Simulation of LIDAR returns from pristine and deformed hexagonal ice prisms in cold cirrus by means of "face tracing"

The simulation of the optical properties of ice clouds plays a crucial role in the interpretation of remote sensing data of cirrus clouds. In this work, a novel simulation code ("face tracing") derived from ray tracing was described and used to compute LIDAR depolarization and the extinction/backscatter ratio at 532 nm, as expected from randomly oriented pristine and slightly deformed hexagonal prisms of various sizes and aspect ratios. By increasing the aspect ratio the depolarization of pristine crystals was found to increase sharply from zero (thin plates) to a maximum value (columns) at an aspect ratio of around I, where an absolute minimum of extinction/backscatter ratio (corresponding to a maximum LIDAR backscatter efficiency) was also found. When including the far-field diffraction in backscattering simulations, pristine particles smaller than 100 mum showed depolarization and extinction/backscatter ratios comparable with the experimental LIDAR data of cold (T < 30 degreesC) polar cirrus. Recent in situ observations showed the widespread presence of nonpristine hexagonal crystals in cirrus, stimulating the calculation of the backscatter properties of deformed particles by "face tracing." Simulations for deformed hexagonal prisms showed in most cases a smaller depolarization and a higher extinction/backscatter ratio compared with those obtained for pristine crystals. A mixture containing variable proportions of pristine and deformed hexagonal prisms (with an aspect ratio of 1-2) resulted in a depolarization-extinction/backscatter scatterplot similar to the experimental one for cold polar cirrus.