Research On The Influence Of Minimum Turbulent Diffusion Coefficient On Near-surface Meteorological Elements And PM_2.5 Simulation In Beijin

WRF（weather research and forecasting）mesoscale weather model is one of the most widely used mesoscale models in meteorological research.In the model,the turbulence is often described by the boundary layer parameterization scheme.Due to the incompleteness of the boundary layer scheme,under the atmospheric condition of weak buoyancy,the calculated turbulent vertical diffusion coefficient may be zero,which means that the vertical diffusion does not exist at all and is inconsistent with the real world.The minimum turbulent diffusion coefficient(K_zmin)is introduced into many boundary layer parameterization schemes.It ensures the turbulent vertical exchange coefficient（_zK）is not lower than this value under strong stable conditions or free atmosphere,that means a minimum vertical diffusion.In this study,WRF and WRF-Chem are used to simulate Beijing and surrounding areas,to research the effects of K_zminsettings in ACM2 boundary layer scheme on temperature,wind speed,water vapor and PM_2.5.It will help researchers understand when and where the common meteorological elements and PM_2.5 in the simulation of similar processes are greatly affected by K_zmin,and which part of the difference in model results may be caused by different settings when using boundary layer schemes with different K_zmin.We found:（1）Different settings of K_zmin will affect the near ground temperature,specific humidity and wind speed.This is because the value of K_zmin will affect the turbulence diffusion intensity.A larger K_zmin means that the lower limit of turbulence diffusion in the model is high.For temperature,larger K_zmin causes higher lower temperature,this difference is greater at night than during the daytime.For water vapor,larger K_zmin causes lower specific humidity.For the wind speed,larger K_zmin causes smaller wind speed in the lower layer at night,and the wind speed in the daytime is larger.Different from temperature and specific humidity,the influence of changingK_zmin on temperature and specific humidity is greater near the ground,and gradually decreases upward.But for the wind speed,due to the friction of the underlying surface,it has little impact on the wind speed near the ground,gradually increases upward,and then gradually decreases.（2）Changing K_zmin has a greater impact on 2m temperature,2m specific humidity and10m wind speed in plain areas than in mountainous areas.During the nighttime,2m temperature and 10m wind speed is more affected than that during the day.It has a greater impact on 2m temperature in urban areas.But 2m specific humidity and 10m wind speed are not sensitive to whether it is an urban underlying surface.The change of K_zmin has a great influence on the 2m specific humidity near the land-sea boundary.（3）The enhancement of turbulent diffusion caused by the increasing K_zmin will reduce the PM_2.5 concentration near the ground,especially in the first and second layers,so that the PM_2.5simulated by the model is no longer concentrated near the ground.In space,increasing K_zminmakes the concentration drop more in the area with high PM_2.5 concentration.Therefore,when simulating the heavy pollution process,we should pay attention to the different settings of K_zminin different boundary layer schemes,which may affect the simulation results.（4）When K_zmin is set as a form proportional to the local urbanization rate(0.01～1.0m² s^-1),compared with the original setting(0.01m² s^-1),the main difference is in urban areas.The 2m temperature and 10m wind speed in urban areas have increased,but there is little change in rural areas.The 2m specific humidity decreases in some areas,but it is not sensitive to the urban underlying surface,which is different from the 2m temperature and 10m wind speed.For PM_2.5,the concentration in urban areas decreased more,and the concentration in rural areas also decreased but less.Compared with increase K_{zm in} to 1.0m² s^-1 in all areas,it has less impact on 2m temperature,2m specific humidity,10m wind speed and PM_2.5.