Search for: All records

Abstract The mechanisms controlling ice crystal growth rates at lowtemperature (T< −40°C) are relatively unknown. A new thermal-gradient diffusion chamber was developed to capture high-resolution images of ice crystals growing from a substrate with minimal vapor competition or shadowing. Time series of dimensional growth rates of columnar ice crystals at cirrus-like temperatures (−67 to −46°C) and moderate to high supersaturation (28 to 80 %) were determined from these images. Results show that growth rates of both primary facet dimensions (aandc) decrease over about the first hour of each experiment, but asymptotically approach constant values. Thea-dimension growth rate is well correlated with the environmental conditions, declining with decreasing temperature and increasing supersaturation. In contrast,c-dimension growth rates from individual experiments are not correlated with temperature and slightly correlated with supersaturation. Together, these trends produce aspect ratios that approach constant values that are negatively correlated with temperature. The ratio of the asymptotic growth rates (dc/da) is tightly correlated with the aspect ratio (ø = c/a), which supports the predictions of crystal growth theory assuming that steps nucleate near facet edges. In contrast, predictions from capacitance theory are not consistent with the measurements. 

Abstract All cloud and climate models assume ice crystals grow as if they were formed from pure water, even though cloud and haze droplets are solutions. The freezing process of a solution droplet is different than that of a pure water droplet, as shown in prior work. This difference can potentially affect the particle’s subsequent growth as an ice crystal. We present measurements of ice crystal growth from frozen sodium chloride (NaCl) solution droplets in the button electrode levitation diffusion chamber at temperatures between −61° and −40°C. Measured scattering patterns show that concentrated solution droplets remain unfrozen with classical scattering fringes until the droplets freeze. Upon freezing, the scattering patterns become complex within 0.1 s, which is in contrast with frozen pure water particles that retain liquid-like scattering patterns for about a minute. We show that after freezing, solution particles initially grow as spherical-like crystals and then transition to faster growth indicative of a morphological transformation. The measurements indicate that ice formed from solution droplets grows differently and has higher growth rates than ice formed from pure water droplets. We use these results to develop a power-law-based parameterization that captures the supersaturation and mass dependencies. 

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

There are few measurements of the vapor growth of small ice crystals at temperatures below -30°C. Presented here are mass-growth measurements of heterogeneously and homogeneously frozen ice particles grown within an electrodynamic levitation diffusion chamber at temperatures between -44 and -30°C and supersaturations ( s i ) between 3 and 29%. These growth data are analyzed with two methods devised to estimate the deposition coefficient ( α) without the direct use of s i . Measurements of s i are typically uncertain, which has called past estimates of α into question. We find that the deposition coefficient ranges from 0.002 to unity and is scattered with temperature, as shown in prior measurements. The data collectively also show a relationship between α and s i , with α rising (falling) with increasing s i for homogeneously (heterogeneously) frozen ice. Analysis of the normalized mass growth rates reveals that heterogeneously-frozen crystals grow near the maximum rate at low s i , but show increasingly inhibited (low α) growth at high s i . Additionally, 7 of the 17 homogeneously frozen crystals cannot be modeled with faceted growth theory or constant α. These cases require the growth mode to transition from efficient to inefficient in time, leading to a large decline in α. Such transitions may be, in part, responsible for the inconsistency in prior measurements of α.