Utilization of equivalent spheres of equal volume and surface area for estimation of the asymmetry parameter from microphysical observations

The asymmetry parameter (g) of ice particle populations has been estimated using equal volume and surface area equivalent spheres. The scattering properties of spheres can be easily calculated using Mie theory and is therefore computationally advantageous. Equal volume and surface area sphere sizes were calculated using particle size distributions measured with instruments on the University of North Dakota Citation aircraft during the Cirrus Regional Study of Tropical Anvils and Cirrus Layers - Florida Area Cirrus Experiment (CRYSTAL-FACE) project in July 2002.

The equal volume and surface area method leads to numerous spheres representing single irregular ice particles. Inner surfaces of concavities are represented by outer surfaces on the spheres leading to an equivalent sphere optical depth (τ) that is higher than that of the ice particle population being modeled. The product τ(1-g) which largely determines the bulk radiative properties of clouds is calculated for the equivalent spheres using Mie theory as well as for g values measured by the Cloud Integrating Nephelometer (CIN) probe. Equivalent sphere results show promise in that they are in the correct range, but do not appear to be directly correlated with CIN calculated values.