New proposal as part of the Research Unit: Experimental studies at the Mainz vertical wind tunnel Stephan Borrmann, Subir K. Mitra, Karoline Diehl, Miklós Szakáll Institute of Atmospheric Physics, University of Mainz Drop shape and oscillation: External influences turbulence electric fields Ice phase: Melting of ice particles

Turbulence AQUARadar project F: single oscillating drops, freely suspended in the vertical wind tunnel, were continuously recorded to determine the axis ratio b/a results: no significant differences between the static mean (drops in equilibrium) and the dynamic mean (oscillating drops) of the axis ratio for all drop sizes between 1 und 7.5 mm laminar conditions: background turbulence level of less than 0.5% in the wind tunnel But: in clouds, in particular in convective clouds, the air is turbulent the laminar air stream in the tunnel allows to produce a well-defined turbulent air stream without super-positions with background turbulence generation of turbulence: placing flow intercepting objects upstream of the experimental region in the air stream of the wind tunnel characterization of turbulence: by the energy dissipation rate measured with a hot wire anemometer: 1 × m 2 s -3 for low air speeds of 1 m s m 2 s -3 for high air speeds up to 8.5 m s -1 (Vohl et al., 1999; Diehl et al., 2000; Vohl et al, 2001)

Electric fields influence of strong electric fields inside clouds on the drop shape preliminary studies (Rasmussen et al., 1985): deformation of drops in strong electrical fields: with increasing electric field strength the axis ratio increases, in particular for drops with radii larger than 2 mm important for polarimetric radar measurements preliminary studies performed at the forerunner type of the Mainz wind tunnel 2 horizontal metal screens with an electric potential difference acted as electrodes to create vertical electric fields electric field strengths between 1000 and 9000 volts cm -1

Melting of ice particles bright band: melting of precipitating ice particles ― important parameters: shape of melting ice particles and snow flakes parameterizations not verified in laboratory experiments axis ratio of melting ice particles important for polarimetric radar shedding of droplets from melting ice particles fall modes during melting Vertical wind tunnel: suitable tool to investigate melting ice particles (Mitra et al., 1990) previous experiments: melting time and the fall behaviour of melting snow flakes natural and laboratory-made aggregates of ice crystals melting under free falling conditions in the wind tunnel warming rate 1.5°C/min, i.e. effective warming rate 0.8°C/100 m fall mode variation, fall velocity, percentage of melted ice

Goals turbulence: gradually increase of turbulence level studying of oscillation frequency, oscillation modes, shape of rain drops continuous time series images of free floating rain drops electric fields: various strengths of electric fields recording of rain drop deformation melting ice particles (graupel, snow flakes) coverage of the melting ice particle with water rotation of the melting ice particle shedding of droplets axis ratio