Summary of single project as part of Research Unit
Stephan Borrmann, Subir K. Mitra
Institute of Atmospheric Physics, University of Mainz
1. State of knowledge and relevant questions
In the subproject F of the AQUARadar research program the shape and oscillation of rain drops
was investigated in laboratory experiments at the Mainz vertical wind tunnel. Deviations from
the spherical shape and oscillations are important for the estimation of the rainfall rate from the
reflectivity of the radar echo. Therefore, by means of a high speed video camera single oscillating
drops were continuously recorded and the dynamic mean of the axis ratio b/a was determined
while the drops were freely suspended in the vertical updraft of the wind tunnel. So far the results
show no significant differences between the static mean (drops in equilibrium) and the dynamic
mean (oscillating drops) for all drop sizes between 1 und 7.5 mm. However, the air stream in the
wind tunnel is laminar with a background turbulence level of less than 0.5%, while in clouds, in
particular in convective clouds, the air is turbulent (Weil et al., 1993). This could affect the
oscillation of rain drops with respect to oscillation modes. Regarding liquid drops another
important factor that affects their shape is the presence of strong electric fields inside clouds.
Preliminary studies (Rasmussen et al., 1985) have shown that electric fields significantly
influence the axis ratios of freely falling water drops: For uncharged drops, with increasing
electrical field strength the axis ratio increases, in particular for drops with radii larger than 2
mm. So within the context of polarimetric radar measurements a detailed study of the shape of
drops in the presence of typical strength electrical fields inside clouds is also needed.
Another important point regarding radar measurements is the ice phase. In mid-latitudes,
precipitation is initiated via the ice phase, i.e. rain is the result of molten ice particles. The
melting layer in the atmosphere is characterized by the so-called bright band, i.e. by a significant
increase of the radar signal followed by a slight decrease of the signal. The signal increase is
affected by the water layer which covers the melting ice particles (Bohren and Battan, 1982;
Korkmaz, 2004). The shape of melting snow flakes was parameterized by Russchenberg and
Lighthart (1996) but not verified in laboratory experiments so far. Furthermore, little is known
about the axis ratio of melting ice particles which might be important in particular for
polarimetric radar. Other important issues are the shedding of droplets from melting ice particles
and the fall modes during melting.
2. Preliminary work
The Mainz vertical wind tunnel allows free suspension of water drops as well as of ice particles
and snow flakes. Therefore, it provides a suitable tool to investigate a number of parameters
which are important for melting ice particles. In earlier experiments, the melting time and the fall
behaviour of melting snow flakes were investigated, partly with the help of movable inner
sections (Mitra et al., 1990). Natural and laboratory-made aggregates of ice crystals were melted
under free falling conditions in the wind tunnel while it was warmed up at rates typical for falling
snow flakes in the atmosphere (1.5°C per min, corresponding to an effective warming rate of
approximately 0.8°C per 100 m). The variation of the fall mode, the fall velocity, and the
percentage of ice melted was recorded with a film camera. Regarding turbulence, the influence of
turbulence on microphysical processes such as collisional growth of cloud droplets, impaction
scavenging of aerosol particles and uptake of trace gases was investigated at the wind tunnel
(Vohl et al., 1999; Diehl et al., 2000; Vohl et al., 2001). The laminar air stream in the tunnel
allows one to produce a well-defined turbulent air stream without superpositions with
background turbulence. The turbulence was generated by placing flow intercepting objects
upstream of the experimental region in the air stream of the wind tunnel. The turbulence was
characterized by the energiy dissipation rate measured with a hot wire anemometer. It was in the
order of 1 × 10-2 m2 s-3 for low air speeds of 1 m s-1 and in the range of 0.5 m2 s-3 for high air
speeds up to 8.5 m s-1 and, thus, in the order of turbulence in clouds. The deformation of drops in
strong electrical fields was investigated in a wind tunnel which was a forerunner type of the
Mainz wind tunnel (Rasmussen et al., 1985). Two horizontal metal screens with an electrical
potential difference acted as electrodes to create vertical electrical fields with strengths lying
between 1000 and 9000 volts cm-1.
3. Goals
Experiments of the influence of turbulence on shape and oscillation of rain drops are planned for
the first 6 months. The turbulence level will be gradually increased while the following will be
studied: the oscillation frequency, the oscillation modes, and the shape of the rain drops. By
means of a high speed video camera continous time series images of free floating rain drops of
different sizes will be recorded. The mean turbulence level will be measured with a hot-wire
anemomenter or a laser Doppler anemomenter. Another 9 months will be needed for experiments
with electrical fields. For various strengths of electrical fields the deformation of rain drops of
different sizes will be recoreded with the high speed video camera. 21 months are planned for
experiments in the ice phase. Melting ice particles (graupel, snow flakes) will be freely
suspended in the wind tunnel while changes during the melting process will be recorded with the
high speed video camera. Observations are planned with regard to the coverage of the melting ice
particle with water, rotation of the melting ice particle and the shedding of droplets, and the axis
ratio. The axis ratio is important in particular in conjection with polarimetric radar.
References
Bohren, C.F., and L.J. Battan, 1982: Radar backscattering of microwaves by spongy ice
spheres. J. Atm. Sci., 39, 2623-2628.
Diehl, K., O. Vohl, S.K. Mitra, and H.R. Pruppacher, 2000: A laboratory and theoretical
study on the uptake of sulfur dioxide gas by small water drops containing hydrogen peroxide
under laminar and turbulent conditions. Atmos. Environ., 34, 2865-2871.
Korkmaz, S.D., 2004: Radar attenuation due to melting: a comparison of physical models
propsed for the melting morphology. Atmos. Res., 70, 261-274.
Mitra, S. K., O. Vohl, M. Ahr, and H.R. Pruppacher, 1990: A wind tunnel and theoretical
study of the melting behavior of atmospheric ice particles. IV: Experiment and theory for snow
flakes. J. Atm. Sci., 47, No.5, 584-591.
Rasmussen, R., C. Walcek, H.R. Pruppacher, S.K. Mitra, J. Lew, V. Levizzani, P.K. Wang,
and U. Barth, 1985: A wind tunnel investigation of the effect of an external vertical electric field
on the shape of electrically uncharged rain drops. J. Atm. Sci., 42, 1647-1652.
Russchenberg, H.W.J., and L.P. Lighthart, 1996: Backscattering by and propagation through
the melting layer of precipitation: A new polarimetric model. IEEE Trans. Geosci. Remote
Sensing, 34, 3-14.
Vohl, O., S.K. Mitra, S.C. Wurzler, and H.R. Pruppacher, 1999: A wind tunnel study of the
effects of turbulence on the growth of cloud drops by collision and coalescence. J. Atm. Sci., 56,
4088-4099.
Vohl, O., S. K. Mitra, K. Diehl, G. Huber, S.C. Wurzler, K.-L. Kratz, and H.R. Pruppacher,
2001: A wind tunnel study of turbulence effects on the scavenging of aerosol particles by water
drops. J. Atmos. Sci., 58, 3064-3072.
Weil, J.C., R. P. Lawson, and A.R. Rodi, 1993: Relative dispersion of ice crystals in seeded
cumuli. J. Appl. Met., 32, 1055-1068.