4 ) the number and mass concentration distributions of the background atmospheric sand particles of ymchuan , bayanhot and tonggunur display a unimodal form , obeying a logarithmic normal distribution , greatly differing f 黄沙的长距离输送和小雨是泥雨形成的原因。云物理和降水与来自沙尘粒子混合，尤其是小于zpm的粒子，导致了华北地区泥雨的发生。
2.
To investigate the space / time distributions and occurrence and evolution of such events , the diffusion pattern over deserts , turbulent transfer features in sandstorm weather , the particle size distribution , mass concentration and its distribution , optic properties , chemical composition and physical factors responsible for the initiation of raising sands , we made integrative observation and sounding of sandstorms deep in the large - scale desert area , including tengri , badanjilin and maowusu , with the items consisting of micrometeorological measurement , 3d wind observation , data from kb - 120 and anderson samplers of mass concentrations of sands with their spectrum , the distribution of aerodynamic particle sizes from the aps3310a , retrieval of aerosols " optic depth from sunphotometer data , assay of the chemical composition by means of neutron activiation analysis ( naa ) and integrated study of all related factors for causing sandstorm to occur , based on the observations of all kinds 利用所取资料，系统分析了不同强度沙尘天气条件下沙尘气溶胶质量浓度和质量浓度谱、粒子谱分布、光学厚度、化学组分等特征；综合分析了影响沙尘起动的诸物理因子在沙尘起动中的作用；用沙尘输送模式对一次沙尘暴天气造成的泥雨过程的形成机制进行了模拟。历史气象资料统计分析表明，沙尘暴有其高发期( 4 、 5月)和高发时段( 14 - 20时) ， 14时到20时之间发生的沙尘暴约占沙尘暴总次数的66 。扬沙和沙尘暴天气条件下，湍流动量通量和湍流感热通量都是重要的湍流交换，沙尘暴发生前近地层的超绝热不稳定对沙尘暴天气有加强作用。
