Sensitivity Of Critical Bulk Richardson Number To Simulation Of Boundary Layer Scalar Transport

The critical bulk Richardson number(Ricr)is an important parameter in YSU boundary layer parameterization scheme which is used to diagnose the boundary layer height(PBLH).As the atmospheric stability changes,the critical bulk Richardson number also changes.In this paper,the WRF-Chem model is uses to investigate the boundary layer height prediction of different critical bulk Richardson numbers and it's influence on the surface meteorological field.Based on the turbulent diffusion equation of YSU boundary layer parameterization scheme,we analyze the influence of the critical bulk Richardson number on the local flux term,non-local flux term and the entrainment flux term and the contribution of each term to the total flux.At the same time,the influence of critical bulk Richardson number on pollutant diffusion id also investigated.The results show that:(1)The WRF-Chem model can simulate the temperature,the relative humidity and the temporal and spatial variation of pollutants.The simulation about the meteorological field of WRF-Chem model is consistent with the observation,the correlation coefficient is above 0.9,and the correlation coefficient for the pollution concentration simulation is about 0.7.The simulation of the boundary layer temperature,humidity and wind speed profiles are consistent with the observation.The model can simulate the variation characteristics of meteorological elements in the boundary layer.(2)The change of the critical bulk Richardson number in the YSU boundary layer parameterization scheme will affect the diagnosis of PBLH.Generally,PBLH increases with the increasing critical Richardson number.The influence of critical bulk Richardson number on the diagnosis of boundary layer height is uneven in time and space.The influence of Ricr on the diagnosis of boundary layer height is mainly reflected in the afternoon 15-16 p.m.and the maxmum difference can reach 500 m.The critical bulk Richardson number can affect the simulation of cloud which is the main reason for the uneven distribution in space.(3)The influence of the critical Richardson number on the ground-to-gas exchange is reflected in the influence on the ground heat flux,which can be divided into direct and indirect effects and the indirect impact is the main influencing mode.The critical bulk Richardson number can affect the simulation of cloud amount,which leads to different solar radiation received by the ground and then affect the calculation of surface flux and affects the ground-to-gas exchange.At the same time,the influence of the critical Richardson number on the ground heat flux will also affect the diagnosis of the boundary layer height,which is the main reason for the uneven spatial distribution of the difference of boundary layer height.The critical bulk Richardson number has little influence on the ground-to-gas exchange and there is no significant difference in different cases.(4)The turbulent diffusion equation of YSU boundary layer parameterization scheme can be divided into three parts,local term,non-local term and entrainment term and Ricr mainly afffects the diffusion of scalars in the boundary layer by changing the entrainment flux.The little Ricr corresponds to a large entrainment flux and the boundary layer height is enhanced by the exchange of boundary layer and free atmosphere.At the same time,the stability of boundary layer is enhanced.(5)The influence of critical bulk Richardson number on pollutant simulation in boundary layer can be divided into direct influence and indirect influence.The direct influence on pollutants is similar to that of water vapor.It affects the caculation of entrainment flux and then affects the concentration of pollutants in the upper part of the boundary layer.The indirect effect of the critical bulk Richardson number on the concentration of pollutants is achieved by affecting the stability of the whole boundary layer.The pollutants accumulate in the lower part of the boundary layer,resulting in a high concentration of pollutants in the middle and lower layers of the boundary layer.