Time-dependent immersion freezing explained by stochastic nucleation processes and substrate variance
Abstract
Atmospheric immersion freezing (IF) is a dominant ice formation pathway impacting the hydrological cycle and climate. Implementation of IF derived from field and laboratory data in cloud and climate models is difficult due to the high variability in spatio-temporal scales and INP composition and morphological complexity. Furthermore, laboratory derived IF data show discrepancies in freezing rates of up to 5 orders of magnitude. Although nucleation is fundamentally a stochastic and time-dependent process, laboratory ice nucleation data are often interpreted using a time-independent concept of ice nucleation active site (INAS) densities. Isothermal (constant temperature) IF experiments clearly demonstrate a time dependency of ice nucleation. We apply isothermal and constant cooling rate IF experiments of illite dust in micrometer-sized water droplets containing different ice nucleating particle (INP) surface areas. We monitor several hundred freezing events for each different illite concentration, thus, INP surface area. Our results demonstrate that IF can be consistently described by a stochastic nucleation process accounting for uncertainties in the INP surface area. By application of a Monte Carlo simulation and a null-hypothesis experiment, we can unambiguously show that (i) the shape of the unfrozen fraction is due to variation in INP surface areas and (ii) experimentally derived variation in freezing rates are smaller than the uncertainty due to the stochastic nature of nucleation. Our findings impact the interpretation and parameterization of IF, affect experimental procedures and design for field and laboratory measurements, have ramifications for the development of a standard INP, and allow us to estimate the smallest achievable uncertainty to predict IF relevant for cloud and climate models.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.A53O3105K
- Keywords:
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- 0305 Aerosols and particles;
- ATMOSPHERIC COMPOSITION AND STRUCTURE;
- 0320 Cloud physics and chemistry;
- ATMOSPHERIC COMPOSITION AND STRUCTURE;
- 3311 Clouds and aerosols;
- ATMOSPHERIC PROCESSES