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The importance of cloud droplet activation in warm and mixed phase clouds
[Thesis]. Manchester, UK: The University of Manchester; 2017.
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Abstract
This thesis investigates the use of aerosol activation as the determining factor between an aerosol particle and a cloud droplet numerical models. The formation of cloud droplets occurs through the condensation of water vapour on ambient aerosol particles. The point at which an aerosol particle becomes a cloud droplet is called activation, and is calculated according to Kohler (or similar) theory. In this work the current representation of cloud droplet activation in state-of-the-art climate and weather models is evaluated using a detailed cloud parcel model. It is found that there is a systematic tendency to over estimate the number of activated droplets when the median aerosol particle diameter is relatively large. It is also demonstrated that the most popular method to representing cloud droplet activation in climate models is not however the most accurate approach available. Although the calculation of cloud droplet activation is important in accurately determining cloud droplet number concentration in weather and climate simulations, this work demonstrates a newly identified aerosol-cloud interaction in which the growth of aerosol particles before activation is the determining factor. It is shown that the competition for water vapour between ambient aerosol particles can lead to the suppression of ice formation by limiting the ability of ice nucleating particles to activate into cloud droplets and therefore freeze. A common assumption in models which include state-of-the-art representation of heterogeneous ice nucleation is that immersion freezing may only occur if an ice nucleating particle has activated into a cloud droplet. Observations from cloud chamber experiments and results from a detailed cloud parcel model indicate that ice nucleation in the immersion mode may take place on ice nucleating particles before they become activated. This means that the number concentration of ice crystals may be significantly under estimated in current weather and climate model simulations where ice nucleation can only take place in activated cloud droplets. Un-activated aerosol particles are not only often excluded from ice nucleation processes but also from precipitation formation. The calculation of rain drop formation, for example by autoconversion parameterisations following Manton and Cotton (1997) or Khairoutdinov and Kogan (2000), in some models depends on the number of concentration of cloud droplets determined by cloud droplet activation. In this work un-activated aerosol particles, which are able to swell to considerable sizes before they activate, are shown to contribute significantly to collision-coalescence which leads to precipitation formation in warm clouds. Overall the work in this thesis highlights the need to consider not just activated, but also un-activated aerosol particles as cloud droplets in calculations of cloud microphysical processes in weather and climate models.