Simulation of the color ratio associated with the backscattering of radiation by ice particles at the wavelengths of 0.532 and 1.064 mm

This study explores a simulation of ice cloud optical properties similar to those observed using the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP), a dual-wavelength (0.532 and 1.064 microns) lidar. The goal is to better understand the sensitivity of the color ratio (the ratio of the backscatter coefficients at the wavelengths of 1.064 and 0.532 microns) to ice particle habit. The single-scattering properties of randomly oriented hexagonal ice particles are simulated from the finite difference time domain (FDTD) method for particles with size parameters less than 50, and from an improved geometric optics method (IGOM) for particles with larger size parameters (a quantity proportional to the ratio of the particle characteristic dimension to the incident wavelength). Based on the assumption that ice particles are hexagonal particles, the color ratio values are found to be less than unity with a peak value near 0.7 for columns and 0.8 for plates. If spherical ice particles are assumed, the color ratio values can be larger than unity and may even approach 2 for many size distributions. The deviation in the value of the color ratio from unity is due to different distributions of size parameters for the two wavelengths, and to smaller single-scattering albedo values for large particles at 1.064 microns than at 0.532 microns. The present simulations of the color ratio for hexagonal columns are qualitatively consistent with measurements from a ground-based lidar, located at Hampton University in Hampton, Virginia, which peak near 0.88, but the two results differ quantitatively by 10–20 percent.