西北地区沙尘天气的时空特征及影响因素分析

利用1960—2005年西北地区135个台站的沙尘日数、月降水和气温资料,以及500 hPa高度场和太平洋海表温度资料,通过常规统计和诊断方法,分析了沙尘天气的时空特征及其影响因素。结果表明,除北疆北部外,西北地区大部分地方年平均沙尘日数在5 d以上,沙尘日数25 d以上的地方主要在南疆、青海西部、甘肃河西及中部地区以及宁夏、内蒙古中西部和陕北一带,而50 d以上沙尘高频区主要集中在塔克拉玛干沙漠周边地区以及巴丹吉林沙漠、腾格里沙漠和毛乌素沙地一带。沙尘天气在春季发生最频繁,而很少发生在秋季。近46 a年沙尘日数总体上呈显著下降趋势,在沙尘天气高发区的下降趋势更为明显。20世纪70年代中期开始,沙尘天气发生频数由较多期跃变为一个相对较少期。当北太平洋地区位势高度偏低,青藏高原至新疆500 hPa高压脊异常偏强,亚洲大陆中高纬高度场较常年偏高时,西北地区沙尘日数偏少,反之亦然。当春季太平洋海温表现为厄尔尼诺(拉尼娜)典型态时,同期沙尘日数偏少（多）。

Spatial-temporal Characteristics of Sand-Dust Events and Influencing Factors in Northwest China

On the basis of the statistics and diagnosis methods, the sand-dust weather of northwest China is studied by the use of the monthly precipitation and temperature data, and sand-dust occurrence frequency data from 1960 to 2005 recorded by 135 stations. The 500 hPa height field data and the sea surface temperature data are from NCEP/NCAR and NOAA, respectively. Results show that the characteristics of sand-dust events in the northwest China are: the numbers of sand-dust occurrence days in the most part of northwest China is more than 5 days. The high frequency areas with sand-dust occurrence days more than 50 days are mainly in the deserts and their surroundings. This shows that the characteristics of underlying surface and the distribution of sand source play an important role in forming sand-dust events. The events are frequent in spring and scarce in autumn. The occurrence days of sand-dust in the last 4 decades appeared a generally decrease trend which was more significant in the high frequency areas. There was a transform from a relative frequent period to a relative scarce period in the middle 1970s, and this decreasing tendency was very remarkable in recent 30 years. There is negative correlation between the numbers of sand-dust days and the precipitation in the most part of northwest China, the higher correlation coefficient areas are mainly located in south Xinjiang, Dunhuang to Qaidam Basin, and west of Inner Mongolia. The correlation coefficient is negative between the sand dust occurrence days and the temperature, except for the south of Qinghai province. The negative anomaly of sand dust days in northwest China companied with the negative anomaly of geopotential height in the north Pacific, the abnormal strong 500 hPa ridge of high pressure, and the abnormal high geopotential height field in the extratropical latitude of Asia, and vice versa. The typical EL Nino (La Nina) style of the sea surface temperature in spring of Pacific consist with the negative (positive) anomaly of sand dust days in the corresponding period.