Abstract
Dust devil in convective boundary layer (CBL) was simulated by Euler-Lagrange approach. By means of large-eddy simulation method and smoothly stretched grid, flow fields of the small scale whirlwind were obtained. Movements of 20,000 dust particles were tracked by computing external forces and velocities with the aid of the simulated high-resolution air flow fields. Characteristics of the simulated airflow were in good accordance with field observations. Distribution of particles reproduced the shape of dust devil. Statistics of particle trajectories revealed basic properties of the movement of dust particles. Small particles with diameter of 0.04 mm were able to form a rolling cylinder and to be lifted easily to a certain height. 0.1 mm dust particles spiraled upwards like a cone with a small cone angle. Particles with diameters of 0.16 mm and 0.3 mm were obviously thrown outward with limited lifting height and fell back to the ground. The negative vertical pressure gradient in the dust devil strengthened particle lifting, unlike in the horizontal wind systems where the vertical pressure gradient isn’t the major driving force. Numerical results showed why total suspended particulates (TSP) concentration exceeded the standard value greatly all the year round on the Loess Plateau, where the loess dust from the local ground was one of the major sources of air pollutants. 90% loess dusts were smaller than 0.04 mm in diameter, which made them easily lifted to a high altitude by dust devils even without obvious horizontal wind. Because thermal plumes are common in CBL, dust devils can occur frequently in multi-locations of the Loess Plateau. According to nature circumstances of the Loess Plateau and thermodynamic characteristics of dust devil, the dust-devil-scale simulation indicated one source of the background TSP on the Loess Plateau and how they were lifted into the atmosphere.
Highlights
Because loess dusts are small and easy to be lifted and dust devils can occur frequently in convective boundary layer (CBL), especially in direct sunlight, dust devil plays an important role in the formation of the background PM10 on the Loess Plateau
Dust devil in the convective boundary layer is simulated by Euler-Lagrange approach
Movements of 20,000 dust particles are tracked by computing external forces and velocities with the aid of simulated high-resolution air flow fields
Summary
It is observed that temperature raises 4-8 K, and pressure drops 2.5-4.5 hPa in the core of the dust devil as compared with ambient air (Ives, 1947; Sinclair, 1973; Hess and Spillane, 1990; Xu, 2002) Both vertical and tangential velocities can reach 15 m/s. Zhao et al (2004) and Gu et al (2006) provided detailed structure of dust devils and the basic characteristics of dust movements in dust devils by large eddy simulation (LES) approach Their results showed the way how dust can be lifted by dust devils without the presence of strong mean horizontal wind. Where d is the particle diameter, VR the slip velocity between the two phases, and Reynolds number Re=| VR | d /ν
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