UNDERSTANDING SATELLITE CIRRUS CLOUD CLIMATOLOGIES WITH CALIBRATED LIDAR OPTICAL DEPTHS

Optical depth measurements of transmissive cirrus clouds were made usi ng coincident lidar and satellite data to improve our interpretation o f satellite cloud climatologies. The University of Wisconsin High Spec tral Resolution Lidar was used to measure the optical depth of clouds at a wavelength of 532 nm, while the GOES and AVHRR window channel ima gers provided measurements at a wavelength of 10.8 mu m. In single-lay er cirrus clouds with a visible optical depth greater than 0.3, the ra tio of the visible to the IR optical depth was consistent with the app roximate 2:1 ratio expected in clouds comprised of large ice crystals. For clouds with visible optical depths < 0.3, the visible/IR ratios w ere nearly always < 2. It is likely that this reflects a measurement b ias rather than a difference in cloud properties, Most cirrus clouds o bserved in this study were more than 1 km thick and were often compris ed of multiple layers. Supercooled liquid water layers coexisted with the cirrus in 32% of the cases examined. In many of these cases the pr esence of water was not evident from the satellite images. Thus, it mu st be concluded that ''cirrus'' climatologies contain significant cont ributions from coexisting scattered and/or optically thin water cloud elements.