Wednesday, 9 July 2014
Single-particle images collected in mid-latitude cirrus during the Small Particles in Cirrus (SPartICus) campaign are analyzed to derive internally consistent and observationally-constrained representations of ice physical and optical properties for a size-resolved (bin) microphysics model. Using measurements gathered during flights through widespread synoptic cirrus, bullet rosettes are found to be the dominant identifiable (pristine) habit among particles larger than 100 µm in randomly-oriented maximum dimension. Properties are therefore derived for bullet rosettes and aggregates of bullet rosettes based on measurements of arm lengths and widths using the Ice Crystal Ruler software developed at the University of Illinois (available for rosettes larger than roughly 200 µm in maximum dimension). It is also assumed that smaller bullet rosettes possess the same number of arms as larger rosettes or else are budding buckyballs with many more arms. Parcel model results for early ice particle size distribution evolution are sensitive to the treatment of ice properties, which remain poorly constrained by the single-particle images. Properties are then approximated for in situ cirrus habits that are categorized as unclassifiable (non-pristine), which represent more than 80% of particles larger than 100 µm. It is found that bullet rosettes falling through deep ice-supersaturated layers enter a plate growth regime. Side planes grow preferentially on bullet sides, therefore tending to increase particle mass without substantial increase in particle maximum dimension, in addition to creating a mixture of plate and column elements that influence particle optical properties. In a deep ice-subsaturated layer, evaporation leads to a quasi-spherical shape commonly at 100-200 µm in maximum dimension.
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