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 wor k, a novel simulation code ("face tracing") derived from ray tracing was de scribed and used to compute LIDAR depolarization and the extinction/backsca tter ratio at 532 nm, as expected from randomly oriented pristine and sligh tly deformed hexagonal prisms of various sizes and aspect ratios. By increa sing 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/backscat ter ratio (corresponding to a maximum LIDAR backscatter efficiency) was als o found. When including the far-field diffraction in backscattering simulat ions, pristine particles smaller than 100 mum showed depolarization and ext inction/backscatter ratios comparable with the experimental LIDAR data of c old (T < 30 degreesC) polar cirrus. Recent in situ observations showed the widespread presence of nonpristine hexagonal crystals in cirrus, stimulatin g the calculation of the backscatter properties of deformed particles by "f ace tracing." Simulations for deformed hexagonal prisms showed in most case s a smaller depolarization and a higher extinction/backscatter ratio compar ed with those obtained for pristine crystals. A mixture containing variable proportions of pristine and deformed hexagonal prisms (with an aspect rati o of 1-2) resulted in a depolarization-extinction/backscatter scatterplot s imilar to the experimental one for cold polar cirrus.