| Estimation of entrainment rate in cumulus,parameterizations of entrainment rate and simulations of cloud and precipitation are included in this study.Firstly,we estimated entrainment rate in precipitating cumulus congestus using the in-situ aircraft observational data during the TOGA-COARE(Tropical Ocean Global Atmosphere Coupled Ocean Atmosphere Response Experiment)field campaign.The probability density function of entrainment rate,the relationships between cloud characteristics and entrainment rate,the response of the calculated entrainment rate to entrained air sources,and the influence of entrainment on relative dispersion were analyzed.Secondly,a Southwest Vortex rainfall process over Sichuan Basin on 8-9 July 2010 was simulated with WRF model to study the characteristics and parameterization of entrainment rate.Thirdly,wide-range strong precipitation weather in Northeast Chirpa on July 1,2010 was simulated using WRF model to analyze the entrainment influences between stratiform cloud and cumulus in mixed cloud systems.The following conclusions are reached.The probability density function of entrainment rate can be well fitted by lognormal,Gamma and Weibull distributions,all with r2 larger than 0.80.Due to dilution and evaporative cooling,entrainment tends to reduce cloud droplet number concentration,liquid water content,temperature,water vapor content in cloud and further suppress precipitation.The negative correlation between volume-mean radius and entrainment rate suggests the dominance of homogeneous mechanism in clouds.In addition,entrainment rate and entrained water vapor content show decreasing tendencies with the distance of the assumed entrained air to cloud edges.But their variation tendencies are non-monotonic due to the short distance between adjacent clouds.We observed that relative dispersion is negatively correlated with entrainment rate,and this is inconsistent with the traditional conclusion that entrainment broadens droplet size distribution.For analyzing the inhibiting effect of entrainment on relative dispersion,cloud droplet size distributions are divided into three portions with droplet radius<3.75 ?m(N1),radius in the range of 3.75 ?m-12.75 ?m(N2)and 12.75 ?m-23.25 ?m(N3),respectively.N1,N2 and N3 herein are the droplet concentrations within the three size ranges,respectively.The number concentrations of cloud droplets with different sizes decrease simultaneously as entrainment rate increases.The variation of standard deviation mainly depends on N3,mean radius reduces with decreasing N3,but increases with decreasing N1.So the influence of entrainment on cloud droplet size distribution causes a decrease in standard deviation but relatively little variation in mean radius,and further leads to the negative correlation between relative dispersion and entrainment rate.Furthermore,a conceptual model of cloud droplet size distribution evolution during entrainment-mixing process is developed to illustrate the possible scenarios entailing different relationships between relative dispersion and entrainment rate.The simulations of WRF indicate that,the liquid water content,vertical velocity and buoyancy in Southwest Vortex cumulus increase with height above cloud base and decrease with height near cloud top.Moisture static energy mainly reduces with height in cloud.Entrainment rate decreases with height above cloud base but increases with height near cloud top.For the clouds with higher tops,the increasing intensity of entrainment rate near cloud top is weaker.In addition,entrainment rate is negatively correlated with both cloud water content and rain water content,which is similar to observations and indicates that entrainment suppresses the development of cloud and precipitation.When we assume the entrained environmental air surrounding the cloud edge,the estimated entrainment rate is larger than the corresponding value when assuming the entrained air is far away from the cloud edge.But the other characteristics about entrainment rate under the two assumptions are similar.Moreover,entrainment rate decreases with time as a whole and this is related to the development and growth of cloud ensemble during this time.The negative correlations between dynamical variables in cloud and entrainment rate simulated with WRF in Southwest Vortex cumuli can be used to parameterize entrainment rate.When entrainment rate is parameterized based on joint probability density function(PDF),for the larger PDF,the slope of the logarithmic linear fitting is also lager.In the process of development of clouds,the change rates of entrainment rate with vertical velocity and buoyancy decrease gradually,and this is related to the increases of vertical velocity and buoyancy and the decrease of entrainment rate in this process.Clouds are further binned and averaged by cloud top heights to parameterize entrainment rate.It is encouraging that our fitted entrainment rates expressed by vertical velocity and buoyancy are both falls in between two previous traditional fitted entrainment rates.In addition,if vertical velocity and buoyancy are simultaneously used to fit entrainment rate by principal components regression,the fitting effect is almost the same as that of only using vertical velocity.But principal components regression links entrainment rate to more dynamical variables and thus is recommended to parameterize entrainment rate for Southwest Vortex convective clouds.In addition,the entrained air of cumulus in mixed cloud is wetter than that of isolated cumulus within the same simulated domain.On the basis of a comparison of mixed cloud cases with the usual isolated cumulus,we reveal longer lifecycle and stronger intensity of the cumulus embedded in stratiform cloud.In addition,for the convection embedded in stratiform cloud,not only the lower level water vapor convergence is stronger,but also the in-cloud liquid water content is larger and more unstable energy aggregates at the low level.Given that the liquid water contents in the two mixed cloud cases are equivalent,the larger in-cloud solid water content causes the precipitation of embedded convection stronger.So the entrained air from stratiform cloud might promote the development of cumulus.On the other hand,in the process of the development of mixed cloud system,the vertical scale,water content and precipitation from the stratiform cloud in mixed cloud system all increase.For the precipitation mechanism,both the in-cloud solid and liquid water content of the stratiform cloud in mixed cloud increase with the development of the mixed cloud system.However,the abovementioned changes in conventional usual stratiform cloud are not obvious.Therefore,the developments of cumulus and stratiform cloud in mixed cloud system are complementary to each other in our simulations. |
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