Impact Factors Of Entrainment Rate In Cumulus And Its Parameterization

Entrainment mixing process in cumulus plays an important role in weather and climate,which affects the vertical transport of atmospheric energy,precipitation and radiation.Entrainment rate is defined to describe this process,which is a key but highly uncertain parameter in convection parameterization schemes.In order to accurately describe entrainment rate in convection parameterization schemes,the impact factors and parameterization of cumulus entrainment rate are studied based on observation and numerical simulation.The main conclusions are as below:For the simulated cumulus in KWAJEX,the mass flux,buoyancy and vertical velocity in the cloud first increase with height and then decrease.The moist static energy decreases first and then increases with height.The entrained air consists of approximately 58%of cloud air and 42%of environmental air.The values of entrainment rate in the cloud are between 0-4 km^-1,which is larger than that in traditional convection parameterization schemes.The entrainment rate decreases with height above the cloud base,with a slower change in the middle of the cloud and a rapid increase near the cloud top.The average entrainment rate in the cloud decreases as the cloud-top height increases.From the relationship between entrainment rate and in-cloud thermodynamic/dynamical quantities,the entrainment rate is negatively correlated with vertical velocity and buoyancy in the cloud,and positively correlated with the inverse of cloud radius and|w^-1dw/dz|.Based on the relationship between entrainment rate and in-cloud thermodynamic/dynamical quantities,a single variable fitting and principal component regression analysis conducted by multiple variables were used to establish parameterization scheme for entrainment rate,respectively.Parametrization with multiple variables is more suitable for the parameterization of entrainment rate.For large-scale models with different resolutions,parameterization of entrainment rate exhibits scale dependence.For the cumulus observed in RACORO and RICO,the entrainment rate is positively correlated with relative humidity of the environment.Physical analysis shows that larger relative humidity promotes entrainment by reducing the buoyancy in the cloud cores and by weakening the downdrafts near the cloud cores.The reduced buoyancy in the cloud cores and weakened downdrafts near the cloud cores further reduce updrafts in the cloud cores;the cloud cores with smaller updraft will be affected by environment more easily,leading to a larger entrainment rate.The relationship between entrainment rate and relative humidity is consistent with buoyancy sorting concept that widely used in convection parameterization schemes.