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 backsc atter depolarization measurements in terms of cloud microphysical content i s treated. The authors rely on scattering principles indicating that polari zation lidar can be applied to identifying cloud phase, and describing ice particle shape and orientation. It is found that 0.694-mum lidar linear dep olarization ratios delta obtained in the zenith display a steady increase w ith height. With respect to temperature, a minimum of d delta = 0.25 is fou nd at -17.5 degreesC, where horizontally oriented planar ice crystals are t o be expected, and the d increase up to 0.45 at -77.5 degreesC. 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-t o-column) but also internal and radial crystallographic features. Evidence for transient supercooled liquid clouds embedded in cirrus is found at temp eratures generally >-20 degreesC. Off-zenith lidar data show that the effec ts of oriented plate crystals in lowering delta are widespread in cirrus, e specially at temperatures >-45 degreesC. Comparisons with other lidar studi es are made, and it is concluded that the depolarization data reveal fundam ental distinctions in cirrus cloud particle properties that vary with tempe rature and probably geographical location. It is important to understand su ch variations in order to improve the ability to model the effects of cirru s clouds on climate.