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GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L11812, doi:10.1029/2007GL029584, 2007

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

Yongxiang Hu

NASA Langley Research Center, Hampton, Virginia, USA


Abstract

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, Sc,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 Sc,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.

Received 6 February 2007; accepted 10 May 2007; published 14 June 2007.

Keywords: depolarization; cloud phase; lidar.

Index Terms: 0319 Atmospheric Composition and Structure: Cloud optics; 0360 Atmospheric Composition and Structure: Radiation: transmission and scattering; 0320 Atmospheric Composition and Structure: Cloud physics and chemistry; 0321 Atmospheric Composition and Structure: Cloud/radiation interaction.

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Citation: Hu, Y. (2007), Depolarization ratio–effective lidar ratio relation: Theoretical basis for space lidar cloud phase discrimination, Geophys. Res. Lett., 34, L11812, doi:10.1029/2007GL029584.