Entropic Aspects of Supercooled Droplet Freezing

Abstract

The freezing of supercooled water droplets in the atmosphere, with an emphasis on the entropic aspects of the problem, is examined. Supercooled water is a metastable state and, therefore, the associated phase transition must be irreversible. Temperature-dependent heat capacities of supercooled water and ice are used to calculate the entropy difference. That difference is then used to establish a lower bound on the amount of latent heat that can be liberated by the freezing droplets. The calculation is compared with tabulated values of the latent heat of fusion with surprising results. Based on a novel physical picture of the freezing process, the authors suggest a simple estimate for the effective latent heat that is suitable for heat budget calculations of glaciating clouds. In addition, the authors arrive at a quadratic dependence on supercooling, (?T)2, for the irreversible contribution to heat exchange during the freezing process. The proportionality factor is estimated as ?0.3 J mol?1 K?2.