Numerical Simulation Of Boundary Layer Height And Its Effect On Dust Weather In Arid And Semi-Arid Regions Of Northwest China

The atmospheric boundary layer is an important medium for the exchange of material and energy between the surface and the atmosphere.The formation of some catastrophic weather phenomena is mostly closely related to the boundary layer process.As a kind of catastrophic weather closely related to the boundary layer,dust weather has a great influence on human life and production and ecological environment.The arid and semi-arid regions of northwest China have become one of the most frequent dust weather areas in China due to its special geographical location and climatic conditions,and have unique atmospheric boundary layer characteristics.It is of great significance to study the characteristics of boundary layer height and its influence on dust weather in order to explore the formation mechanism of dust weather,early warning,disaster prevention and mitigation.Firstly,this paper studies the spatial and temporal distribution characteristics of the boundary layer and the optimal combination of the parametric scheme for the deep boundary layer simulation in the arid and semi-arid region of northwest China by using ERA-Interim reanalysis data and WRF model.Secondly,the spatial and temporal distribution of dust weather in 5 representative areas and the characteristics of boundary layer height change under dust weather conditions are analyzed.Then the causes and effects of a typical dust weather in northwest China are analyzed by using a variety of meteorological data.Finally,use WRF-Chem model to simulate the dust transmission process,boundary layer height characteristics of the typical dust weather,as well as the influence of surface heat flux on the boundary layer height and the accompanying change of the vertical distribution of dust,the main conclusions are as follows:(1)the boundary layer height in 5 representative areas is higher in late spring and summer(the highest is 2,485~3,502m in June)and lower in autumn,winter and early spring(the lowest is758~907m in December).The seasonal variations of the boundary layer height are smaller at 02:00BJT and 08:00 BJT,while the variations are relatively larger at 14:00BJT and 20:00 BJT.The atmospheric boundary layer with heights more than 4,000m generally exists in late spring and summer.The boundary layer heights is higher in the arid region than in the semi-arid region as well as the deep-developed boundary layer lasts longer in the arid area.However,at 08:00 BJT and 20:00 BJT the boundary layer heights in Minqin and Yuzhong are relatively higher in summer,and there is little difference between areas in winter.Deep atmospheric boundary layer height persists for a long time during daytime in arid areas in summer,the average duration of atmospheric boundary layer over 2000 m in Dunhuang from May to August in extreme arid area is more than 6 hours,the higher atmospheric boundary layer lasted for more than 6 hours only in June to July in Jiuquan and Minqin.(2)The numerical simulation results show that there is a significant difference between different combinations of parameterization schemes to simulate the deep-developed boundary layer in these areas.Among the three types of parameterization schemes that are relatively closely related to the development of the boundary layer,the selection of short-wave radiation scheme,land surface process scheme and boundary layer scheme has different influence on the numerical simulation results of boundary layer height.The influence of the simulation results on the deep boundary layer in Dunhuang,Minqin,Yuzhong and Lanzhou from large to small,which is the boundary layer scheme,the land surface process scheme and the short-wave radiation scheme,while Jiuquan is the boundary layer scheme,the short-wave radiation scheme and the land surface process scheme.The combination Goddard+SLAB+ACM2 performs better in the extreme arid area Dunhuang,and the arid area Jiuquan and Minqin,while the simulation effect of combination Dudhia+Noah+ACM2 is better in the semi-arid areas,Yuzhong and Lanzhou.The difference of simulation results is related to the method of determining the height of the boundary layer by the boundary layer scheme,the consideration of the nature of the underlying surface by the land surface process scheme,and the treatment of the absorption of short-wave radiation by the absorbing material in the short-wave radiation scheme.(3)The interannual distribution of dust weather in Dunhuang,Jiuquan and Minqin shows a decreasing trend,Yuzhong and Lanzhou shows fluctuating changes;Dunhuang has the most dust weather,with an average of 34 times per year.Dunhuang and Jiuquan are prone to floating dust and blowing sand,Minqin is prone to blowing sand and sandstorm,Yuzhong and Lanzhou are prone to floating dust.Dust weather mainly occurs in February to May in all areas.The number of dust weather in spring accounts for more than 50%of the total number of dust weather in the whole year,and in autumn it is less than 10%.In terms of the distribution characteristics of each time,there is little difference in floating dust,sand blowing and dust storm are mainly concentrated in the afternoon and evening.At night,the boundary layer height during dust weather is significantly higher than that at sunny days in Dunhuang and Minqin,and the higher the intensity of dust weather,the higher the boundary layer height.During the daytime,the boundary layer height of strong dust weather is higher than the boundary layer height of floating dust.During the typical dust cases(January 24-26,2017),the upper-level trough moves eastward and develops over the Northwest China to lead a cold air moving southward,the ground high pressure system and the front moving eastward.Strong wind near surface is forced by strong pressure gradient behind the cold front.The simulation results of HYSPLIT model shows that the dust source is located in west of Gansu and dust transmits through Hexi corridor to the southeast of Gansu province along the northwest path.Before the dust weather,temperature rises in the source resulting in the decrease of the soil moisture and the instability of the lower atmosphere,which is conducive to dust emission.During the dust weather,the horizontal wind speed at all levels are relatively large,which is favorable to the dust horizontal transportation.In vertical direction,the upper and lower layers of the dust source and the enhancement zone such as Minqin has obvious ascending movement,which is advantageous to bring the surface dust particles to the upper air.Other representative areas in the transmission path has shown descending movement,which brings the upper dust in the path to the local surface to form dust weather.During the dust weather,the daytime boundary layer height in dust source area and Minqin increased,while the daytime boundary layer height in Jiuquan and Zhangye decreased,and the night boundary layer height in Lanzhou increased.(4)Dust source and Minqin have large dust emission flux and 10 m wind speed during the dust weather.The dust emission flux of the central region in the dust source is more than 3000?g/m~2/s,and the wind speed in the area is up to 20 m/s.In the control experiment,the high dust concentration in dust source area can reach more than 1000 m above the ground,and the concentration reaches 24000?g/m~3 or more,and the strong ascending motion can transmit dust to more than 3000 m above the ground.The high value area of dust concentration in Minqin is always maintained above 1000 m above the ground.In the dust emission stage,the boundary layer height in the area with high dust concentration is higher than that in the surrounding area,and higher than the average boundary layer height of the corresponding moment in the place.Sensitivity experiment shows that the dust area range decreases significantly after closing surface heat flux,the ground dust concentration is greatly reduced,the maximum concentration difference between the two can be more than 4 times.The boundary layer height near the high dust concentration value area in sensitivity experiment is obviously lower than that in control experiment,however,when the dust concentration increases to a certain extent,the boundary layer height difference between the two decreases.The difference of boundary layer height between the high dust concentration area and its surrounding area is relatively small and the distribution of boundary layer height is relatively uniform in the sensitivity experiment.In the vertical direction,the high dust concentration value area in dust source always keeps below 400 m above the ground during the dust emission stage in sensitivity experiment.The high dust concentration center in control experiment is about 500 m higher than that in sensitivity experiment in Minqin.The boundary layer height has a significant impact on the vertical distribution of dust,The higher the boundary layer height is,the more conducive to the upward transportation of dust,and dust concentration can also has an impact on the boundary layer height.