The Optical Properties of Equatorial Cirrus from Observations in the ARM Pilot Radiation Observation Experiment

Abstract

The optical properties of equatorial cirrus were studied during a three-week period of the ARM Pilot Radiation and Observation Experiment at Kavieng, Papua New Guinea, in January and February 1993. The experiment consisted of vertical lidar (532 nm) and passive infrared filter radiometer (10.84 ?m) observations of cirrus clouds. The observations gave values of cloud height, depth, structure, infrared emittance, infrared absorption, and visible optical depth and linear depolarization ratio. A standard lidar?radiometer analysis, with some improvements, was used to calculate these quantities. The cirrus was found to vary in altitude from a maximum cloud top of 17.6 km to a minimum cloud base of 6 km with equivalent temperatures of ?82°C to ?7°C respectively. The cirrus also varied widely in depth (0.7 to 7.5 km). The mean emittance (for each temperature interval) of the cooler clouds was found to be higher than that observed previously at tropical and midlatitude sites and at equivalent temperatures. The mean infrared absorption coefficients were similar to those of midlatitude clouds, except at the extreme temperature ranges, but were higher than those observed in tropical synoptic clouds over Darwin. Infrared optical depths varied from 0.01 to 2.4 and visible optical depths from 0.01 to 8.6. Plots of integrated attenuated backscatter versus infrared emittance, for various ranges of cloud temperature, showed characteristic behavior. Values of the measured quantity k/2?, where k is the visible backscatter to extinction ratio and ? a multiple scattering factor, were found to increase with temperature from 0.14 at ?70°C to 0.30 at ?20°C. Values of the quantity 2α?, where α is the ratio of visible extinction to infrared absorption coefficient, varied from about 1.7 to 3.8, depending somewhat on the cloud temperature. Deduced values of α were as high as 5.3 at the lower temperature ranges, indicating smaller particles. The lidar integrated attenuated depolarization ratio ? decreased with temperature, as found previously in midlatitude cirrus. Values of ? varied from 0.42 at ?70°C to 0.18 at ?10°C. Data obtained from the NOAA/ETL microwave radiometer gave values of water path, varying from 4 to 6 cm precipitable water. A value of the water vapor continuum absorption coefficient at 10.84 ?m equal to 9.0 ± 0.5 g?1 cm2 atm?1 was obtained in agreement with previous observations.