Measurements of the Near-Infrared Radiative Properties of Laboratory Generated Water Ice Clouds

Cirrus clouds are known to have a significant impact on climate, yet large uncertainties remain in the radiation budget due to clouds and their radiative properties. We present results of preliminary laboratory experiments designed to measure near-infrared scattering phenomena at high spectral resolution in the vicinity of water vapor absorption features at 1.37 æm. In addition to the fundamental importance of determining the radiative properties of ice clouds, these experiments characterize the effects of ice particles on the performance, precision and accuracy of an in situ near-infrared spectrometer, the JPL Laser Hygrometer (JLH). JLH scans across a strong water vapor absorption line at 1.37 æm wavelength in an open path sample cell to measure atmospheric water vapor. The extent of radiative interference effects of ice crystals on JLH measurements within cirrus clouds is quantified here. An ice cloud was generated in a chamber cooled to below -41§C, the homogeneous nucleation temperature of water. The cloud was formed by injecting water droplets, approximately 3 æm to 6 æm diameter, into the chamber. The droplets froze upon contact with the cold air. Light attenuation was measured simultaneously by JLH placed inside the chamber and by a visible (red) diode laser and detector. To extract any meaningful data, the microphysical properties of the ice cloud and the particle concentration at each moment in time needed to be measured. Following the procedure of Barkey et al., 2000, replicas of ice crystals were made immediately before and after reflectance measurements were made. The replicas were studied microscopically to determine size distribution. By measuring the amount of visible laser light extinction, and using the derived ice crystal size distribution, an estimate was made of the particle concentration as a function of time. Implications for JLH data, and plans for future experiments, will be discussed. Barkey, B., Liou, K.N., Takano, Y., Gellerman, W., Experimental and theoretical spectral reflection properties of ice clouds generated in a laboratory chamber. Appl. Opt. 39 (21), 3561-3564 (2000)