Effective Density Derived from Laboratory Measurements of the Vapor Growth Rates of Small Ice Crystals at −65° to −40°C

Abstract

Abstract An electrodynamic levitation thermal-gradient diffusion chamber was used to grow 268 individual, small ice particles (initial radii of 8–26 μm) from the vapor, at temperatures ranging from −65° to −40°C, and supersaturations up to liquid saturation. Growth limited by attachment kinetics was frequently measured at low supersaturation, as shown in prior work. At high supersaturation, enhanced growth was measured, likely due to the development of branches and hollowed facets. The effects of branching and hollowing on particle growth are often treated with an effective density ρeff. We fit the measured time series with two different models to estimate size-dependent ρeff values: the first model decreases ρeff to an asymptotic deposition density ρdep, and the second models ρeff by a power law with exponent P. Both methods produce similar results, though the fits with ρdep typically have lower relative errors. The fit results do not correspond well with models of isometric or planar single-crystalline growth. While single-crystalline columnar crystals correspond to some of the highest growth rates, a newly constructed geometric model of budding rosette crystals produces the best match with the growth data. The relative frequency of occurrence of ρdep and P values show a clear dependence on ice supersaturation normalized to liquid saturation. We use these relative frequencies of ρdep and P to derive two supersaturation-dependent mass–size relationships suitable for cloud modeling applications.