The Asymmetry Parameter of Cirrus Clouds Composed of Hollow Bullet Rosette–Shaped Ice Crystals from Ray-Tracing Calculations

Abstract

Cirrus clouds in the midlatitude and Arctic regions are often composed of bullet rosette?shaped ice crystals. Bullet rosette?shaped ice crystals are composed of a number of bullets radiating from a central point. The bullets that make up the rosette will grow to be hollow in some conditions. To understand better the radiative impact of cirrus clouds, the authors have used a ray-tracing code to calculate the scattering properties of solid and hollow bullet rosettes at visible wavelengths. Results show that hollow bullet rosettes exhibit a broader forward-scattering peak than do solid bullet rosettes. This difference results in an asymmetry parameter that is as much as 0.08 lower for hollow bullet rosettes than for solid rosettes. The effective asymmetry parameter of spheres with the same particle volume and total surface area of the bullet rosettes has also been calculated. Asymmetry parameter estimates for equivalent spheres were similar to those calculated using the ray tracing. Asymmetry parameter calculations were used in combination with direct aircraft measurements from the Atmospheric Radiation Measurement Program intensive operational period in March of 2000. Asymmetry parameter estimates were used with particle size distributions for three cirrus cloud flights for which the observed large particles were predominantly bullet rosettes. Calculated asymmetry parameter values (0.80?0.84) agreed poorly with published cirrus parameterizations (0.75?0.84) when applied to the same aircraft data. Differences lead to 4.5?9 W m?2 differences in reflected and transmitted visible light energy for a cloud of 0.5 optical depth.