Abstract
Several cloud optical quantities were measured for the first time in midlevel, mixed-phase clouds. These included cloud infrared emittance and absorption coefficient (10–12 μm), effective backscatter-to-extinction ratio, and lidar depolarization ratio. Contrary to expectations, the supercooled water clouds were not always optically thick and therefore had measurable infrared absorption coefficients. At times, the water clouds had quite low emittances, whereas ice clouds had emittances that sometimes approached unity. On average, the cloud emittances were greater than those measured previously at lower temperatures in cirrus, but with considerable variability. At higher temperatures, the emittance values were skewed toward unity. The infrared absorption coefficients, for the semitransparent cases, showed a similar trend. The effective isotropic backscatter-to-extinction ratio was also measured. When separated into temperature intervals, the ratio was surprisingly constant, with mean values lying between 0.42 and 0.43, but with considerable variation. These ratios were most variable (0.15–0.8) in the −20° to −10°C temperature range where various ice crystal habits can occur. When multiple scattering effects were allowed for, values of backscatter-to-extinction ratio in the supercooled water clouds agreed well with theory. Multiple scattering factors based on previously obtained theoretical values were used and, thus, validated.
Characteristic and well-known patterns of lidar backscatter coefficient and depolarization ratio were used to separate out the incidence of supercooled water and ice layers and to identify layers of horizontal planar hexagonal crystals. This approach allowed the most detailed examination yet of such incidence by ground-based remote sensing. Water was detected for 92% of the time for the temperature interval of −5° to 0°C. Between −20° and −5°C, percentages varied between 33% and 56%, dropping to 21% between −25° and −20°C and to zero below −25°C. Oriented hexagonal plate crystals were present for 20% of the total time in ice layers between −20° and −10°C, the region of their maximum growth. The depolarization ratio varied significantly among different ice fall streaks, indicating considerable variation in ice crystal habit. Although the dependence of depolarization ratio on optical depth had been predicted theoretically, the first experimental validation in terms of IR emittance was obtained in this study.
* Current affiliation: Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado.
* Corresponding author address: Dr. Stuart A. Young, CSIRO Atmospheric Research, PMB 1, Aspendale, VIC 3195, Australia.
