中国西北不同沙源地区起沙阈值的对比分析与研究

An estimation of the threshold friction velocities over the three different dust storm source areas in northwest China

沙尘粒子进入大气所受到的空气动力因子和阴力因子分别用摩擦速度u,和临界摩擦速度u(?)来定量表示。临界摩擦速度是影响沙尘释放和输送的重要参数。文中利用2004年春季浑善达克沙地和2006年春季黄土高原与戈壁3种不同沙源地区开展的微气象学和沙尘暴观测资料,分析了沙尘天气过程中沙尘浓度随摩擦速度的变化关系,估算了临界起沙摩擦速度u(?),并与不同学者给出的结果进行对比。结果表明:沙尘天气过程中,临近起沙阶段,沙尘浓度较低,摩擦速度增加快,为起沙阶段提供了利的动力条件;起沙阶段,动力和热力湍流混合使得沙尘浓度迅速升高;沙尘天气减弱阶段,由于重力沉隆作用,沙尘浓度随摩擦速度近似呈线性递减关系。以空气中的沙尘浓度大于200μg/m^3作为起沙的判据条件,得到浑善达克沙地和戈壁地区的临界起沙摩擦速度分别为0.6和0.45 m/s。由于受地形及非均匀下垫面的影响,黄土高原的临界起沙摩擦速度与风向有关。西北风反映了来自西北方向毛乌素沙地的影响,u(?)为0.35 m/s,偏南风时对应观测站点南部黄土丘陵沟壑区的起沙特征,u(?)为0.7 m/s。黄土丘陵沟壑区的地表粗糙度值和土壤粒子间的相互作用力较大,从而导致了较大的临界起沙摩擦速度。不同的起沙阈值条件和风速频率分布与沙尘释放能力的不同相对应。

The emission of dust panicles into the atmosphere is governed by the aerodynamic and resistant factors,which are quantified by the friction velocity u.and the threshold friction velocity u_（-t）,respectively.The threshold friction velocity u_（？） influences the vertical flux and dust transport.Based on the micrometeorological data ohtained in the spring seasons of 2004 and 2006 over the Hunshandake desert area,the Loess Plateau and the Gobi area,the relationship between dust concentration and friction velocity for the two dust events occurred over the Hunshandake desert area was investigated,and the threshold friction velocities over the three different dust source areas were estimated and compared with other authors＇ results.The results show that during the pre-emission stage of a dust storm event,although the value of dust coneentration was low,the rapid increase of friction velocity provides favorable dynamic conditions for dust emission.During the dust emission stage,the dust concentration rose sharply due to the mechanical and thermal turbulent mixing.At the calm-down stage,the dust concentration dropped nearly linearly with the decreasing friction velocity,on account of the gravitational deposition of larger dust particles.The dust concentration higher than 200μg/m^3 is considered as the criterion for a dust emission process to occur.According to this criterion, the values of threshold friction velocity over the Hunshandake desert area and the Gobi region are 0.6 and 0.45 m/s.respectively. The threshold friction velocity over the Locss Plateau depends on the wind direction,due to the complex terrain and in（？） mogeneous surface.The northwest wind represents the effects of the Mu Us desert in the northwest with the corresponding threshold friction velocity of 0.35 m/s.The south wind reflects the characteristics of the Loess hilly dunes in the area south of the monitoring site with the threshold friction velocity is 0.7 m/s.The large roughness length of the Loess hilly dunes and the large inter-particle cohesion for the clay soil texture increases the local friction velocity.Different threshold friction velocities and occurrence frequencies of strong wind account for the different dust emission capabilities for source areas.