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1. Re-evaluating cloud chamber constraints on depositional ice growth in cirrus clouds – Part 1: Model description and sensitivity tests

   https://doi.org/10.5194/acp-23-6043-2023  

   Lamb, Kara D.; Harrington, Jerry Y.; Clouser, Benjamin W.; Moyer, Elisabeth J.; Sarkozy, Laszlo; Ebert, Volker; Möhler, Ottmar; Saathoff, Harald (January 2023, Atmospheric Chemistry and Physics)

   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. 

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2. An Approximate Criterion for Morphological Transformations in Small Vapor Grown Ice Crystals

   https://doi.org/10.1175/JAS-D-23-0131.1  

   Harrington, Jerry Y; Pokrifka, Gwenore F (February 2024, Journal of the Atmospheric Sciences)

   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. 

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

   https://doi.org/10.1175/JAS-D-22-0077.1  

   Pokrifka, Gwenore F.; Moyle, Alfred M.; Harrington, Jerry Y. (February 2023, Journal of the Atmospheric Sciences)

   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. 

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4. Columnar Ice Dimensional Growth Rates at Temperatures below −40°C: Measurements in a Novel Thermal Gradient Diffusion Chamber

   https://doi.org/10.1175/JAS-D-25-0030.1  

   Pokrifka, Gwenore F; Moyle, Alfred M; Harrington, Jerry Y (August 2025, Journal of the Atmospheric Sciences)

   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. 

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5. The Causes and Consequences of Ordovician Cooling

   https://doi.org/10.1146/annurev-earth-040523-114630  

   Bergmann, Kristin D; Macdonald, Francis A; Swanson-Hysell, Nicholas L (May 2025, Annual Review of Earth and Planetary Sciences)

   A long-term cooling trend through the Ordovician Period, from 487 to 443 Ma, is recorded by oxygen isotope data. Tropical ocean basins in the Early Ordovician were hot, which led to low oxygen concentrations in the surface ocean due to the temperature dependence of oxygen solubility. Elevated temperatures also increased metabolic demands such that hot shallow water environments had limited animal diversity as recorded by microbially dominated carbonates. As the oceans cooled through the Ordovician, animal biodiversity increased, leading to the Great Ordovician Biodiversification Event. The protracted nature of the cooling suggests that it was the product of progressive changes in tectonic boundary conditions. Low-latitude arc-continent collisions through this period may have increased global weatherability and decreased atmospheric CO₂levels. Additionally, decreasing continental arc magmatism could have lowered CO₂outgassing fluxes. The Ordovician long-term cooling trend culminated with the development of a large south polar ice sheet on Gondwana. The timescale of major ice growth and decay over the final 2 Myr of the Ordovician is consistent with Pleistocene-like glacial cycles driven by orbital forcing. The short duration of large-scale glaciation indicates a high sensitivity of ice volume to temperature with a strongly nonlinear response, providing a valuable analog for Neogene and future climate change.▪Oxygen isotope data record progressive and protracted cooling through the Ordovician leading up to the onset of Hirnantian glaciation.▪The gradual cooling trend is mirrored by an Ordovician radiation in biological diversity, consistent with temperature-dependent oxygen solubility and metabolism as a primary control.▪Long-term cooling occurred in concert with low-latitude arc-continent collisions and an increase in global weatherability. Although CO₂outgassing may have also decreased with an Ordovician decrease in continental arc length, in the modern, CO₂outgassing is variable along both continental and island arcs, leaving the relationship between continental arc length and climate uncertain.▪Evidence for significant ice growth is limited to less than 2 Myr of the Hirnantian Stage, suggesting a high sensitivity of ice growth topCO₂and temperature.▪Independent estimates for ice volume, area, and sea level change during the Hirnantian glacial maximum are internally consistent and comparable to those of the Last Glacial Maximum. 

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