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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 Arctic single‐layer mixed‐phase clouds were studied using a one‐dimensional model that incorporated the adaptive habit growth model for ice microphysics. The base case was from the Indirect and Semidirect Aerosol Campaign, and it was perturbed over a range of cloud‐average temperatures, maximum (per model run) ice nuclei (IN) concentrations, and large‐scale subsidence velocities. For each parameter combination, the model was iterated out to 48 hr, and the time, called the glaciation time, to complete disappearance of liquid recorded if this occurred within the 48 hr. Dependence of glaciation times on cloud‐average temperatures from −30°C to −5°C, maximum IN concentrations from 0.10 to 30 L⁻¹, and strong–no subsidence, with both isometric and habit‐dependent ice crystal growth, were investigated. For isometric crystal growth, the relationship between the critical maximum IN concentration (IN_crit), the maximum (per model run) IN concentration above which a mixed‐phase cloud glaciated within a fixed model runtime, and cloud‐average temperature was monotonic. IN_critdecreased with decreasing cloud‐average temperature. Strengthening of subsidence led to a further decrease in IN_critfor every cloud‐average temperature. For habit‐dependent ice crystal growth, the relationship between IN_critand cloud‐average temperature was nonmonotonic. Ice crystals develop dendritic and columnar habits near −15°C and −7°C, respectively, and at these two temperatures, ice crystals grew and depleted supercooled liquid water faster than the case when ice crystals grew isometrically. This led to deep local minima in IN_critaround these two temperatures in the model runs. Habit‐dependent ice crystal growth, coupled with changes in cloud‐average temperature, IN_crit, and subsidence strength, led to significant changes in Arctic single‐layer mixed‐phase cloud lifetimes. 

Abstract Variability of ice microphysical properties like crystal size and density in cirrus clouds is important for climate through its impact on radiative forcing, but challenging to represent in models. For the first time, recent laboratory experiments of particle growth (tied to crystal morphology via deposition density) are combined with a state‐of‐the‐art Lagrangian particle‐based microphysics model in large‐eddy simulations to examine sources of microphysical variability in cirrus. Simulated particle size distributions compare well against balloon‐borne observations. Overall, microphysical variability is dominated by variability in the particles' thermodynamic histories. However, diversity in crystal morphology notably increases spatial variability of mean particle size and density, especially at mid‐levels in the cloud. Little correlation between instantaneous crystal properties and supersaturation occurs even though the modeled particle morphology is directly tied to supersaturation based on laboratory measurements. Thus, the individual thermodynamic paths of each particle, not the instantaneous conditions, control the evolution of particle properties. 

Abstract Aircraft measurements reveal ice supersaturation statistics in cirrus (ISSs) with broad maxima around ice saturation and pronounced variance. In this study, processes shaping ISSs in midlatitude and tropical upper tropospheric conditions are systematically investigated. Water vapor deposition and sublimation of size‐resolved ice crystal populations are simulated in an air parcel framework. Mesoscale temperature fluctuations (MTFs) due to gravity waves force the temporal evolution of supersaturation. Various levels of background wave forcing and cirrus thickness are distinguished in stochastic ensemble simulations. Kinetic limitations to ice mass growth are brought about by supersaturation‐dependent deposition coefficients that represent efficient and inefficient growth modes as a function of ice crystal size. The simulations identify a wide range of deposition coefficients in cirrus, but most values stay above 0.01 such that kinetic limitations to water uptake remain moderate. Supersaturation quenching times are long, typically 0.5–2 hr. The wave forcing thus causes a remarkably large variability in ISSs and cirrus microphysical properties except in the thickest cirrus, producing ensemble‐mean ISSs in line with in‐situ measurements. ISS variance is controlled by MTFs and increases with decreasing cirrus integral radii. In comparison, the impact of ice crystal growth rates on ISSs is small. These results contribute to efforts directed at identifying and solving issues associated with ice‐supersaturated areas and non‐equilibrium cirrus physics in global models. 

Abstract Observations and measurements show that crystals remain relatively compact at low ice supersaturations, but become increasingly hollowed and complex as the ice supersaturation rises. Prior measurements at temperatures >−25°C indicate that the transition from compact, solid ice to morphologically complex crystals occurs when the excess vapor density exceeds a threshold value of about 0.05 g m⁻³. A comparable threshold is not available at low temperatures. A temperature-dependent criterion for the excess vapor density threshold (Δρ_thr) that defines morphological transformations to complex ice is derived from laboratory measurements of vapor grown ice at temperatures below −40°C. This criterion depends on the difference between the equilibrium vapor density of liquid () and ice (ρ_ei) multiplied by a measurement-determined constant,. The new criterion is consistent with prior laboratory measurements, theoretical estimates, and it reproduces the classical result of about 0.05 g m⁻³above −25°C. Since Δρ_thrdefines the excess vapor density above which crystals transition to a morphologically complex (lower density) growth mode, we can estimate the critical supersaturation (s_crit) for step nucleation during vapor growth. The derived values ofs_critare consistent with previous measurements at temperatures above −20°C. No direct measurements ofs_critare available for temperatures below −40°C; however, our derived values suggest some measurement-based estimates may be too high while estimates from molecular dynamics simulations may be too low. 

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. 

Abstract. Ice growth from vapor deposition is an important process for the evolution of cirrus clouds, but the physics of depositional ice growth at the low temperatures (<235 K) characteristic of the upper troposphere and lower stratosphere is not well understood. Surface attachment kinetics, generally parameterized as a deposition coefficient αD, control ice crystal habit and also may limit growth rates in certain cases, but significant discrepancies between experimental measurements have not been satisfactorily explained. Experiments on single ice crystals have previously indicated the deposition coefficient is a function of temperature and supersaturation, consistent with growth mechanisms controlled by the crystal's surface characteristics. Here we use observations from cloud chamber experiments in the Aerosol Interactions and Dynamics in theAtmosphere (AIDA) aerosol and cloud chamber to evaluate surface kinetic models in realistic cirrus conditions. These experiments have rapidly changing temperature, pressure, and ice supersaturation such that depositional ice growth may evolve from diffusion limited to surface kinetics limited over the course of a single experiment. In Part 1, we describe the adaptation of a Lagrangian parcel model with the Diffusion Surface Kinetics Ice Crystal Evolution (DiSKICE) model (Zhang and Harrington, 2014) to the AIDA chamber experiments. We compare the observed ice water content and saturation ratios to that derived under varying assumptions for ice surface growth mechanisms for experiments simulating ice clouds between 180 and 235 K and pressures between 150 and 300 hPa. We found that both heterogeneous and homogeneous nucleation experiments at higher temperatures (>205 K) could generally be modeled consistently with either a constant deposition coefficient or the DiSKICE model assuming growth on isometric crystals via abundant surface dislocations. Lower-temperature experiments showed more significant deviations from any depositional growth model, with different ice growth rates for heterogeneous and homogeneous nucleation experiments.