Scattering properties of complex ice crystal in cirrus cloud

During the March 2000 Cloud Intensive Operational Period, the University of North Dakota Citation executed spiral descents through mid-latitude cirrus of a non-convective origin over the Atmospheric Radiation Measurement program's Southern Great Plains site. Aggregates of bullet rosettes (ABRs) observed during the spiral descents using a Cloud Particle Imager (CPI) 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=7.14L^0.455, where 100 µm ≤ L ≤ 600 µm. A representation of aggregates of bullet rosettes (rABR) is developed by attaching six bullet rosettes consisting of bullets with six different sizes together randomly without overlap. Using a geometric ray tracing method, the phase function, asymmetry parameter (g), and single scattering albedo of 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 non-absorbing wavelength (0.55 µm). For longer wavelengths, 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 and 1.2 (1.3; 2.4)% less in the forward direction compared to 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% respectively at 0.55 µm, and by 2.0% and 0.3% at 2.11 µm and 1.1% and 0.5% at 3.78 µm. As an implication for remote sensing studies, the bidirectional reflectance distribution function (BRDF) of ice clouds is calculated using a Monte Carlo radiative transfer code revealing that an incorrectly parameterized ice crystal habits and hence scattering properties can cause up to a 107% error in BRDF at moderately absorbing (2.11 µm) and up to 35% and 28% errors at non-absorbing (0.55 µm) and strongly absorbing (3.78 µm) wavelengths.