Modeling The Characteristics And Mechanism Of Low-Level Jets And Their Effects On Dust Activity In Taklimakan Desert

The Low-level jets(LLJs)widely exist over the Taklimakan Desert(TD),affecting weather phenomena such as precipitation,pollutant dispersion,flight safety,wind power equipment,and human activities in the Tarim Basin(TB).TD is the most significant area of the contribution to dust aerosols in China,and the dust aerosols have triggered huge environmental impacts for the whole East Asia under the combined effects of topography,thermal conditions,and background circulation,including reduced atmospheric visibility and the outbreak of human respiratory disease.Previous studies on LLJs in the TD mainly focused on the climatic characteristics of the LLJs in different seasons,such as wind speed and height.However,these studies lacked discussions on the characteristics of diurnal variations and the formation mechanisms,as well as the effect of LLJs on dust activity,which has not received sufficient attention.Based on previous studies,this paper systematically studys the diurnal variation characteristics and formation mechanisms of LLJs in the Taklimakan region in different seasons,explores the simulation effect of WRF model on LLJs with different configurations,clarifys the daily variation mechanism of LLJs and its influence on dust activity,and quantifys the contribution of the LLJs and frontal cyclone activity to dust emission in the Taklimakan region in summer,using a combination of multi-source remote sensing datasets,various ground sites observation datasets and reanalysis datasets,as well as the mesoscale Weather Research and Forecasting(WRF)/Chemical model(WRF-Chem).The main conclusions are as follows:(1)The characteristics of diurnal variation and the formation mechanisms of LLJs over the TD in different seasons are revealed.LLJs over the TD only exhibit significant periodicity on a diurnal time scale,and the diurnal trends of wind speed and height of strong and weak LLJs are similar in different seasons,but there are differences in the magnitude of wind speed,which are stronger in spring and summer than in autumn and winter.Strong LLJs mainly exist in the hinterland of TD,with a clear east-northeast dominant wind direction,while weak jets mainly exist in the edge of TD,with a discrete wind direction.The formation of LLJs over the Taklimakan region is the result of four mechanisms:(i)the oblique topographic compressibility generated by the plateau-basin,(ii)the downhill flow formed cross the Kuruqtag Mountains,(iii)the thermal difference between the desert and the mountains to the east of TB,and(iv)the diurnal variation of the turbulent diffusion coefficient.(2)The effects of different configurations on the simulation of LLJs and nearsurface wind in the Taklimakan region with WRF model are evaluated.Results show that the use of the National Centers for Environmental Prediction Final(NCEP-FNL)reanalysis data as the initial field and boundary conditions for WRF dynamic downscaling can yield the best results for studying the vertical structure of LLJs and near-surface wind after sunrise in the TD.A horizontal resolution of a few dozen kilometers and a default vertical grid are sufficient to capture the main features of the LLJs over Taklimakan,but a finer horizontal resolution and vertical resolution encryption to 20 layers below 1000 m from the ground can yield more accurate near-surface wind speeds when simulating the effects of LLJs on near-surface wind speeds after sunrise.The Yonsei University(YSU)non-local boundary layer parameterization scheme is found to be better than the Mellor-YamadaNakanishi-Niino(MYNN)local boundary layer parameterization scheme for LLJs simulations in the Taklimakan region only.Through topography sensitivity experiments and surface heat flux sensitivity experiments,it was found that the diurnal variation of turbulent viscosity in the boundary layer contributes more to the formation of LLJs than the role of topography.(3)The mechanism of the diurnal variation of Nocturnal Low-Level Jets(NLLJs)over the TD and its influence on dust emission and transport are clarified.By using the WRF-Chem model,the formation and diural variation of NLLJs in the Taklimakan region are successfully simulated.The study finds that the downward transfer of NLLJs momentum to the surface through turbulent mixing after sunrise is the direct cause of dust emission over TD in the early morning.Moreover,the wind speed variation of NLLJs is mainly induced by the inertial oscillation of non-geostrophic wind.The diurnal variation of turbulence viscosity largely accounts for the daily variation of NLLJs.The dust aerosols suspended in the residual layer during the night can be transported to the Pamir Plateau in the west by the NLLJs directly or to a wider downstream area by the westerly wind in the upper air.(4)The contribution of LLJs and frontal cyclone activity to the dust emission over the TD are quantified.There are two peaks of dust emission in the diurnal variation in the TD,one appears after sunrise and the other occurs in the afternoon.The dust emission occurring in the morning contribute 41% of the total dust emission in the Taklimakan region.About 38% of the dust emission in the Taklimakan are associated with LLJs,while only about 2% of the dust emission come from cyclone-related frontal activity.The remaining approximately 60% of the dust emissions occur mainly in the afternoon,with a high probability of originating from undefined processes such as dry and wet convection and mountain-plain wind circulation within the boundary layer.