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Infrared and visible cirrus cloud optical properties were measured using ground-based high spectral resolution remote sensing data. The optical properties include: downwelling atmospheric radiance at infrared wavelengths and atmospheric aerosol backscatter cross-section and depolarization at a visible wavelength. Data was acquired at the University of Wisconsin-Madison, during a 3 month case study from October through early December, 1995. The instrument suite included: a High Spectral Resolution Lidar (HSRL), an Atmospheric Emitted Radiance Interferometer (AERI), and a Cross-chain Loran Atmospheric Sounding System (CLASS).
The high spectral resolution AERI allowed data analysis in 19 spectral intervals between water vapor lines in the infrared atmospheric window located between 770 and 1200 cm. AERI measured downwelling atmospheric radiance provided the downwelling brightness temperature. AERI radiance combined with radiosonde data, HSRL measured cloud boundaries, and Fast Atmospheric Signature Code (FASCOD3P) allowed inversion of infrared cloud optical depth at each of these spectral regions. The infrared results were compared against 0.532 m HSRL measured optical depth and derived brightness temperatures. HSRL derived brightness temperatures were determined using visible optical depth data, radiosonde data, and FASCOD3P model simulated clear sky radiance and transmission below the cloud. A comparison of visible to infrared optical depths demonstrated a spectral dependence in the data. Mie theory applied to ice spheres suggested a spectral particle size sensitivity that was apparent in the data and consistent with 35 to 50 m radius particles. Optical depth and brightness temperature results are presented for various cases which demonstrate close agreement between the instruments. Combining the data from each instrument yields a weighted cloud extinction cross-section which improved the results relative to Mie theory.
An estimation of the expected error is also given, where the primary error sources were water vapor continuum uncertainties in the FASCOD3P model derived values and instrument field of view and temporal averaging differences. Brightness temperature errors, associated with FASCOD3P uncertainties, approached 4 K for a boundary layer relative humidity of less than 70 percent. Instrumental field of view and dwell-time characteristics exhibited large differences in the results for non-uniform cloud conditions. Field of view differences were shown to account for an 80 percent difference in measured optical depth, while instrumental dwell-times were shown to account for a several-fold difference in optical depth. Both examples were for the extreme case of broken cirrus.