Characteristics And Effects Of Turbulent Structure In The Atmospheric Boundary Layer Driven By Weak (fog-haze Events) And Strong (typhoon Process) Forces

Atmospheric boundary layer turbulence governs the vertical exchange of momentum,energy,and composition between underly surface and atmosphere,which in turn affects weather processes.This study devotes to clarify the atmospheric boundary layer turbulence structure and its impact on weather processes under weak and strong forces.The observations in farmland in Baoding,China during fog-haze episodes in December 2016,and simulations of the landfall of Hurricane Harvey(2017)using the WRF-LES model are analyzed in this study.During this severe air pollution period,more than 73.5%of the PM2.5 concentration is higher than 100?g·m-3,with a maximum of 522?g·m-3.During the pollution development-maintenance period,there are southerlies and lower wind speeds in the lower troposphere.Slow southerlies transport the southern pollutants to North China.In contrast,the high-pressure system at the 500 h Pa level and increasing temperature(caused by air pollutant absorbed radiation)at 850 h Pa suppress the convection and lead to pollutants accumulation over the ground.The analysis of the turbulence spectrum during fog-haze episodes implied that the lower winds could suppress large-scale turbulence eddies,which are more likely inhomogeneous,break into small-scale eddies.At night,air pollutants'radiation scatters effect suppresses the land surface cooling and decreases the air temperature difference as well as the vertical turbulence intensity difference.The atmospheric boundary layer vertical turbulence effect analysis suggests that not only the turbulence intensity but also the percentage of large-scale turbulence eddies changes atmospheric vertical transport capacity.During the daytime,the air pollutant concentration is controlled by the wind speed and vertical turbulence intensity.At night,the vertical turbulence is weak and leads to lower atmospheric boundary layer height,and the percentage of large-scale eddies controls the atmospheric vertical transport capacity.Increased number of large-scale turbulence eddies driven by the low wind at night could generate stronger vertical transport of air pollutants and decrease the air pollutant concentration near the ground.The hurricane simulation results implied that the turbulence in LES was mainly attributed to roll vortices.The turbulence intensity of roll vortices is proportional to the hurricane intensity.Before and during the hurricane landfall,the strong inflow convergence can lead to vertical perturbation and finally generate the roll vortices with suitable thermodynamic Rf(?)at around-0.2—0.2)and dynamic conditions(strong negative inflow wind shear).After the hurricane landfall,the decayed inflow weakens the inflow convergence and leads to the quick reduction of strong roll vortices.The analysis of vertical turbulence energy indicates that the atmospheric pressure perturbation,which caused by air density perturbation,leading to strong roll vortices with a mean wavelength of about 1 km.The strong roll vortices are supported by the atmospheric pressure transport term in the TKE equation.With roll vortices,the LES provided a better hurricane wind vertical structure and precipitation simulations.While the simulation with the YSU scheme overestimated the precipitation for hurricane oversea,the flux analysis indicated that the roll vortices introduced negative momentum flux maintain the maximum of tangential wind at around 400 m above ground.During hurricane landfall,this negative flux enhanced the inflow and maintained the higher wind in LES simulation.The overestimated low-level momentum flux in the YSU scheme overestimated the hurricane intensity for hurricane oversea but accelerated the decay of hurricane during landfall.The analysis of the rainfall reflected that the roll vortices lead to a weak updraft and insufficient water vaper support.For the simulation with the YSU scheme,the strong updraft combined with surplus water vaper support eventually leading to heavy rainfall for hurricane oversea.We can conclude that the large-scale turbulence in the atmospheric boundary layer plays an essential role in vertical mixing both under weak and strong forces.Therefore,the large-scale turbulence mixing effort should be considered when making weather predictions.