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Abstract Prediction of ice formation in clouds presents one of the grand challenges in the atmospheric sciences. Immersion freezing initiated by ice-nucleating particles (INPs) is the dominant pathway of primary ice crystal formation in mixed-phase clouds, where supercooled water droplets and ice crystals coexist, with important implications for the hydrological cycle and climate. However, derivation of INP number concentrations from an ambient aerosol population in cloud-resolving and climate models remains highly uncertain. We conducted an aerosol–ice formation closure pilot study using a field-observational approach to evaluate the predictive capability of immersion freezing INPs. The closure study relies on collocated measurements of the ambient size-resolved and single-particle composition and INP number concentrations. The acquired particle data serve as input in several immersion freezing parameterizations, which are employed in cloud-resolving and climate models, for prediction of INP number concentrations. We discuss in detail one closure case study in which a front passed through the measurement site, resulting in a change of ambient particle and INP populations. We achieved closure in some circumstances within uncertainties, but we emphasize the need for freezing parameterization of potentially missing INP types and evaluation of the choice of parameterization to be employed. Overall, this closure pilot study aims to assess the level of parameter details and measurement strategies needed to achieve aerosol–ice formation closure. The closure approach is designed to accurately guide immersion freezing schemes in models, and ultimately identify the leading causes for climate model bias in INP predictions.
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Time-dependent freezing rate parcel model
The time-dependent freezing rate (TDFR) model here described represents the formation of ice particles by immersion freezing within an air parcel. The air parcel trajectory follows an adiabatic ascent and includes a period in time when the parcel remains stationary at the top of its ascent. The description of the ice nucleating particles (INPs) in the air parcel is taken from laboratory experiments with cloud and precipitation samples and is assumed to represent the INP content of the cloud droplets in the parcel. Time dependence is included to account for variations in updraft velocity and for the continued formation of ice particles under isothermal conditions. The magnitudes of these factors are assessed on the basis of laboratory measurements. Results show that both factors give rise to three-fold variations in ice concentration for a realistic range of the input parameters. Refinements of the parameters specifying time dependence and INP concentrations are needed to make the results more specific to different atmospheric aerosol types. The simple model framework described in this paper can be adapted to more elaborate cloud models. The results here presented can help guide decisions on whether to include a time-dependent ice nucleation scheme or a simpler singular description in models.
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- Award ID(s):
- 1034858
- PAR ID:
- 10033868
- Date Published:
- Journal Name:
- Atmospheric Chemistry and Physics
- Volume:
- 15
- Issue:
- 4
- ISSN:
- 1680-7324
- Page Range / eLocation ID:
- 2071 to 2079
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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null (Ed.)Abstract. Glaciation in mixed-phase clouds predominantly occurs through theimmersion-freezing mode where ice-nucleating particles (INPs) immersedwithin supercooled droplets induce the nucleation of ice. Modelrepresentations of this process currently are a large source of uncertaintyin simulating cloud radiative properties, so to constrain these estimates,continuous-flow diffusion chamber (CFDC)-style INP devices are commonly usedto assess the immersion-freezing efficiencies of INPs. This study explored anew approach to operating such an ice chamber that provides maximumactivation of particles without droplet breakthrough and correction factorambiguity to obtain high-quality INP measurements in a manner thatpreviously had not been demonstrated to be possible. The conditioningsection of the chamber was maintained at −20 ∘C and water relative humidity (RHw) conditions of 113 % to maximize the droplet activation,and the droplets were supercooled with an independentlytemperature-controlled nucleation section at a steady cooling rate(0.5 ∘C min−1) to induce the freezing of droplets andevaporation of unfrozen droplets. The performance of the modified compactice chamber (MCIC) was evaluated using four INP species: K-feldspar,illite-NX, Argentinian soil dust, and airborne soil dusts from an arableregion that had shown ice nucleation over a wide span of supercooledtemperatures. Dry-dispersed and size-selected K-feldspar particles weregenerated in the laboratory. Illite-NX and soil dust particles were sampledduring the second phase of the Fifth International Ice Nucleation Workshop(FIN-02) campaign, and airborne soil dust particles were sampled from anambient aerosol inlet. The measured ice nucleation efficiencies of modelaerosols that had a surface active site density (ns) metric were higher but mostly agreed within 1 order of magnitude compared to results reported in the literature.more » « less
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Abstract Ice‐nucleating particles (INPs) play a key role in ice formation and cloud microphysics and thus significantly impact the water cycle and the climate. However, our understanding of atmospheric INPs, particularly their sources, emissions, and spatiotemporal variability, is incomplete. While the enhancement of atmospheric INP concentrations with rainfall has been previously shown, a mechanistic understanding of the process is lacking. Here, we link detailed precipitation observations with near‐surface atmospheric INP concentrations at a semiarid grassland site in Colorado. Considering the during‐precipitation air samples, INP concentrations positively correlate with cumulative rainfall kinetic energy and amount, suggesting that INP aerosolization is induced by raindrop and hailstone impact. By additionally analyzing the INP content of precipitation water, terrestrial source samples, and heat‐treated samples, we demonstrate that local plants are the most plausible source of rain‐induced INPs during a precipitation event. Should INPs aerosolized by precipitation rise to cloud height, they could influence cloud ice fraction and initiate precipitation resulting in an aerosol‐cloud‐precipitation feedback.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.5194/acp-15-2071-2015
