A Midlatitude Cirrus Cloud Climatology from the Facility for Atmospheric Remote Sensing. Part II: Microphysical Properties Derived from Lidar Depolarization

In Part II of this series of papers describing the results of the extended time observations of cirrus clouds from the University of Utah Facility for Atmospheric Remote Sensing (FARS), the information content of laser backscatter depolarization measurements in terms of cloud microphysical content is treated. The authors rely on scattering principles indicating that polarization lidar can be applied to identifying cloud phase, and describing ice particle shape and orientation. It is found that 0.694-μm lidar linear depolarization ratios δ obtained in the zenith display a steady increase with height. With respect to temperature, a minimum of δ = 0.25 is found at −17.5°C, where horizontally oriented planar ice crystals are to be expected, and the δ increase up to 0.45 at −77.5°C. This trend indicates a basic transition in cirrus ice crystal shape with temperature, likely reflecting not only the effects of crystal axis ratio (i.e., plate-to-column) but also internal and radial crystallographic features. Evidence for transient supercooled liquid clouds embedded in cirrus is found at temperatures generally >−20°C. Off-zenith lidar data show that the effects of oriented plate crystals in lowering δ are widespread in cirrus, especially at temperatures >−45°C. Comparisons with other lidar studies are made, and it is concluded that the depolarization data reveal fundamental distinctions in cirrus cloud particle properties that vary with temperature and probably geographical location. It is important to understand such variations in order to improve the ability to model the effects of cirrus clouds on climate.