Radiative Transfer Applications to the Remote Sensing of Ice Clouds: Theory and Experiment

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This project is a continuation of the Principal Investigator's ongoing research on radiative transfer in ice clouds, with specific applications to remote sensing in both cirrus cloudy atmospheres and a laboratory cloud chamber. The Principal Investigator and his graduate students have developed a method to infer ice crystal size and shape from airborne and satellite polarization and reflectance observations at visible and near-infrared wavelengths. They also employ a new theoretical approach using the reflected line spectrum in the 1.38 micrometer band to determine the vertical profile of ice crystal size in cirrus clouds. Laboratory activities include construction of a cloud chamber used to generate ice crystals of various shapes and sizes and equipped with a special radiation sensing platform. The observational apparatus measures the reflected light and polarization simultaneously at several scattering angles and at specific wavelengths in the visible and near-infrared by using imaging grating spectrometers, in a way that reproduces the bidirectional reflection geometry observed by satellite and aircraft. The reflected spectrum in the 1.38 micrometer region is measured by a Fourier Transform Infrared Spectrometer for application to vertical profiling. The broader impacts of the research include advancing the understanding of radiative transfer in ice clouds containing nonspherical crystals of intricate shape, size, and orientation. The combined theoretical and experimental approach provides the physical foundation and validation data for the development of new methods of remote sensing to infer cirrus cloud composition from satellites and space-borne radar and lidar in support of weather and climate research. It also supplies the correct light scattering and radiative properties of ice clouds for their parameterization in climate models, which information is critical for understanding the effect of widespread cirrus clouds on the reflection of sunlight and the trapping of thermal infrared radiation associated with greenhouse warming and climate feedback processes.