A Quantitative Investigation of Entrainment and Detrainment in Numerically Simulated Cumulonimbus Clouds

Abstract

A method is developed that uses numerical tracers to make accurate diagnoses of entrainment and detrainment rates and of the properties of the entrained and detrained air in numerically simulated clouds. These rates and properties are averaged horizontally and over time, and are produced independently of each other. There are no restrictions on the types of clouds to which the procedure can be applied. Cumulonimbus clouds are simulated with a variety of initial thermodynamic soundings. In the simulations, updraft entrainment rates are large near and above cloud base, through the entire depth of the conditionally unstable layer. Stronger updrafts in a more unstable environment are better able to entrain relatively undisturbed environmental air, while weaker updrafts in a less unstable environment can entrain only air that has been modified by the clouds. Smaller convective clouds in more stable environments mix more with their environment but do not necessarily have larger entrainment rates. How much air is entrained depends on the low-level convective available potential energy (CAPE) and on the convective inhibition of the environmental air. Strong updrafts that are produced when the low-level CAPE is large include parcels with a wide range of equivalent potential temperature and are more likely to have an undilute core and to reach or exceed their level of neutral buoyancy than the weaker and more horizontally uniform updrafts that are produced when low-level CAPE is small. These results help to explain previous observations that convective updraft cores are stronger in midlatitude continental clouds than they are in tropical maritime clouds.