Surface Roughness of Small Cirrus Ice Particles

The microphysical properties of ice crystals play an important role in the radiative impact of cirrus clouds as they determine the angular single scattering phase function of the particles and in this way the shortwave bulk optical properties of the clouds. In order to compile representative parameterizations of single scattering properties of natural ice particles, detailed in situ microphysical and optical measurements are required which are also prerequisites for the evaluation of remote sensing measurements. Such in situ investigations are mostly restricted to particles larger than about 50 μm, mainly because of optical resolution limitations and potential impacts by ice crystal breakup on the probe inlets. Therefore, the microphysical properties of sub-50 µm cirrus crystals are still poorly understood. In recent years, satellite observations have provided indirect evidence for i) small ice particles and ii) highly distorted ice crystal structures with hollowness and surface roughness [1]. There are a few reports on crystal distortion and hollowness observed for small ice particles in ice replicas from cirrus clouds [2]. In case of crystal surface roughness, recent measurements of spatial light scattering patterns of individual ice particles by the Small Ice Detector 3 (SID3) indicate prevailing surface roughness or highly distorted ice crystals in mid-latitude cirrus clouds [3]. In this contribution cloud chamber studies of the microphysical properties of small cirrus ice particles are discussed. The SID3 method was applied in these experiments to investigate the dependence of ice particle surface roughness or crystal distortion on the prevailing thermodynamic conditions. Clear correlations between surface roughness and ice supersaturation by temperature were observed which could be explained by different ice crystal growth rates. SID3 measurements during the MACPEX and ML-CIRRUS aircraft studies are presented and discussed in the context of the results from cloud chamber measurements.

References

[1] B. Cole, P. Yang, B. A. Baum, J. Riedi, L. Labonnote, F. Thieuleux, and S. Platnick, J. Appl. Meteor. Clim. 52, 186 (2013)

[2] C. G. Schmitt and A. J. Heymsfield, J. Atmos. Sci. 64, 4514 (2007)

[3] Z. Ulanowski, P. H. Kaye, E. Hirst, R. S. Greenaway, R. J. Cotton, E. Hesse, and C. T. Collier, Atmos. Chem. Phys. 14, 1649 (2014)