Backscatter radar-lidar ratios for sizing ice crystals of cirrus clouds

Cirrus clouds consisting mainly of ice crystals are important components of the atmosphere which essentially modulate the radiative budget of the Earth. Until now, the microphysical properties (i.e., size and shape) of the ice crystals, as well as their number density are poorly known because of their great variability in time and space and difficulties of field measurements. At present, cirrus clouds are widely studied by various ground-based, airborne and spaceborne instruments. Among such instruments, lidars and radars are promising devices providing active remote sensing of the clouds. In the report, we present the results of the calculations the radar-lidar ratio explicitly at a reasonable model for the size and shapes of the cirrus ice crystals using the physical-optics approximation. We show that it is the radar-lidar ratio that is mainly informative for retrieving crystal sizes. Also, we calculate the depolarization ratios for both lidar and radar. We obtain that the lidar depolarization ratio is effective for estimating crystal shapes in cirrus clouds. Such data would be useful for interpreting any data obtained simultaneously by radars and lidars.