Simulation Study On The Formation Of Deep Boundary Layer And Its Effects On Near-Surface Ozone Concentration

The atmospheric boundary layer is an important pathway for energy and material exchange between the surface and the free atmosphere.The deep planetary boundary layer(PBL)in winter and spring over the Tibetan Plateau(TP)can influence the process in the upper troposphere and lower stratosphere regions and provide a favorable condition for the stratosphere-troposphere exchange(STE).This paper first analyze the spatial and temporal distribution characteristics of the deep boundary layer on the Tibetan Plateau using the ERA5 reanalysis data from 1979-2018.Then we investigate the influence of thermal and dynamical factors on the development of the boundary layer.The pre-monsoonal data carried out by the Sino-Japanese Collaborative Center for Meteorological Disasters(JICA)research project in Gerze on the Tibetan Plateau and surface heat flux on March 10,2008 and the Large Eddy Model(LEM-version 2.4)of the UK Met Office are used to analyse the characteristics and evolution of boundary layer.The effect of high altitude wind shear on the turbulent structure within the boundary layer and the formation of the deep boundary layer is analyzed by sensitivity tests of varing the heigh of the jet core using LEM.The effect of the lower air density of the plateau on the formation of the deep boundary layer is analysed by sensitivity tests of varying the atmospheric density.On this basis,the effects of the deep boundary layer on the near-surface ozone concentration on the Tibetan Plateau are analyzed using the ERA5 reanalysis data.The quantitative effects of the relative positions of the top of the boundary layer and tropopause in the folding zone on the near-surface ozone are investigated using WRF-Chem model simulations on February 27,2008.The results of the LEM model simulation reveal that the deep boundary layer turbulence is helpful to transport ozone from high altitude to the ground.The main conclusions obtained from the paper are as follows:(1)The frequency of the deep boundary layer(PBL height > 4000 m,referred to as DPBL cases in this paper)on the Tibetan Plateau is significantly higher in winter and spring than in summer and autumn.The frequency increases from December to March and decreases thereafter.In terms of spatial distribution,the DBLH mainly occurs in the western part of the plateau.The surface sensible heat flux is small in winter on the plateau,while the wind shear plays a major role in the development of convective boundary layer.The surface sensible heat flux increases and becomes the main factor driving the development of boundary layer in spring.When the wind shear at high altitude is large,the atmospheric stability at high altitude will decrease,and the turbulent kinetic energy in its horizontal direction is obviously stronger than that in the vertical direction.The turbulence at high altitude formed by the wind shear mixes downward and couples with the turbulence developing upward in the boundary layer,which is conducive to the development of the deep boundary layer.(2)The high altitude of Tibetan Plateau,thin air and low air density are also important reasons for the formation of deep boundary layer.The results of sensitivity tests using LEM show that the boundary layer is significantly thickened when the atmospheric density decreases.The increase of the buoyancy and shear terms in the turbulent kinetic energy equation is most obvious in the entrained layer.The thin air on the Tibetan Plateau is subject to disturbed thermally and dynamically,thus the turbulence is more likely to form.The entrainment at the top of the boundary layer is enhanced,which promotes the formation of a deep boundary layer.(3)The top of the boundary layer over Gerze on the Tibetan Plateau on February27,2008 simulated by the WRF-Chem model is close to the tropopause,then the ozone within the boundary layer increases significantly.The ozone column from tropopause to the ground is formed in some areas.The ozone near the ground increases by 5 ppbv due to the vertical transport of ozone by turbulent mixing when the tropopause folds and rolls air into the boundary layer.The rate of the mixing term increases as the distance between the tropopause and the boundary layer decreases.The results of LEM simulation show that a turbulent layer that discontinuous with the boundary layer appears at 6000 m altitude during the simulation,then the turbulence at high altitude develops upward and downward with time.This turbulent layer is formed by the stronger horizontal wind shear near the tropopause.The Ozone in the stratosphere is transported downward through this turbulent layer.When the high-altitude turbulence couples with the boundary layer turbulence,the stronger turbulence in the boundary layer transports the upper-level ozone to the near ground,and the near-ground ozone concentration will increase rapidly.