In-situ Observations of Ice Crystal Habits, Their Surface Roughness and Light Scattering Properties During ACRIDICON-CHUVA

Cirrus clouds in the tropics are important for the radiative balance of the Earth since up to 70% of tropics are covered with cirrus cloud systems (e.g. Nazaryan et al., 2008; Guignard et al., 2012). Convection plays an important role in the formation of ice crystals and their transportation to upper troposphere. To study the microphysics of these convective outflow systems in-situ, the German-Brazilian cooperative aircraft campaign ACRIDICON-CHUVA on the German research aircraft HALO was conducted over the Amazonas in September/October 2014.

During the campaign, we operated the novel Particle Habit Imaging and Scattering Probe (PHIPS-HALO), which simultaneously takes stereoscopic images of individual ice crystals and measures their scattering phase function. The stereoscopic images were used to identify the ice crystals habits in convective outflows and in-situ cirrus. Additionally, the surface roughness and crystal complexity of small (<50 μm) ice crystals were determined from SID3 diffraction patterns. We found a clear separation of the ice particle habits in convective anvils and in-situ cirrus. The dominant ice particle habit in anvils were plates and aggregates of plates, indicating the growth of the ice particles had taken place in the convective cells at temperatures around -20°C, whereas the in-situ cirrus ice particle habits were mainly columns and bullet rosettes that were grown under cold cirrus temperatures. We frequently found high aerosol concentrations (>1000 cm-3) at the boundary layer below the convective cells. The high aerosol number concentrations correlated with a high fraction of plate-like ice particles. Generally, little or almost no indications of homogeneous freezing in the form of frozen droplets were found. Surprisingly, the scattering properties of the measured in-situ and outflow cirrus did not show difference, although the clear separation of the particle habits. We measured in all cases a smooth and featureless scattering phase function with high fraction of backward scattering. The similarity of the scattering phase function is argued to be due to high degree of surface roughness and crystal complexity observed during the campaign.

References

Guignard, A., et al. "Bulk microphysical properties of semi-transparent cirrus from AIRS: a six year global climatology and statistical analysis in synergy with geometrical profiling data from CloudSat-CALIPSO." Atmospheric Chemistry and Physics 12.1 (2012): 503-525.

Nazaryan, Hovakim, M. Patrick McCormick, and W. Paul Menzel. "Global characterization of cirrus clouds using CALIPSO data." Journal of Geophysical Research: Atmospheres (1984–2012) 113.D16 (2008).