影响中国西北及青藏高原沙尘天气变化的因子分析

利用1948—2006年的NCEP(2.5°×2.5°)月平均再分析资料,分析了影响西北及青藏高原沙尘天气变化的动力、热力因子。结果表明:①200 hPa副热带西风急流是影响沙尘天气的动力因子,高层天气系统的季节性变化,导致了其位置及强弱的季节性变化,从而导致了高原南部扬沙、沙尘暴季节性南北移动;急流的动力结构使局地环流得以形成,局地环流的下沉支流使得高空动量下传,使地面风速增大,从而使扬沙和沙尘暴发生。②浮尘和扬沙、沙尘暴天气成因有所不同,地表温度等热力因素对浮尘天气有直接影响;而急流等动力因子则影响浮尘天气的频率,对发生范围影响较小;动力因子是扬沙、沙尘暴发生的直接原因;500 hPa锋生函数大值带的季节性南北移动也是扬沙、沙尘暴南北季节性变化的重要原因。③500 hPa水汽输送带的边缘是扬沙和沙尘暴容易发生的区域。④地表湿度是沙尘天气发生的一个因子,当地表较干时,沙尘天气发生频率增加,而当地表湿度增大时,沙尘天气发生的频率减小。

Impacting Factors of Sand-dust Weather Variation over Northwest China and Tibetan Plateau

The dynamical and thermodynamic factors affecting the variation of the sand-dust weather are analyzed over Northwest China and Tibetan Plateau by using the NCEP(2.5°×2.5°) long-term mean data. The results show: ①The 200 hPa westerly jet stream is the dynamical reason for the change of sand-dust weather. The seasonal changes of position and intensity of upper jet stream are caused by the weather systems at high level. This is just the reason why the sand-dust storms extend to the north and the east from the southern part of Tibetan Plateau. The regional circumfluence is formed by the dynamical structure of upper jet stream, and the down airflow of the circumfluence induces the down-transmission of high momentum, which causes the aggrandizement of wind on surface and the occurrence of sand-dust storms. ②The forming causes are different among the floating dust and the sand-dust storm. The thermodynamic reasons such as surface temperature has immediate influence on the occurrence of floating dust. The dynamical factors such as the upper jet stream affects the intensity of floating dust, but it has feeble influence on the range of the occurrence, whereas it is the immediate reason for the happening of sand-dust storm. The south to north transfer of the 500 hPa frontal belt is also an important reason for the variation of sand-dust storm. ③The edge of the 500 hPa vapour transporting belt is the incidental region for sand-dust storm. When the low level vapour increases within a certain limitation, it can induce the increment of potential instability, which could make the occurring of convection and the sand-dust storm. When the vapour increases to a certain degree, it restrains the occurrence of the sand-dust storm. ④The surface moisture is a factor aggrandizing the happening of sand-dust weather. When the soil was drier, the frequency of the sand-dust weather increases; the soil becomes wetter, the frequency decreases.