From drop to rain: turbulence matters

Warm clouds, i.e. clouds with cloud top temperatures higher than 0 ◦C, are responsible for more than 30% of the total rain on Earth. Observations show that from cloud formation to the onset of rain it takes less than 30 minutes, thus droplets need to grow fast. In order to explain droplet growth to reach raindrop size (a) the process of collision and coalescence - when two droplets collide and merge to form one larger drop - is needed. Secondly, it should be linked with turbulence (b), which speeds up the growth process to match the short time frames as observed in nature. While theoretical and modelling studies try to quantify the collision and coalescence process and the impact of turbulence therein, direct observational evidence is missing. To fill this gap, the Turbulent Leipzig Aerosol Cloud Interaction Simulator (LACIS-T), a unique turbulent moist-air wind tunnel, will provide direct measurements of droplet collision and coalescence under different meteorological conditions. With LACIS-T this project will deliver observations on the relevant highly resolved time and lengths scales, combined with highly resolved simulations using computational fluid dynamics, to quantify the collision and coalescence process in turbulent environments, providing the basis to understand fast rain formation. The results will enable us to improve the microphysical parametrisations in weather and climate forecast models, and thus, help to improve cloud and rain forecasts.