THE OPTICAL-PROPERTIES OF EQUATORIAL CIRRUS FROM OBSERVATIONS IN THE ARM-PILOT-RADIATION-OBSERVATION-EXPERIMENT

The optical properties of equatorial cirrus were studied during a thre e-week period of the ARM Pilot Radiation and Observation Experiment at Kavieng, Papua New Guinea, in January and February 1993. The experime nt consisted of vertical lidar (532 nm) and passive infrared filter ra diometer (10.84 mu m) observations of cirrus clouds. The observations gave values of cloud height, depth, structure, infrared emittance, inf rared absorption, and visible optical depth and linear depolarization ratio. A standard lidar-radiometer analysis, with some improvements, w as 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 degrees C to -7 degrees 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 cloud s was found to be higher than that observed previously at tropical and midlatitude sites and at equivalent temperatures. The mean infrared a bsorption coefficients were similar to those of midlatitude clouds, ex cept at the extreme temperature ranges, but were higher than those obs erved 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 behavio r. Values of the measured quantity k/2 eta, where k is the visible bac kscatter to extinction ratio and eta a multiple scattering fatter, wer e found to increase with temperature from 0.14 at -70 degrees C to 0.3 0 at -20 degrees C. Values of the quantity 2 alpha eta, where alpha is the ratio of visible extinction to infrared absorption coefficient, v aried from about 1.7 to 3.8, depending somewhat on the cloud temperatu re. Deduced values of alpha were as high as 5.3 at the lower temperatu re ranges; indicating smaller particles. The lidar integrated attenuat ed depolarization ratio Delta decreased with temperature, as found pre viously in midlatitude cirrus. Values of Delta varied from 0.42 at -70 degrees C to 0.18 at -10 degrees C. Data obtained from the NOAA/ETL m icrowave radiometer gave values of water path, varying from 4 to 6 cm precipitable water. A value of the water vapor continuum absorption co efficient at 10.84 mu m equal to 9.0 +/- 0.5 g(-1) cm(2) atm(-1) was o btained in agreement with previous observations.