Single‐scattering properties of aggregates of plates

Abstract

During the 2006 Tropical Warm Pool International Cloud Experiment (TWP-ICE) sponsored by the Atmospheric Radiation Measurement programme, the Scaled Composites Proteus aircraft executed spiral profiles and flew horizontal legs through aging anvils, fresh anvils, and cirrus of unknown origin in the vicinity of Darwin, Australia. Pristine ice crystals including both plates and bullet rosettes, their aggregates, and unclassifiable ice crystals were observed by a Cloud Particle Imager. The widths of observed plates ranged between 80 µm and 200 µm. When a fresh dissipating anvil was observed on 2 February, aggregates of plates contributed 46.2% of the total area of measured ice crystals with maximum dimensions greater than 200 µm, while it was only 7.2% and 1.0% for 27 and 29 January, respectively, when aged cirrus was sampled. Because aggregates of plates have been observed to make large contributions to projected ice crystal area near convection during TWP-ICE and other projects, their single-scattering properties that have not been previously examined are investigated here.The dependence of the scattering phase function P 11 , asymmetry parameter g, and single-scattering albedo ω 0 on three parameters (the area ratio AR, normalized projected area A n , and a newly defined aggregation index AI which varies between 0 and 1 with ice crystals with more compact shape having lower AI ) defining the three-dimensional shapes of aggregates of plates were calculated using a geometric ray-tracing code at wavelengths λ of 0.55, 1.38 and 2.13 µm. The scattering properties depended on the crystal morphology with, for example, the g of an aggregate of plates with a high AI of 0.818 differing by +6.89% (+6.44%; +4.55%) from that with a lower AI of 0.378 at λ of 0.55 µm (1.38 µm; 2.13 µm), but by only +0.29% (+0.25%; −0.03%) from those of the component plates. The ω 0 at absorbing λ increased with AI, 1 − AR, and A n . Adding ray distortion to the aggregates caused a decrease in forward scattering and an increase in lateral and backward scattering, decreasing g, an effect that weakened with λ due to absorption. The aggregates of plates with low AI were more influenced by ray distortion compared to those with high AI. The dependence of scattering properties on crystal morphology noted here should be considered when computing bulk scattering properties of ice clouds to determine its importance for climate and remote-sensing studies.

Dimensions and aspect ratios of natural ice crystals

“…The average D (µm) of crystals (columns; plates; bullets) in non-anvil clouds (154.0; 122.3; 132.6) are larger than those in anvil clouds (109.8; 104.0; 99.9) during TWP-ICE, whereas those in anvil clouds (187.2; 117.5; 158.5) are larger than those in non-anvil clouds (158.1; 116.3; 132.8) during SPARTICUS at −67 < T < −35 • C. shows a strong dependence on the type of cirrus, and is much higher in anvil clouds compared to non-anvil clouds for TWP-ICE and SPARTICUS, which agrees with previous studies (e.g., Connolly et al, 2005;Um and McFarquhar, 2009;Lawson et al, 2010). In general, crystal dimensions are found to increase with temperature (e.g., Fig.…”

“…During TWP-ICE, Um and McFarquhar (2009) showed much higher occurrence of plates and their aggregates on 2 February compared with the other 2 days, 27 and 29 January, when aged cirrus were sampled. The analyzed plates at T <∼ −55 • C were sampled within convective anvils on 2, 6, 10, and 12 February during TWP-ICE (see Table 4).…”

“…The ice crystals imaged by the CPI were classified using the habit classification scheme described by Um and McFarquhar (2009). Ice crystals classified as columns, plates, and bullet rosettes were then visually inspected to restrict subsequent analysis to only well-defined or pristine shapes without riming from which dimensions could be unambiguously measured.…”

“…In general, the aspect ratio (AR) of each ice crystal is defined as W/L or L/W for columns and plates, whereas it is (H + L)/W or W/(H + L) for bullets. Um and McFarquhar (2009) showed several definitions of AR that have been used in theoretical studies and in situ data analysis, which might impact derived scattering properties of ice crystals (Korolev and Isaac, 2003). In order to characterize aspect ratio in a form that is appropriate for numerical models, a power law relationship of the form αL β , where α and β are constants determined empirically from observations, is typically used (e.g., Auer and Veal, 1970;Heymsfield, 1972;Hobbs et al, 1974;Davis, 1974;Mitchell and Arnott, 1994).…”

The effect of observed vertical structure, habits, and size distributions on the solar radiative properties and cloud evolution of cirrus clouds

“…Given that ice particles in cirrus are comparable or larger than solar wavelengths, the optical properties are often dependent on ice particle details such as particle size distribution, particle shape, surface roughness and concavity. Ice particles have been observed to vary in shape from individual columns to quite complex shapes, such as irregular or aggregate (Field and Heymsfield, 2003;Baran and Labonnote, 2007;Um and McFarquhar, 2009). Particle sizes range from a few micrometres in high tropical cirrus (McFarquhar and Heymsfield, 1996) to several hundred micromeytres in mid-latitude frontal cirrus (Heymsfield and Miller, 1990;Wang and Sassen, 2001).…”