Small irregular ice crystals in tropical cirrus

Images acquired by a Cloud Particle Imager (CPI) are analyzed to compile a statistical covariance function of radius for an ensemble of small, irregular, quasi-spherical ice crystals in tropical cirrus measured during the Department of Energy Atmospheric Radiation Measurement Program's Tropical Warm Pool International Cloud Experiment. The infrequent occurrence of multiple particles in single CPI frames suggests that most crystals sampled were natural ice crystals rather than artifacts from large particles shattering on probe tips. The covariance function is used in conjunction with the Gaussian random sphere geometry to generate threedimensional model ice particles for ray-optics simulations at 550-nm wavelength. The crystal shapes and single-scattering properties are compared with those obtained by the same methodology for midlatitude cirrus sampled over Oklahoma. The small tropical ice crystals are closer to spherical than their midlatitude counterparts and, consequently, their asymmetry parameters are larger, but the differences are not significant from the standpoint of climate studies. Because some quasi-spherical ice crystals seempartially faceted, a convex hull transformation is introduced that increases the asymmetry parameter from 0.785 to 0.808. Furthermodifying the covariance function to promote sixfold symmetry in the model crystals increases the asymmetry parameter to 0.818. The introduction of internal scatterers, such as air bubbles, has a larger impact, decreasing the asymmetry parameter by up to tens of percent, depending on their amount and characteristics. Unfortunately, no data are available to determine realistic values for the internal scatterers to assess their likely actual impact.