Single-Scattering Properties of Aggregates of Bullet Rosettes in Cirrus

Abstract

Abstract 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.