Observational Study And LES Modeling On Structure Of Dust Devils In The Taklimakan Desert

Dust devils, vertical convective vortices found in the convective boundary layer, are caused by heating of near-surface air by insolation. In a certain angular momentum, a rotating upward convection vortex is formed with the help of the thermal convection,which is formed under the action of buoyancy. It is visible for carrying a large number of dust particles to the upper air during rotation. But the understanding of dust devil and its dust transport capacity is quite limited at present. We observed dust devils in the Taklimakan Desert, China July 2014. By analyzing the observation results, simulating dust devils using WRF-LES, assessing the contribution of dust devils to the global dust emission, this study presents the following unique and important results and interpretations:1.Formation environments and structure of dust devils in the Taklimakan DesertDust devils appear during 11 to 17 PM in the summer. The ambient wind speed is in the range of 2.5m s-1～3.2m s-1, and surface and near-surface 2m air temperature is about 15℃ when observing dust devils in the Xiaotang region of the Taklimakan Desert. The structure of dust devils is determined by the observation of dust devils’ opacity in the Taklimakan Desert. The opacity values gradually decrease with height, the difference of the values in the central position is smaller than both sides, and the lapse rate is also different at different altitudes; The horizontal opacity values increase first and then decrease from the center part to both sides at the base of the dust devil, implying that the dust devil is quasi-symmetric, and the values in the east side is greater than the west side, indicating that the dust devil is not absolutely symmetrical; A new method to convert 2D structure of opacity to 3D is developed, the results retain almost all the basic features and the inhomogeneity of the dust devil.2. Large Eddy Simulation of dust devilsThe horizontal wind speed of dust devil increases first and then decreases from the center to both sides, and it is also increases first and decreases with height; Vertical wind speed decreases with the increase of height, sinking air exists in the center of the vertical direction; The temperature is highest in the center of the dust devil, decreasing gradually from the center to both sides, and the temperature difference between the central area and the surrounding environment is about 15-3K, it decreases with height in the vertical direction; The air pressure in the central part of the dust devil is low, and the air pressure difference between the central area and the surrounding environment is about 1.5-5Pa, in the vertical direction it increases gradually with height; There exists a transition zone where wind velocity, temperature and pressure are not changed with the height in the vertical direction; Dust devils can be judged by temperature difference and pressure difference; The successful simulation by setting mean wind to zero demonstrates that mean wind is not the main factor of the formation of dust devil.3.Evaluation of the contribution of dust devils to global dust emissionThe value of dust devil’s dust flux is calculated by the opacity of dust devils observed in the Taklimakan Desert. The calculation results through the empirical formula show that the range of dust devil’s dust flux is 1.6×10-4～7.0×10-4 kgm-2s-1, and the average value is 3.0×10-4 kg m-2s-1; And based on dust devil’s dust flux calculated before, we provide a best estimate of the contribution of dust devils to global dust emission of 10.7%±7.9%, indicating that the dust devil has a certain contribution to the global dust emission,and it is an indispensable source of global dust emission.