Which parameters govern the strength of entrainment?

One of the key small-scale processes that necessarily require parameterisation is the turbulent mixing of cloudy and cloud-free air, i.e. entrainment and detrainment (in the following simplified as entrainment), which describe the in-mixing of ambient air into the cloud and the mixing of cloudy air into the environment surrounding the cloud, respectively. Entrainment changes cloud particle properties such as number concentrations and sizes, which also modifies the radiative properties of the cloud, and has important implications for cloud lifetime. No reliable formulation exists to date that allows understanding and describing entrainment in terms of cloud- and environmental physical quantities. This is despite the fact that a wide variety of entrainment parameterisations exists. However, their dependencies on meteorological parameters remain controversial. Indeed, the mixing processes (including entrainment) and their treatment in the numerical models have been found to be responsible for much of the large spread found in climate sensitivity estimates. A combination of measurements in the turbulent wind tunnel LACIS-T at TROPOS and computational fluid dynamics (CFD) simulations is used to identify the main parameters that govern the strength of entrainment. Here, we will present first results from the observations in LACIS-T, where the two air streams of the wind tunnel are used to mimic in-cloud and out-of-cloud conditions. Conditions in one of the air streams are varied to test the impact of the corresponding parameters (i.e. temperature, relative humidity, air speed) on the entrainment. Cloud droplets are induced by a droplet generator. The measurements are accompanied by CFD simulations which are verified by the point measurements in the wind tunnel and allow to retrieve full 3D fields.