Rates of phase transformations in mixed-phase clouds

This paper presents a theoretical framework describing the thermodynamics and phase transformations of a three phase component system consisting of ice particles, liquid droplets and water vapour. The instant rates of change of water ((q)overdot(w)), ice ((q)overdot(i)) and vapour ((q)overdot(v)) mixing ratios are described based on the quasi-steady approximation. The local thermodynamic conditions required for the equilibrium of liquid ((q)overdot(w) = 0), ice ((q)overdot(i) = 0) and vapour ((q)overdot(v) = 0) phases are analysed. It is shown that there are four different regimes of the partitioning of water between liquid, ice, and gaseous phases in mixed clouds. The Wegener-Bergeron-Findeisen (WBF) process is identified as being relevant to two of those regimes. The efficiency of the WBF process in characterizing the capability of ice crystals to deplete the water evaporated from liquid droplets is introduced here. It is shown that the WBF process has maximum efficiency at approximately zero vertical velocity. The analysis of the dependences of. (q)overdot(w) (q)overdot(i) and (q)overdot(v) on the vertical velocity, temperature, pressure and the integral radii of the cloud particles is presented. It is shown that the maximum rates of ice growth and droplet evaporation do not necessarily occur at T = -12 degrees C where the maximum difference between saturation vapour pressure over ice and that over liquid is observed. Copyright (C) 2008 Crown in the right of Canada. Published by John Wiley & Sons, Ltd.