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

The emission of dust particles 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 *t influences the vertical flux and dust transport. Based on the micrometeorological data obtained 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 concentration 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 200μ/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 Loess Plateau depends on the wind direction, due to the complex terrain and inhomogeneous 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.