Deformations of frozen droplets formed at −40°C

Abstract

The optical properties of cirrus clouds are frequently computed with respect to various ice‐crystal size distributions and shapes; small ice crystals are generally considered to be quasi‐spherical. This approximation can lead to significant errors in the analysis of the in‐situ microphysical data and also in the theoretical study of the radiative properties of cirrus clouds. Effective size and aspect ratio of the particles are the key parameters required to determine the optical parameters that are included in modeling and prediction of climate; their parameterizations need to be as accurate as possible. In this work, laboratory experiments were conducted to study the ice crystal shape formed from the freezing of water droplets at −40°C. Liquid and ice clouds were separately formed and sampled in a cloud chamber at −30 and −40°C, respectively. The effective water droplet diameter ranged from 8 to 20μm, while the effective ice particle diameter ranged from 8 to 30 μm. The average effective diameters were 11 μm for water drops and 14 μm for frozen droplets. The deformation of the frozen droplets was evident during inspection under microscope; bulges and spikes protuberances were found in many of the observed ice particles. The results show that the aspect ratio of the frozen droplets is 1.2.