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

The asymmetry parameter of particle populations has been calculated from in-situ aircraft measurements taken during the CRYSTAL-FACE field project. Equivalent spheres with equal total volume and equal total surface area were used. The equivalent sphere size was estimated for each size bin in the measured particle size distribution and a cloud asymmetry parameter was estimated by calculating a projected area weighted average. The asymmetry parameter was calculated using Mie theory and the results were scaled by a similarity relationship to account for the change in optical depth caused by using the equivalent spheres. The resulting calculations were compared to direct measurements of asymmetry parameter from the CIN probe. The results show that the EVSA theory estimates are in the correct range, but the results were not correlated to the measured values. This work demonstrates that the EVSA sphere theory shows promise for the estimation of the asymmetry parameter from direct aircraft measurements. It is thought that the current state of the particle probe measurements may be insufficient to capture all of the detail necessary for accurate calculations using the EVSA technique. Particle volume and surface area were calculated from parameterizations that did not take particle habit or irregular particle complexity into account. Future work will include surface area estimates that are based on CPI data to better understand the particle complexity. The CPI images will be used directly to more accurately estimate the ratio of particle surface area to projected area. This will refine the calculation of EVSA sphere size. Total ice water content and cloud extinction measurements can also be used to calculate one EVSA sphere size to represent an entire cloud, thus providing a more realistic potential explanation of the effective radius that is derived from remote sensing measurements. Additional research should be done to investigate the EVSA theory at different absorbing wavelengths and to evaluate the EVSA theory for calculations of radar reflectivities.