关键词: 尘卷风, 热对流, 分层现象, 电场, 荷质比

Abstract: Dust devils are very common meteorological phenomena on the Earth as well as on Mars. They are an abbreviated wind-sand conveyance system. The moving particles in dust devils may become electrically charged，to the point of arcing to spacesuit or vehicle，and creating electromagnetic interference. In this paper，taking the effect of thermal flux from the surface to the atmospheric boundary layer into consideration，a dynamic model is suggested to simulate a dust devil and to obtain its fine structure with large eddy simulation （LES）. And this model not only reveals the formation mechanism of dust devil，but also can realize its process of occurrence，development. Then，in the dust devil the large amount of sands collide and chafe each other，it causes a part of sands charged positive or negative，and the charged sands will produce electric field in dust devil. So based on Coulomb’s law，the electric field model is established to simulate the electric field and its distribution in dust devils. The numerical results show that the formation mechanisms of dust devil can be explained with theory of thermal convection，and the hemispheric isothermal distribution is formed at the beginning of the simulation and isotherms deliver upward step by step. Then isotherms declutch at a height，and the high temperature delivers further upward. This indicates that high temperature of the local surface heats the lower atmosphere and the heated atmosphere will transfer upward heat. This process of heat diffusion causes unstable atmosphere state and generates intense turbulence and convective plumes，air converging into these plumes tends to conserve any initial angular momentum and a spinning vortex develops. In dust devil，when the wind speed exceeds the threshold velocity and sand grains begin to be lifted from bed with a random lift-off velocity. With more and more sand grains with different diameters being picked up into rotational wind field，the number of grains in air becomes to attain a dynamic constant. As sand grains with different diameters move in a dust devil，stratification will occur: smaller sand grains are usually above bigger sand grains. In dust devils，sand grains will have charge via frictional or triboelectric processes，and sand grains with different diameters may have different charges. Usually small sand grains have negative charge and big sand grains have positive charge. When the percent of charged sands is about 23.4% and charge-to-mass is about，the stable electric field in dust devil is in a good agreement with the observation values. And at the beginning of the evolution of a dust devil the electric field strengthens with time，but after 60 s the electric field changes little，which indicates that electric field of dust devil has trended to stabilization. The electric field is symmetric about dust devil center，and the electric field has a maximum value  at a certain height，then it weakens rapidly outward to be zero. In the meantime，the electric field increases to maximum and then decreases with the increment of height. In the interior of a dust devil the electric field increases to maximum and then decreases as height increases and the electric field attains maximum at about 6 m height. Outside the dust devil the variational trend of electric field is similar to that of inside the dust devil，but there does not exist a certain height where the electric field attains maximum. This research will improve the dust devil - electric field prediction model，and in future it plays an important role in studying the electromagnetic interference and damage of large dust devil on Mars. At the same time，the research also has the positive significance to study the internal structure and electromagnetic field of large tornado on the earth.

Key words: dust devil, thermal convection, stratification, electric field, charge-to-mass