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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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No anomalous supersaturation in ultracold cirrus laboratory experiments
Abstract. High-altitude cirrus clouds are climatically important: their formationfreeze-dries air ascending to the stratosphere to its final value, and theirradiative impact is disproportionately large. However, their formation andgrowth are not fully understood, and multiple in situ aircraft campaigns haveobserved frequent and persistent apparent water vapor supersaturations of5 %–25 % in ultracold cirrus (T<205 K), even in the presence of iceparticles. A variety of explanations for these observations have been putforth, including that ultracold cirrus are dominated by metastable ice whosevapor pressure exceeds that of hexagonal ice. The 2013 IsoCloud campaign atthe Aerosol Interaction and Dynamics in the Atmosphere (AIDA) cloud andaerosol chamber allowed explicit testing of cirrus formation dynamics atthese low temperatures. A series of 28 experiments allows robust estimationof the saturation vapor pressure over ice for temperatures between 189 and235 K, with a variety of ice nucleating particles. Experiments are rapidenough (∼10 min) to allow detection of any metastable ice that mayform, as the timescale for annealing to hexagonal ice is hours or longer overthe whole experimental temperature range. We show that in all experiments,saturation vapor pressures are fully consistent with expected values forhexagonal ice and inconsistent with the highest values postulated formetastable ice, with no temperature-dependent deviations from expectedsaturation vapor pressure. If metastable ice forms in ultracold cirrusclouds, it appears to have a vapor pressure indistinguishable from that ofhexagonal ice to within about 4.5 %.
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- Award ID(s):
- 1743753
- PAR ID:
- 10284991
- Date Published:
- Journal Name:
- Atmospheric Chemistry and Physics
- Volume:
- 20
- Issue:
- 2
- ISSN:
- 1680-7324
- Page Range / eLocation ID:
- 1089 to 1103
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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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.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.5194/acp-20-1089-2020
