| The vertical transport of various physical quantities caused by atmospheric turbulence in the boundary layer plays an important role in the atmospheric process.It is important to study the turbulence characteristics of the boundary layer to analyze the pollutant diffusion conditions,to study the transport mechanism of land and gas,and to improve the simulation capability.The semi-arid region of the Loess Plateau is located in a transitional zone between semi-humid and arid climates and is sensitive to climate change.Studying the characteristics of earth-atmosphere exchange in the semi-arid region of the Loess Plateau has important scientific significance for studying the response of ecosystems to climate change and revealing the characteristics of regional dry-up.However,the underlying surface of the Loess Plateau semi-arid area is uneven and the boundary layer features are complex,which restricts people's understanding of surface processes and atmospheric interactions in this area.To this end,the boundary layer characteristics of the area must be studied in depth to properly understand the material and energy transport between the earth and atmosphere.At present,the study of atmospheric boundary layer turbulence is mainly based on observational data and mesoscale numerical simulation.Although significant progress has been made,there are still many problems.For example,the spatial resolution of turbulent data in the Loess Plateau is low,and it is difficult to describe the fine spatial structure of turbulence.The research on the basic structures such as wind speed,temperature and boundary layer height in the atmospheric boundary layer is mostly limited to a single site,and the spatial distribution characteristics of the atmospheric boundary layer need further research.Atmospheric boundary layer simulation experiments with high spatial and temporal resolution are carried out using the WRF Large-Eddy Simulation(WRF-LES).In the two sets of simulationexperiments,by analyzing the temperature profile,water vapor mixing ratio profile and wind component profile with or without sand dust,the spatial distribution of atmospheric boundary layer structure and meteorological field in the Loess Plateau is analyzed.Constrained by the fact that flux observation data is difficult to obtain,the observation time is short and the spatial resolution is low in many areas.To gain an insight into the structure and characteristics of atmospheric boundary layer,the characteristics of turbulent flow driven by heat under the condition of summer temperature and humidity profile of the Loess Plateau are simulated by numerical model WRF Large-Eddy Simulation(WRF-LES).The main results are as follows:(1)The sounding data of the Yuzhong station showes that in the sunny day,temperature near the ground increased from 312.0 K to 316.0 K;the water vapor mixing ratio decreased rapidly with the increase of the height.And rapidly reduced from 2.6 g/kg to 1.9 g/kg at a height of 100 m.Then,with increasing altitude,the water vapor mixing ratio showed a downward trend,gradually decreasing from 1.9g/kg to 1.0 g/kg.In the case of dust,the potential temperature is basically maintained at 297.0 K below 2000 m,and then gradually increases to 312.0 K.The water vapor mixing ratio in the near-surface layer also decreases rapidly with height,from 3.0 g/kg to 2.6 g/kg.From 100 m to 2500 m,the water vapor mixing ratio does not vary with height and is maintained at around 2.7 g/kg.The potential temperature during dust weather is about 15.0 K lower than that of sunny days,and the difference between the two decreases with height.(2)The temperature from WRF-LES is 312.9~314.4 K,and the boundary layer mixing is not strong within 100 m from the ground.The horizontal distribution of potential temperature is basically consistent and potential temperature gradient is large at the surface.In contrast,the high-level mixing effect is stronger and the distribution of potential temperature is more uniform.When there is dust,the temperature is 299.0~300.8 K,which is 13.9 K lower than the sunny days.The low-level temperature field is centered at the northeast corner of the simulation domain and has an irregular circular staggered structure.Compared with the dust-free case,the distribution of potential temperature and wind field change faster with height.(3)Vertical distribution profile of north-south direction potential temperature simulated by WRF-LES inferred that,in the clear weather,the vertical profile of thepotential temperature in the southern part of the simulated area is 312.9~314.0 K.The near-ground potential temperature is the highest and the distribution is the most uneven.It is characterized by alternating distribution of cold and warm water along the east-west direction,and the horizontal temperature gradient is large.The vertical profile of the temperature in the middle of the simulation area is 313.1~314.0 K.The vertical development height is much larger than the vertical section of the north and south boundary of the simulation area.The high temperature air extends continuously from the ground to 1500 m.In the dusty weather,the vertical temperature profile of the southern,central and northern parts of the simulated area is 298.9~300.3 K,and the high temperature center can develop upward to about 1200 m from the ground.The spatial distribution of the profiles at different locations with sand dust is more similar,indicating that the vertical development intensity of different locations in the simulated area is more balanced when there is dust.(4)The dust-free situation simulated by WRF-LES shows that the water vapor mixing ratio is in the range of 2.0~2.8 g/kg at a height of 10 m from the ground.The horizontal distribution presents an irregular network structure,and the water vapor mixing ratio in the eastern part of the simulation domain is large.At a height of 100 m,the water vapor mixing ratio is in the range of 1.8 to 2.4 g/kg,and the high-humidity region with a network distribution in the ground gradually decreases with height,and the network structure is no longer obvious.Most of the area in the simulation zone is a uniformly distributed low-humidity area with a water vapor mixing ratio of 1.8 g/kg.At a height of 1000 m,the water vapor mixing ratio is in the range of 1.9 to 2.1 g/kg.At this height,the water vapor mixing ratio distribution is more uniform and the turbulent scale is reduced.When there is dust,the mixing ratio of water vapor at each height increases significantly.At a height of 10 m,the mixing ratio of water vapor is3.0~3.3 g/kg,which is about 1.0 g/kg higher than that without sand.At a height of100~2000 m,the water vapor mixing ratio is in the range of 3.0~3.1 g/kg,and the water vapor mixing ratio is very uniform.(5)At 10 o'clock in the local summer,the height of the boundary layer of the study area is 1052~1122 m.The maximum height of the boundary layer appears at14:00,2700 m,and the boundary layer height is less than 100 m from 3 to 6 in the morning.The turbulence intensity in the vertical direction increases from the surface to 1000 m with height,and reaches a maximum value of 0.07 near 1000 m,and thendecreases as the height increases.Combined with the simulation results of the boundary layer height,1000 m is the height of the mixed layer at SACOL at 10 am,and the regional averaged temperature profile has a significant gradient discontinuity at the height of 1000 m,indicating that the vertical turbulence intensity at the top of the mixed layer is the largest.The turbulent vertical mixing is the most intense.The horizontal turbulence intensity decreases slightly with height in the near-surface layer,and the upward turbulence intensity first approaches a constant(0.46)and then decreases,and increases again at 2000 m.From the regional averaged potential temperature profile,there is a thicker inversion layer above 2000 m height.(6)The ground temperature from WRF-LES is 301.7~302.3 K,and the temperature of 1000 m height at the top of the mixed layer is 7.7 K lower than the ground,which is 294.0~294.6 K.The ground temperature field simulated by WRF-LES exhibits an irregular network structure.The high temperature and low temperature regions are interlaced and disordered,showing a typical large eddy structure of the convective boundary layer.After replacing the roughness of 0.100 m with the actual roughness of 0.062 m,the ground temperature simulated by WRF-LES is 0.4 K lower than the previous one,which is closer to the observation data of SACOL,indicating that the WRF is improved with reasonable roughness. |
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