Results from 22 airborne field campaigns, including more than 10 million high‐resolution particle images collected in cirrus formed in situ and in convective anvils, are interpreted in terms of particle shapes and their potential impact on radiative transfer. Emphasis is placed on characterizing ice particle shapes in tropical maritime and midlatitude continental anvil cirrus, as well as in cirrus formed in situ in the upper troposphere, and subvisible cirrus in the upper tropical troposphere layer. There is a distinctive difference in cirrus ice particle shapes formed in situ compared to those in anvils that are generated in close proximity to convection. More than half the mass in cirrus formed in situ are rosette shapes (polycrystals and bullet rosettes). Cirrus formed from fresh convective anvils is mostly devoid of rosette‐shaped particles. However, small frozen drops may experience regrowth downwind of an aged anvil in a regime with RH_ice > ~120% and then grow into rosette shapes. Identifiable particle shapes in tropical maritime anvils that have not been impacted by continental influences typically contain mostly single plate‐like and columnar crystals and aggregates. Midlatitude continental anvils contain single‐rimed particles, more and larger aggregates with riming, and chains of small ice particles when in a highly electrified environment. The particles in subvisible cirrus are < ~100 μm and quasi‐spherical with some plates and rare trigonal shapes. Percentages of particle shapes and power laws relating mean particle area and mass to dimension are provided to improve parameterization of remote retrievals and numerical simulations.

During the March 2000 Cloud Intensive Operational Period, the University of North Dakota Citation executed spiral descents through midlatitude cirrus of a nonconvective origin over the Atmospheric Radiation Measurement Program’s Southern Great Plains site. Aggregates of bullet rosettes (ABRs) observed during the descents using a cloud particle imager are used to derive a relationship between the length L and width W of bullets that are the fundamental components of the ABRs, which is given by W = 7.14L0.455, where 100 μm ≤ L ≤ 600 μm. To derive a representation of an aggregate of bullet rosettes, six bullet rosettes, each of which is composed of six bullets of the same size but each of which has different-sized bullets from the other bullet rosettes, are attached together randomly without overlap. Using a geometric ray-tracing method, the phase function, asymmetry parameter g, and single-scattering albedo of the representation of ABR (rABR) and the component bullets and bullet rosettes are calculated at wavelengths λ of 0.55, 0.64, 1.38, 1.62, 2.11, and 3.78 μm. As the aspect ratio of the component bullets increases, the forward scattering increases by up to 1.3% and the lateral and backward scattering decrease by up to 8.9% and 10.2%, respectively, for a bullet rosette at a nonabsorbing λ (0.55 μm). For longer λ, light absorption decreases the rate at which these scatterings change with aspect ratio. The shape of the aggregates also affects the scattering properties. The rABR constructed here scatters up to 4.4% (7.0%; 20.4%) and 34.2% (11.1%; 32.7%) more light in the lateral and backward directions, respectively, and 1.2% (1.3%; 2.4%) less in the forward direction in comparison with the component bullets (component bullet rosettes; equivalent projected area bullet rosette), resulting in up to 2.5% (1.6%; 3.8%) decrease in g at 0.55 μm. In addition, as the aspect ratio and number of attached bullets in ABRs increase, g increases by up to 1.8% and decreases by up to 2.0% at 0.55 μm, increases by 2.0% and decreases by 0.3% at 2.11 μm, and increases by 1.1% and decreases by 0.5% at 3.78 μm, respectively. As an implication for remote sensing studies, the difference in the bidirectional reflectance distribution function calculated using the rABR and a bullet rosette is shown to vary by up to 107% at moderately absorbing (2.11 μm) wavelengths and by up to 35% and 28% at nonabsorbing (0.55 μm) and strongly absorbing (3.78 μm) wavelengths.

During the 2006 Tropical Warm Pool International Cloud Experiment (TWP-ICE) in the tropics, the 2008 Indirect and Semi-Direct Aerosol Campaign (ISDAC) in the Arctic, and the 2010 Small PARTicles In CirrUS (SPARTICUS) campaign at mid-latitudes, high-resolution images of ice crystals were recorded by a Cloud Particle Imager at temperatures (T) between −87 and 0 °C. The projected maximum dimension (D'), length (L'), and width (W') of pristine columns, plates, and component bullets of bullet rosettes were measured using newly developed software, the Ice Crystal Ruler. The number of bullets in each bullet rosette was also measured. Column crystals were further distinguished as either horizontally oriented columns or columns with other orientations to eliminate any orientation effect on the measured dimensions. The dimensions and aspect ratios (AR, the dimension of the major axis divided by the dimension of the minor axis) of crystals were determined as functions of temperature, geophysical location, and type of cirrus. Dimensions of crystals generally increased with temperature. Columns and bullets had larger dimensions (i.e., W') of the minor axis (i.e., a axis) for a given dimension (i.e., D' orL') of the major axis (i.e., c axis), and thus smaller AR, as T increased, whereas this trend did not occur for plate crystals. The average number of branches in bullet rosettes was 5.50 ± 1.35 during three campaigns and 6.32 ± 1.34 (5.46 ± 1.34; 4.95 ± 1.01) during TWP-ICE (SPARTICUS; ISDAC). The AR of bullets increased with the number of branches in bullet rosettes. Most dimensions of crystals and ARs of columnar crystals measured during SPARTICUS were larger than those measured during TWP-ICE and ISDAC at −67 < T < -35 °C and at −40 < T < −15 °C, respectively. The relative occurrence of varying pristine habits depended strongly on cirrus type (i.e., anvil or non-anvil clouds), with plates especially occurring more frequently in anvils. The L–W relationships of columns derived using current data exhibited a strong dependence on temperature; similar relationships determined in previous studies were within the range of the current data.

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 P₁₁, asymmetry parameter g, and single-scattering albedo ω₀ 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 ω₀ 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. Copyright © 2009 Royal Meteorological Society

The projections of small ice crystals (with maximum dimension <50 μm) appear quasi-circular when imaged by probes on aircraft flying through cloud. Therefore, idealized models constructed to calculate their single-scattering properties have included quasi-spherical models such as Chebyshev particles, Gaussian random spheres, and droxtals. Recently, an ice analogue grown from sodium fluorosilicate solution on a glass substrate, with several columns emanating from a common center of mass, was shown to be quasi-circular when imaged by state-of-the-art cloud probes. In this study, a new idealized model, called the budding Bucky ball (3B) that resembles the shape of the small ice analogue is developed. The corresponding single-scattering properties (scattering phase function P₁₁ and asymmetry parameter g) are computed by a ray-tracing code. Compared with previously used models, 3B scatters less light in the forward and more light in the lateral and backward directions. The Chebyshev particles and Gaussian random spheres show smooth and featureless P₁₁, whereas droxtals and 3Bs, which have a faceted structure, show several peaks in P₁₁ associated with angles of minimum deviation. Overall, the difference in the forward (lateral; backward) scattering between models are up to 22% (994%; 132%), 20% (510%; 101%), and 16% (146%; 156%) for small ice crystals with respective area ratios of 0.85, 0.77, and 0.69. The g for different models varies by up to 25%, 23%, and 19% for particles with area ratios of 0.85, 0.77, and 0.69, respectively. Because the single-scattering properties of small ice crystals depend both on the choice of the idealized model and the area ratios used to characterize the small ice crystals, higher resolution observations of small ice crystals or direct observations of their single-scattering properties are required.