Besides clouds being highly complex systems in themselves, they occur sporadically in locations usually hard to reach. Therefore, investigating atmospheric clouds in-situ is an ambitious, expensive and often impossible task. To make things even worse, measurements in atmospheric clouds suffer from a lack of reproducibility regarding initial and boundary conditions. Due to these issues, the examination of individual cloud processes in the laboratory is mandatory for increasing our understanding of cloud microphysical processes, and their interactions with turbulence (Stratmann et al., 2009).
The newly built turbulent moist air wind tunnel LACIS-T (Turbulent Leipzig Aerosol Cloud Interaction Simulator) is an ideal facility for pursuing mechanistic understanding concerning these processes and interactions under well-defined and reproducible laboratory conditions. Within LACIS-T, we are able to precisely adjust turbulent temperature and water vapor fields, so as to achieve supersaturation levels allowing for, e.g., the detailed investigation of aerosol particle activation to cloud droplets.
LACIS-T is designed as a closed loop (Göttingen-type) wind-tunnel, in which the air circulates continuously with a total flow rate of up to 10.000 l/min. The actual measuring section of the tunnel is 2 m long, 80 cm wide and 20 cm deep. Cloud formation occurs via turbulent mixing of three conditioned flows (i.e. two particle-free air streams, and one aerosol stream), and is initiated at the inlet of the measuring section. The turbulence required for mixing the flows is generated by two passive planar grids in the air streams (approx. 5.000 l/min each). The air streams are humidified by Nafion humidifiers and tempered by two separate heat exchangers. The aerosol flow is introduced into the mixing zone of the two air streams. In the measuring section, the characterization of the respective fluid and thermodynamic states, as well as the microphysical properties of the cloud formed (droplet size, number of droplets, etc.), is carried out. After passing through the measuring section, the entire flow is dried, split up again into two streams driven by blowers and cleaned by filters before humidification takes place. For the investigations presented here, we used a welas 2300 optical system (from Palas GmbH) and a digital in-line holographic system at LACIS-T to characterize the (cloud) particle size distributions.
We will present results, which clearly document the feasibility of LACIS-T for investigating cloud microphysical processes under turbulent conditions. Specifically, we will deal with the deliquescence and hygroscopic growth behavior of size-segregated sodium chloride particles, and thereby observed indications of possible influences of relative humidity fluctuations on deliquescence. We also present first evidence concerning the effects of turbulence on cloud droplet activation.
This study is part of a project that has received funding from the European Union’s Horizon 2020 research and innovation program and grant agreement No 730997.
Bodenschatz, E., S. P. Malinowski, R. A. Shaw, and F. Stratmann (2010), Can we understand clouds without turbulence? Science, 327, 970-971.
Stratmann F., O. Möhler, R. Shaw, and H. Wex (2009), Laboratory Cloud Simulation: Capabilities and Future Directions, in Clouds in the Perturbed Climate System, MIT Press, Cambridge, MA, 149-172.