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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
https://doi.org/10.5194/acp-2018-5
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
Research article
02 Feb 2018
Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).
Microphysical Characteristics of Frozen Droplet Aggregates from Deep Convective Clouds
Junshik Um1,2, Greg M. McFarquhar2,3,4, Jeffrey L. Stith4, Chang Hoon Jung5, Seoung Soo Lee6, Ji Yi Lee7, Younghwan Shin8, and Yun Gon Lee9 1Department of Atmospheric Sciences, Pusan National University, Busan, South Korea
2Cooperative Institute for Mesoscale Meteorological Studies, University of Oklahoma, Norman, Oklahoma, USA
3School of Meteorology, University of Oklahoma, Norman, Oklahoma, USA
4National Center for Atmospheric Research, Boulder, Colorado, USA
5Department of Health Management, Kyungin Women’s University, Incheon, South Korea
6Earth System Science Interdisciplinary Center, College Park, Maryland, US
7Department of Environmental Engineering, Chosun University, Gwangju, South Korea
8Department of Agricultural and Biological Engineering, University of Illinois, Urbana, Illinois, USA
9Department of Atmospheric Sciences, Chungnam National University, Daejeon, South Korea
Abstract. During the 2012 Deep Convective Clouds and Chemistry (DC3) experiment the National Science Foundation/National Center for Atmospheric Research Gulfstream-V (G-V) aircraft sampled the upper anvils of two storms that developed in eastern Colorado on 6 June 2012. A cloud particle imager (CPI) mounted on the G-V aircraft recorded images of ice crystals at altitudes of 12.0–12.4 km and T = −61– −55 °C. A total of 22,393 CPI crystal images were analyzed, all with maximum dimension Dmax < 433 m with an average Dmax of 80.7 ± 45.4 m. The occurrence of well-defined pristine crystals (e.g., columns and plates) was less than 0.04 % by number. Single frozen droplets and frozen droplet aggregates (FDAs) were the dominant habits with fractions of 73.0 % (by number) and 46.3 % (by projected area), respectively. The relative frequency of occurrence of single frozen droplets and FDAs depended on temperature and position within the anvil cloud.

A new algorithm that uses the circle Hough transform technique was developed to automatically identify the number, size, and relative position of element frozen droplets within FDAs. Of the FDAs, 42.0 % had two element frozen droplets and the frequency of occurrence decreasing as the number of element frozen droplets increasing with the average of 4.70 ± 5.0 element frozen droplets. Based on the number, size, and relative position of the element frozen droplets within the FDAs, possible three-dimensional (3-D) realizations of FDAs were generated and characterized by two different shape parameters, the aggregation index (AI) and the fractal dimension (Df), that describe 3-D shapes and link to scattering properties with an assumption of spherical shape of element frozen droplets. The AI of FDAs is seen to decrease with an increase in the number of element frozen droplets, with larger FDAs with more element frozen droplets have more compact shapes. It is shown that Df of FDAs is about 1.20–1.43 smaller than that of BC aggregates (1.53–1.85) determined in previous studies. Such smaller Df of FDAs indicates that FDAs have more linear chain-like branched shapes than the compact shapes of BC aggregates. Determined morphological characteristics of FDAs along with the proposed reconstructed 3-D representations of FDAs in this study have important implication to improve the calculations of the microphysical (e.g., fall velocity) and radiative (e.g., asymmetry parameter) properties of ice crystals in upper anvil clouds.


Citation: Um, J., McFarquhar, G. M., Stith, J. L., Jung, C. H., Lee, S. S., Lee, J. Y., Shin, Y., and Lee, Y. G.: Microphysical Characteristics of Frozen Droplet Aggregates from Deep Convective Clouds, Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2018-5, in review, 2018.
Junshik Um et al.
Junshik Um et al.
Junshik Um et al.

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Short summary
During the 2012 Deep Convective Clouds and Chemistry experiment upper anvils of two storms were sampled. The occurrence of well-defined pristine crystals was low in the anvils, while single frozen droplets and frozen droplet aggregates (FDAs) were the dominant habits. New algorithm was developed to automatically identify the number, size, and relative position of element frozen droplets within FDAs. The morphological characteristics of FDAs were compared with those of black carbon aggregates.
During the 2012 Deep Convective Clouds and Chemistry experiment upper anvils of two storms were...
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