Quantitative Calculation Of Entrainment And Disengagement And Its Influence Mechanism On Cloud Microphysical Quantities

The entrainment-mixing processes(entrainment rate,detrainment rate,and entrainmentmixing mechanisms)have an important influence on the cloud macro and micro processes.To solve the problem of the inability to simultaneously estimate the entrainment and detrainment rates based on observations,and the impact of the entrainment and detrainment on the cloud microphysics,a new approach is developed for estimating entrainment and detrainment rates in cumulus clouds based on aircraft observations.The correlations between entrainment,detrainment,and cloud microphysics are explored,especially the influence of the entrainment on the spectral width of cloud droplet spectra.For cumulus clouds and stratocumulus clouds,this paper focuses on the lateral and the cloud-top entrainment and detrainment,respectively.The main conclusions are as follows:(1)A new approach for estimating the entrainment and detrainment rates in cumulus clouds based on aircraft observations has been established.The previous approaches are only suitable for high-resolution simulations,or only the entrainment rate can be estimated based on observations,while the detrainment rate cannot be estimated.Based on the cumulus air column model and the definition of the entrainment and detrainment rate,the new approach theoretically derives the relationship between the entrainment and detrainment rates and the physical properties inside and outside the cloud,which makes up for this deficiency.Applying this approach to the aircraft observations of cumulus clouds,it is found that both the entrainment and detrainment rates decrease with the increase of height,and the trend is not affected by the nature of detrained air.The entrainment and detrainment rates calculated by the new approach can reproduce the observation results of total moisture and moist static energy in cumulus clouds,which verifies the reliability of the new approach.On this basis,the relationship between the entrainment and detrainment rates and the environmental relative humidity is revealed: when the environmental relative humidity is high,the entrainment rate is high,and the detrainment rate is low.(2)The correlation between the entrainment and detrainment of the cumulus clouds and the microphysical properties is revealed.From the perspective of dynamics,the larger the entrainment rate corresponds to the stronger the mixing dilution and evaporation,thus the number concentration,liquid water content,and cloud droplet size are smaller;the detrainment rate has little effect on the microphysical properties.From the perspective of microphysics,as the height increases,the entrainment-mixing mechanism changes from inhomogeneous mechanism to homogeneous mechanism.The main reason is that the cloud droplet size increases with height,which causes the cloud droplet evaporation time scale to increase,and the homogeneous mechanism is easier to happen.The entrainment-mixing mechanism is more homogeneous in the cloud core where the number concentration and liquid water content are larger and the relative dispersion is smaller.From the perspective of dynamics and microphysics,the larger the entrainment rate,the more homogeneous the entrainment-mixing mechanism is,which is contrary to the theoretical expectation that the more environmental air is entrained,the more inhomogeneous the entrainment-mixing mechanism is.Environmental relative humidity plays a leading role: on the one hand,the larger the environmental relative humidity,the slower the cloud droplet evaporation,and the more homogeneous entrainment-mixing mechanism;on the other hand,the increase in the environmental relative humidity affects the buoyancy and the downdraft convective near cloud core and increases the entrainment rate.(3)The influence mechanism of the entrainment on the spectral width of cloud droplet spectra in stratocumulus cloud is clarified.The entrainment near the cloud top and in the cloud holes increases the spectral width.On the one hand,due to the entrainment of unsaturated environmental air,the cloud droplets evaporate,and the cloud droplet spectrum expands toward the small droplet,increasing the spectral width.This effect increases with the increase in entrainment intensity.On the other hand,the entrainment reduces the vertical velocity and thereby increases the spectral width of cloud droplet spectra,as the lower the vertical velocity,the lower the cloud supersaturation.Besides,cloud droplet number concentration and cloud droplet size are also important factors affecting the spectral width of cloud droplet spectra.