NASA - National Aeronautics and Space Administration

There has been a long-standing problem in discrepancy between theoretical and observed backscatter by water clouds. Lidar ratios from water clouds are often much lower than what calculated out by Mie theory. In this study, we find the low lidar ratios of water clouds in CALIPSO lidar observations can be explained if we assume some large droplets with liquid spherical shells and ice cores. Using a light scattering model for layered spherical particles, we can produce lidar ratios similar to those from the CALIPSO data while assuming large droplets with ice cores in clouds. This study explains the reason of low lidar ratios for water clouds and provides a method for remotely sensing partially melting droplets.

Lidar can be used for measuring cloud thermodynamic phase [1,2]. Backscattered lidar signal from cloud particles is also used for understanding cloud microphysical properties, such as the particle size, extinction coefficient, liquid water content, and droplet number concentration, etc. [3,4]. However, there has been a long-standing problem in discrepancy between theoretical and observed backscatter by water clouds.

NASA’s CALIPSO [1] mission was developed to provide global profiling measurements of cloud and aerosol distribution and properties. CALIOP, the primary instrument carried by CALIPSO, is a lidar operating at the wavelengths of 1064 nm and 532 nm that can measure atmospheric particles’ extinction to laser beam and the backscatter of the laser beam from the particles. Traditionally, the ratio of lidar returns at different wavelengths is called lidar color ratio, and the ratio of the particles’ extinction cross-section and the particles backscatter cross-section is defined as lidar ratio. In analyzing the CALIPSO lidar data from clouds, as shown in Fig. 1, it is found that lidar backscatter from water clouds with large droplets are significantly larger than what the Mie theory [5] predicts for pure water droplets or ice particles, and this means much lower lidar ratios from measurements than from Mie theory results. Fig. 1 shows the lidar ratio (i.e. the ratio of extinction cross section and backscatter cross section of cloud particles) for water clouds at a wavelength of 532 nm. For cloud particles with large effective radius Re , lidar ratios from the CALIPSO satellite data are significantly smaller than the ones calculated from Mie theory. Changing size distributions of particles does not significantly