Large Eddy Simulation Study On The Influencing Factors Of Macro And Micro Physical Quantities Of Shallow Convection

Shallow convective clouds play an important role in the atmospheric dynamical and thermodynamic processes.However,the theoretical understanding of macro and microphysical factors affecting shallow convection is still uncertain,especially the understanding of the processes related to relative dispersion of cloud droplet size distributions and the effects of aerosol.In this study,the Large Eddy Simulation version of Weather Research and Forecasting model(WRF-LES)with Spectral Bin Microphysics scheme(SBM)is used to simulate a shallow convective cloud system observed by aircraft in the field campaign of Small Cumulus Microphysics Study in 1995(SCMS95).The influencing factors of macro and microphysical properties in shallow convection are analyzed.The main conclusions are as follows:(1)The factors affecting the cloud spectral relative dispersion in shallow convection are revealed.The simulation is first validated with aircraft observations by comparing liquid water potential temperature,water vapor mixing ratio in cloud and environment,and cloud water mixing ratio.Secondly,the simulation results indicate that cloud droplet relative dispersion is found to be negatively correlated with both vertical velocity and adiabatic fraction.Larger positive vertical velocity leads to larger supersaturation,which further causes narrower cloud droplet size distributions and smaller relative dispersion.Larger adiabatic fraction means less entrained dry air,resulting in less cloud evaporation and smaller relative dispersion.Furthermore,relative dispersion becomes smaller from cloud edge to cloud core as entrainment becomes weaker.Thirdly,the negative relationships between relative dispersion and vertical velocity/adiabatic fraction are relatively weak near the cloud base;after that,the negative relationships strengthen first and then weaken with the increasing height.The reason why the negative relationships are weak near cloud base is that,on the one hand,larger vertical velocity leads to more droplet activation to increase relative dispersion,and on the other hand,the increase of vertical velocity also decreases relative dispersion by increasing supersaturation.The results will be helpful to improving parameterizations related to relative dispersion(e.g.,autoconversion and cloud effective radius)in large-scale models.(2)The influences of aerosol number concentration on macro and microphysical properties of shallow convection,especially on the cloud droplet relative dispersion,are investigated.From the macro viewpoint,mean vertical velocity increases with the increase of aerosol number concentration,because fraction of updraft increases due to the reduction of the cloud fraction(especially for downdraft);the cloud fraction reduction is caused by the accelerated evaporation due to the decrease of cloud radius.Adiabatic fraction becomes larger with the increase of aerosol number concentration,i.e.,entrainment and evaporation become weaker because of the larger vertical velocity in cloud.From the micro viewpoint,mean cloud radius and standard deviation both decrease with the increase of aerosol number concentration.Relative dispersion generally decreases because relative dispersion has negative relationships with vertical velocity and adiabatic fraction,respectively.The increase of vertical velocity and adiabatic fraction due to the increase of aerosol number concentration,causes stronger condensation,weaker entrainment,and smaller relative dispersion.