Aerosol and Cloud Physics

Ice Nuclei Research Unit (INUIT)

During INUIT-1 (http://www.ice-nuclei.de/the-inuit-project/) we have investigated the impact of ice nuclei on mixed-phase convective clouds. Experiments of immersion and contact freezing were carried out by means of the Mainz vertical wind tunnel and an acoustic drop levitator.

During INUIT experiments, the acoustic levitator was installed inside a walk-in cold chamber at ambient temperatures down to -30°C. Inside the levitator a standing ultrasonic wave is produced by interference where drops can be floated at the nodes (Diehl et al., 2009). The set-up further includes a platinum-resistor thermometer Pt100 to measure the ambient temperature, a digital video camera to observe the freezing process and to determine the drop sizes, and an infra-red thermometer to directly and contact-free measure the surface temperature of the freezing drops. The transition from the liquid to the ice phase was clearly defined by a sudden increase of the drop temperature recorded from the infra-red thermometer.

In the Mainz wind tunnel drops of sizes from 680 to 760 micron in diameter were freely floated at their terminal velocities in the vertical air stream. Thus, ventilation and heat transfer were similar to the situation as in the real atmosphere. The onset of freezing was determined by direct observation. It is characterized by an opaque look of the drops and a different floating behavior.

Contact freezing experiments were performed in the acoustic levitator at constant temperatures lowered in steps of 1K. At each temperature, individual drops were placed in the levitator and supercooled to the ambient temperature which was reached after 10 to 30 s. Afterwards, the selected particles were blown as one burst onto the drop surface. If a drop did not freeze it was removed and counted as unfrozen, and afterwards, the next drop was injected.

Immersion freezing experiments were studied with both methods (i.e. acoustic levitator and wind tunnel). The drops were formed from solutions which contain the selected particles in defined concentrations. In the wind tunnel, the experiments were performed isothermal at constant ambient temperatures, i.e. the wind tunnel was pre-cooled to certain temperatures. Single drops were injected into the wind tunnel and freely floated for approximately 40 s. Drops which did not freeze within this timed period were counted as unfrozen. For experiments in the acoustic levitator, the cold chamber was pre-cooled to low temperatures corresponding to the expected lowest freezing temperatures. Individual drops were levitated one after another and cooled down until they froze.

Raindrop Microphysics

Raindrop microphysical studies had been carried out in our laboratory since 2006 as we had started to participate in the research project AQUARadar funded by the German Research Foundation (DFG) Radar polarization measurable quantities are sensitive functions of raindrop size and shape which is generally characterized by the drop axis ratio. Drops with radii larger than 0.5 mm are no longer spheres but show the shape of oblate spheroids. They start to oscillate, in the beginning with high frequency and small amplitude. With increasing drop size the oscillation frequency decreases while the amplitude increases. These changes of the drop shape are greatly important for applications in radar meteorology. With the drop shape the internal circulation is also changing from a fixed scheme to a turbulent mixing; this influences the uptake of trace gases. The internal circulation of cloud and raindrops as well as the shape and oscillation of raindrops with radii up to 3.5 mm were investigated with a high speed video camera (Szakall et al., 2009; Thurai et al., 2009; Szakall et al., 2010).

Drop-drop collision experiments are carried out at the vertical wind tunnel and the oscillation parameters of the colliding drops are investigated. Larger drops are freely floated at their terminal velocities while smaller droplets, carried along with the air stream, are colliding with them. These collisions result in coalescence and filament break-up events. The oscillation behavior of the drops is recorded by a high speed video camera with 2000 frames-per-second and evaluated by frequency analyses.

Photoacoustic Spectroscopy