The Survival of Ice Particles Falling from Cirrus Clouds in Subsaturated Air

Abstract

A theoretical study has been carried out to determine the relevant microphysical processes which control the survival distance of ice particles failing from cirrus clouds in subsaturated air, and to determine the atmospheric conditions which are necessary for such particles to ?seed? lower level supercooled clouds and thereby initiate glaciation. Differential equations were developed which describe the heat and mass transfer during the evaporation of cirrus ice particles. In these equations forced convection and kinetic effects were included. Spherical, columnar and plate-like ice particles were considered. The effect of radiative heat exchange between an ice particle and its environment was studied in terms of maximum and minimum physical limits for the upward and downward radiation fluxes. Using these limits and the known emission and absorption Properties of ice, we concluded that radiative heat transfer changes the survival distance of columnar ice crystals falling from cirrus clouds by less than 10% if the relative humidity of the environmental air is less than 70%. Considering the radiative effects and a wide range of values for the initial size and ice particle bulk density, and for the temperature and humidity conditions of the ambient air, the present theoretical model showed that ice particles could survive distances of up to 2 km when the relative humidity with respect to ice was below 70% in a typical mid-latitude atmosphere. Larger survival distances are only possible if the ambient air has relative humidities larger than 70%. The theoretical model is compared to several field observations on evaporating cirrus ice particles. Good agreement was found with observational data when the atmospheric temperature and humidity profiles were available for the site at which the ice particles were sampled.