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Author: Shaw, Raymond A. × Has files: Yes ×

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    Characterization and first results from LACIS-T : a moist-air wind tunnel to study aerosol–cloud–turbulence interactions
    (Katlenburg-Lindau : Copernicus, 2020) Niedermeier, Dennis; Voigtländer, Jens; Schmalfuß, Silvio; Busch, Daniel; Schumacher, Jörg; Shaw, Raymond A.; Stratmann, Frank
    The interactions between turbulence and cloud microphysical processes have been investigated primarily through numerical simulation and field measurements over the last 10 years. However, only in the laboratory we can be confident in our knowledge of initial and boundary conditions and are able to measure under statistically stationary and repeatable conditions. In the scope of this paper, we present a unique turbulent moist-air wind tunnel, called the Turbulent Leipzig Aerosol Cloud Interaction Simulator (LACIS-T) which has been developed at TROPOS in order to study cloud physical processes in general and interactions between turbulence and cloud microphysical processes in particular. The investigations take place under well-defined and reproducible turbulent and thermodynamic conditions covering the temperature range of warm, mixed-phase and cold clouds (25∘C>T>−40∘C ). The continuous-flow design of the facility allows for the investigation of processes occurring on small temporal (up to a few seconds) and spatial scales (micrometer to meter scale) and with a Lagrangian perspective. The here-presented experimental studies using LACIS-T are accompanied and complemented by computational fluid dynamics (CFD) simulations which help us to design experiments as well as to interpret experimental results. In this paper, we will present the fundamental operating principle of LACIS-T, the numerical model, and results concerning the thermodynamic and flow conditions prevailing inside the wind tunnel, combining both characterization measurements and numerical simulations. Finally, the first results are depicted from deliquescence and hygroscopic growth as well as droplet activation and growth experiments. We observe clear indications of the effect of turbulence on the investigated microphysical processes.
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    Turbulence-induced cloud voids: Observation and interpretation
    (Katlenburg-Lindau : EGU, 2019) Karpińska, Katarzyna; Bodenschatz, Jonathan F.E.; Malinowski, Szymon P.; Nowak, Jakub L.; Risius, Steffen; Schmeissner, Tina; Shaw, Raymond A.; Siebert, Holger; Xi, Hengdong; Xu, Haitao; Bodenschatz, Eberhard
    The phenomenon of "cloud voids", i.e., elongated volumes inside a cloud that are devoid of droplets, was observed with laser sheet photography in clouds at a mountain-top station. Two experimental cases, similar in turbulence conditions yet with diverse droplet size distributions and cloud void prevalence, are reported. A theoretical explanation is proposed based on the study of heavy inertial sedimenting particles inside a Burgers vortex. A general conclusion regarding void appearance is drawn from theoretical analysis. Numerical simulations of polydisperse droplet motion with realistic vortex parameters and Mie scattering visual effects accounted for can explain the presence of voids with sizes similar to that of the observed ones. Clustering and segregation effects in a vortex tube are discussed for reasonable cloud conditions. © Author(s) 2019.
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    Evidence for inertial droplet clustering in weakly turbulent clouds
    (Milton Park : Taylor & Francis, 2017) Lehmann, Katrin; Siebert, Holger; Wendisch, Manfred; Shaw, Raymond A.
    Simultaneous observations of cloud droplet spatial statistics, cloud droplet size distribution and cloud turbulence were made during several cloud passages, including cumulus clouds and a stratus cloud. They provide evidence that inertial droplet clustering occurs even in weakly turbulent clouds. The measurements were made from the Airborne Cloud Turbulence Observation System suspended from a tethered balloon. For a profile through a stratus cloud with gradually changing droplet Stokes number, droplet clustering, quantified by the pair correlation function, is observed to be positively correlated with the droplet Stokes number. This implies that the droplet collision rate, which is relevant to drizzle formation via droplet coalescence, depends not only on the droplet size distribution, but also on the cloud turbulence. For cumulus clouds, the relation between droplet clustering and Stokes number seems more complicated. Stokes number is determined by measuring droplet size and local energy dissipation rate, the latter requiring highresolution air velocity measurements not possible on fast-flying aircraft.