Entrainment vs. Dilution in Tropical Deep Convection

Entrainment and detrainment describe how a cloud exchanges mass with its surrounding environment. These important processes need to be better understood in order to improve parameterizations of moist convection in global weather and climate models. Entrainment is often described as being synonymous with dilution, but no study has tried to verify this quantitatively. To do this we need independent definitions of entrainment, and recently developed methods of directly measuring entrainment in model simulations makes this possible. This study investigates the distinction between entrainment and dilution in the context of tropical deep convection.

 A method for estimating the rate of dilution by entrainment and detrainment is presented and calculated for a radiative convective equilibrium (RCE) simulation and a series of bubble simulations with a range of initial radii. Entrainment generally causes dilution of convection, but the two quantities are not well correlated unless the convection is sufficiently strong. In other words, more entrainment does not always imply more dilution. Detrainment is found to have a small enhancing or "concentrating" effect on convection that balances out the dilution by entrainment.

Dilution of the cloud core is shown to be significantly reduced by the presence of a shell of moist air around the core. In the case of RCE convection, entrainment enhances the core properties due to the tendency for clouds to form in the areas of previous deep convection. A toy Lagrangian thermal model is used to demonstrate the importance of the cloud shell as a thermodynamic buffer to reduce dilution by entrainment.

The relationship between entrainment and cloud radius is also investigated. Both entrainment and dilution are found to scale inversely with cloud radius, but the relationship is difficult to quantify.

Overall, the results underscore the importance of considering the cloud shell should in future convective parameterization development.