Investigations And Improvements Of The Parameterizations Of Cloud Liquid-phase Microphysical And Entrainment Processes

Based on the commonly used numerical model Weather Research and Forecasting(WRF),three uncertain cloud physical processes in physical schemes are investigated,including activation of cloud condensation nucleus(CCN),collision-coalescence of cloud droplets and raindrops,and entrainment between cloud and environment(entrainment-mixing process and entrainment rate);their parameterizations are improved.The influences of these three processes on cloud microphysical characteristics,radiation characteristics,and precipitation are also studied.The main results are as follows:(1)CCN activation parameterization and its effects on cloud microphysical and optical properties.The interference of unactivated particles in the CCN measurement results may cause errors in activation parameterization,which in turn increases the uncertainty of the indirect effects of aerosols in models.We revise the CCN activation parameterization scheme,and find that the influence of revised CCN activation parameterization scheme on cloud microphysical and optical properties strongly depend on the distribution of supersaturation.The impact of the revised CCN activation parameterization scheme on cloud microphysics is the reducing cloud droplet number concentration and increasing cloud droplet radius under low supersaturation conditions,and at the same time,it will weaken aerosol indirect effect.With the increase of supersaturation,the influence of the revised CCN activation parameterization scheme will be weakened.The influence of revised CCN activation parameterization will decrease with the increase of aerosol concentration.(2)Cloud collision-coalescence parameterizations and their influences on precipitation and cloud optical properties.The accretion scheme considering the number concentration and the radius of cloud droplets and raindrops can effectively alleviate the phenomenon of overestimation of precipitation,at mean time,cloud water path,cloud optical depth and cloud effective radius increase,which is most consistent with the observations.This parameterization suppresses spurious accretion and does not produce liquid-phase precipitation until cloud droplets are big enough.The size of cloud and raindrops will obviously affect the efficiency of accretion,and the particle size is related to the mass concentration and number concentration of cloud and raindrops.Therefore,in the future development of accretion schemes,the mass concentration and number concentration of droplets should be considered at the same time.It is also confirmed that increasing the model resolution can reduce precipitation overprediction.Results from the case study are confirmed by the use of a one-month simulation.(3)Parameterizations of the entrainment process and impacts on cloud microphysics,cloud optical properties and precipitation.Entrainment is the interaction between cloud and environment which is one of the most uncertain processes in the cloud physical processes.The entrainment process in the model is mainly divided into the entrainment-mixing process and the entrainment rate.In terms of entrainment-mixing parameterization,we establish a high-resolution cloud cover parameterization using relative humidity and mixing ratio of cloud droplet,and then improve the entrainment-mixing parameterization scheme.Entrainment-mixing parameterization reduce the concentration of cloud droplets,increase the cloud mean-volume radius and cloud effective radius,but the influences are different for cumulus and stratocumulus.The effect of entrainment-mixing parameterization relies on the relative humidity of the air,turbulence dissipation rate,aerosol concentration and so on.The entrainment-mixing process in stratocumulus clouds is more inhomogeneous than that in cumulus clouds.Changing various factors also has a more obvious impact on cloud microphysical and optical properties in stratocumulus clouds.Compared with cumulus clouds,the parameterization of entrainment-mixing process for stratocumulus clouds is necessary.In addition,with the increase of aerosol number concentration,the influence of entrainmentmixing parameterization increases in both cumulus and stratocumulus.In terms of entrainment rate parameterization,we estimate the entrainment rates in the three-dimensional convective cloud,they are found to be negatively correlated with both vertical velocity and buoyancy,and positively correlated with the vertical divergence of the vertical velocity and with the reciprocal of cloud radii.The physical mechanisms underlying these relationships are interpreted.It is found that the parameterizations with multiple properties perform better than those with a single property.Entrainment rates and relative humidity of entrained air are positively correlated at temperature higher than 0?,but negatively correlated at temperature lower than 0?.Relative humidity is also included in the parameterization of entrainment rates,which differs from our previous work on shallow cumulus clouds and other studies.Finally,two forms of parameterization for entrainment rate are recommended.The first treats the entrainment rate as a function of the vertical velocity and buoyancy for temperature higher than 0°C,but as a function of relative humidity and buoyancy for temperature lower than0?.The second involves an equation that relates entrainment rates to vertical velocity and buoyancy regardless of temperature.The entrainment rate calculated by the two entrainment parameterizations are both negative correlated with cloud number concentrations,cloud radius,cloud optical depth,and precipitation. activation of cloud condensation nucleus,collision-coalescence of cloud droplets and raindrops,and entrainment process affects cloud optical properties and precipitation through cloud microphysics such as alleviating the underestimation of cloud droplet effective radius in the model,these parameterization improvements may have positive effects on improving the simulation ability of cloud related processes in the model.