Microphysics of Frozen Droplets Formed in Deep Convective Clouds

Deep convective cloud systems are an important source of ice particles in the upper troposphere. Field measurements have shown that single frozen droplets are the dominant particle type in the anvils and in the upper parts of these cloud systems (e.g. Stith et al., 2013). Although frozen droplets are frequently measured, our understanding of the microphysical and optical properties of these ice particles is weak. The resolution of the ice particle imaging used in aircraft measurements is not detailed enough to give information on the deviation of frozen droplets from the perfect spherical geometry. Microscopic structures, like surface roughness, as well as detailed information on the aspect ratios of the frozen droplets found in deep convective clouds are key parameters required to determine the optical parameters that are included in the modeling and prediction of the climate effect of these cloud systems.

There is evidence (e.g. Rosenfeld and Woodley, 2000) that homogeneous freezing of droplets is an important source of ice crystals in the upper regions of deep convective systems. We simulated the homogeneous freezing process taking part in these clouds in cloud chamber studies. These studies were conducted at the AIDA cloud chamber located in Karlsruhe and at the CERN CLOUD chamber. In all of the studies we started with water subsaturated conditions at -30°C. Supercooled liquid water droplets with maximum sizes of 20 μm were formed by activating sulphuric acid solution particles at water saturated conditions, which were reached by expansion cooling. By further cooling of the chamber volume, the homogeneous nucleation temperature of pure water of -36°C was reached and all the supercooled liquid droplets were turned into frozen droplets with sizes ranging from 6-50 μm. The size distributions as well as two-dimensional diffraction patterns of the cloud particles were measured with a Particle Phase Discriminator (PPD, Kaye et al., 2008). We analyzed the diffraction patterns of the cloud particles to classify them based on shape (spherical or non-spherical particles) and surface roughness (smooth or rough).

All of the frozen droplets developed rough surface features almost immediately upon freezing. We furthermore studied the development of the surface roughness, when the ice particles were in ice sub-saturated conditions. These ice sub-saturated conditions are found in the outflow regions of the deep convective clouds. We detected that the rough surface features were smoothened out in the sublimation process, in a way that at the end of the sublimation cycle 50% of the ice particles were classified as smooth spheres. The smooth frozen droplets showed diffraction patterns that differed slightly from diffraction patterns of perfect spheres indicating that the liquid droplets were deformed in the freezing process. The aspect ratios of the frozen droplets were measured to frequently deviate from 1.

Additionally to chamber measurements, we measured frozen droplets during MACPEX aircraft studies. The results from the aircraft campaign will be presented and discussed in the context of what we learned from chamber studies.