Depolarization ratio–effective lidar ratio relation: Theoretical basis for space lidar cloud phase discrimination

This paper introduces a technique for cloud phase discrimination based on linear polarization measurements made by space‐based lidar. Using CALIPSO Level 2 data products, a 3‐dimensional histogram of the depolarization ratio, δ , and the effective lidar ratio, S c,eff , is derived for all optically thick clouds measured during July 2006. A second histogram is derived using data from early November, 2006. By analysis of the relationship between δ and S c,eff , and an examination of their spatial correlations, water clouds, ice clouds with randomly oriented particles and ice clouds with horizontally oriented particles are clearly differentiated. For clouds having the same depolarization ratios, the effective lidar ratios of water clouds are significantly smaller than the effective lidar ratios of ice clouds comprised of randomly oriented particles. The variance of the depolarization ratios within water clouds is significantly larger than it is in ice clouds containing horizontally oriented particles. Depolarization ratios and effective lidar ratios are negatively correlated for water clouds, and positively correlated for ice clouds with horizontally‐oriented particles. For the optically thick ice clouds observed by CALIPSO, approximately half contain some fraction (up to 100%) of horizontally oriented particles.