Effective Density Derived from Laboratory Measurements of the Vapor Growth Rates of Small Ice Crystals at 2658 to 2408C

An electrodynamic levitation thermal-gradient diffusion chamber was used to grow 268 individual, small ice particles (initial radii of 8–26 mm) from the vapor, at temperatures ranging from 2658 to 2408C, 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 reff. We fit the measured time series with two different models to estimate size-dependent r_eff values: the first model decreases reff to an asymptotic deposition density r_dep, and the second models r_eff by a power law with exponent P. Both methods produce similar results, though the fits with r_dep 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 r_dep and P values show a clear dependence on ice supersaturation normalized to liquid saturation. We use these relative frequencies of r_dep and P to derive two supersaturation-dependent mass–size relationships suitable for cloud modeling applications.