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Microphysical characteristics of frozen droplet aggregates from deep convective clouds
Um, Junshik,McFarquhar, Greg M.,Stith, Jeffrey L.,Jung, Chang Hoon,Lee, Seoung Soo,Lee, Ji Yi,Shin, Younghwan,Lee, Yun Gon,Yang, Yiseok Isaac,Yum, Seong Soo,Kim, Byung-Gon,Cha, Joo Wan,Ko, A-Reum Copernicus GmbH 2018 Atmospheric Chemistry and Physics Vol.18 No.23
<P>Abstract. During the 2012 Deep Convective Clouds and Chemistry (DC3) experiment the National Science Foundation/National Center for Atmospheric Research Gulfstream V (GV) 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 GV aircraft recorded images of ice crystals at altitudes of 12.0 to 12.4 km and temperatures (T) from −61 to −55 ∘C. A total of 22 393 CPI crystal images were analyzed, all with maximum dimension (Dmax)<433 µm and with an average Dmax of 80.7±45.4 µm. 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 with an average of 4.7±5.0 element frozen droplets. The frequency of occurrence gradually decreased with the number of 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 decreased with an increase in the number of element frozen droplets, with larger FDAs with more element frozen droplets having more compact shapes. The Df of FDAs was about 1.20-1.43 smaller than that of black carbon (BC) aggregates (1.53-1.85) determined in previous studies. Such a 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 implications for improving the calculations of the microphysical (e.g., fall velocity) and radiative (e.g., asymmetry parameter) properties of ice crystals in upper anvil clouds. </P>
비구형 빙정의 단일산란 특성 계산: 물리적으로 일관된 구름 미세물리와 복사를 향하여
엄준식(Junshik Um),장성현(Seonghyeon Jang),김정규(Jeonggyu Kim),박성민(Sungmin Park),정희정(Heejung Jung),한수지(Suji Han),이윤서(Yunseo Lee) 한국기상학회 2021 대기 Vol.31 No.1
The impacts of ice clouds on the energy budget of the Earth and their representation in climate models have been identified as important and unsolved problems. Ice clouds consist almost exclusively of non-spherical ice crystals with various shapes and sizes. To determine the influences of ice clouds on solar and infrared radiation as required for remote sensing retrievals and numerical models, knowledge of scattering and microphysical properties of ice crystals is required. A conventional method for representing the radiative properties of ice clouds in satellite retrieval algorithms and numerical models is to combine measured microphysical properties of ice crystals from field campaigns and pre-calculated single-scattering libraries of different shapes and sizes of ice crystals, which depend heavily on microphysical and scattering properties of ice crystals. However, large discrepancies between theoretical calculations and observations of the radiative properties of ice clouds have been reported. Electron microscopy images of ice crystals grown in laboratories and captured by balloons show varying degrees of complex morphologies in sub-micron (e.g., surface roughness) and super-micron (e.g., inhomogeneous internal and external structures) scales that may cause these discrepancies. In this study, the current idealized models representing morphologies of ice crystals and the corresponding numerical methods (e.g., geometric optics, discrete dipole approximation, T-matrix, etc.) to calculate the single-scattering properties of ice crystals are reviewed. Current problems and difficulties in the calculations of the single-scattering properties of atmospheric ice crystals are addressed in terms of cloud microphysics. Future directions to develop physically consistent ice-crystal models are also discussed.
항공기 구름 관측에 사용되는 전방산란 관측 기기의 정확도 향상을 위한 구름입자의 광학적 특성 계산
엄준식(Junshik Um) 한국기상학회 2020 대기 Vol.30 No.1
Current in-situ airborne probes that measure the sizes of ice crystals smaller than 50㎛ are based on the concept that the measured intensity of light scattered by a particle in the forward and/or backward direction can be converted to particle size. The relationship between particle size and scattered light used in forward scattering probes is based on Mie theory, which assumes the refractive index of particle is known and all particles are spherical. Not only are small crystals not spherical, but also there are a wide variety of non-spherical shapes. Although it is well known that the scattering properties of non-spherical ice crystals differ from those of spherical shapes, the impacts of non-sphericity on derived in-situ particle size distributions are unknown. Thus, precise relationships between the intensity of scattered light and particle size and shape are required, as based on accurate calculations of scattering properties of ice crystals. In this study, single-scattering properties of ice crystals smaller than 50㎛ are calculated at a wavelength of 0.55㎛ using a numerically exact method (i.e., discrete dipole approximation). For these calculations, hexagonal ice crystals with varying aspect ratios are used to represent the shapes of natural small ice crystals to determine the errors caused by non-spherical ice crystals measured by forward scattering probes. It is shown that the calculated errors in sizing non-spherical ice crystals are at least 13% and 26% in forward (4~12°) and backward (168~176°) directions, respectively, and maximum errors are up to 120% and 132%.