆地区,水雨层云、冰雨层云和深对流云以及水高层云、冰高层云、卷层云的云量高值区在天山—昆仑山—祁连山一带以及陕南和/或陇南地区,因此上述地区也是有利于人工增水作业的地区。近20年中,高云量在3个气候区都呈明显下降趋势,中、低云量之和则呈上升趋势。西北地区云与地气系统之间可能存在这样一个过程:地面气温的升高,促使地面蒸发加剧,从而导致中、低云量增多而使降水增多,同时高云云量减少。

Distribution and Variation Trend of Cloud over Northwestern China

Using the monthly mean data of ISCCP D2 and NCEP/NCAR from July 1983 to September 2001,the distribution of 15 different cloud types in three climatic subregions in Northwestern China were analyzed.Also the trend of cloud amount and their possible formation process were discussed.The results show that the optical thickness and water path of liquid stratus,ice stratus,liquid nimbostratus,ice nimbostratus and deep convective clouds are the highest.Liquid stratus mainly occurs over Tianshan Mountains,Northern Xinjiang and Southern Shanxi and ice stratus mostly occurs over Northern Xinjiang.Liquid nimbostratus,ice nimbostratus and deep convective cloud,and also liquid altostratus,ice altostratus and cirrostratus,with values of optical thickness and water path at the second highest level,have a similar distribution.Their high value areas are over Tianshan Mountains,Kunlun Mountains,Qilian Mountains,Southern Shanxi and Southern Gansu.Therefore,those regions have advantage for artificially enhanced precipitation.During the nearly 20 year periods,the trend of the high-level cloud amount significantly decreased while low-middle-level cloud amounts increased in all of the three climatic regions.Such a process probably exists between the cloud and earth-atmospheric system in northwestern China that the increase of surface temperature enhances evaporation,thus causing the increase of the low-middle-level cloud amounts and leading to precipitation to increase and the high-level cloud amount to decrease.