Understanding Satellite Cirrus Cloud Climatologies with Calibrated Lidar Optical Depths

Abstract

Optical depth measurements of transmissive cirrus clouds were made using coincident lidar and satellite data to improve our interpretation of satellite cloud climatologies. The University of Wisconsin High Spectral Resolution Lidar was used to measure the optical depth of clouds at a wavelength of 532 nm, while the GOES and AVHRR window channel imagers provided measurements at a wavelength of 10.8 µm. In single-layer cirrus clouds with a visible optical depth greater than 0.3, the ratio of the visible to the IR optical depth was consistent with the approximate 2:1 ratio expected in clouds comprised of large ice crystals. For clouds with visible optical depths <0.3, the visible/IR ratios were nearly always <2. It is likely that this reflects a measurement bias rather than a difference in cloud properties. Most cirrus clouds observed in this study were more than 1 km thick and were often comprised of multiple layers. Supercooled liquid water layers coexisted with the cirrus in 32% of the cases examined. In many of these cases the presence of water was not evident from the satellite images. Thus, it must be concluded that ?cirrus? climatologies contain significant contributions from coexisting scattered and/or optically thin water cloud elements.