Cirrus Structure and Radiative Parameters from Airborne Lidar and Spectral Radiometer Observations: The 28 October 1986 FIRE Study

Abstract

Remote sensing lidar and imaging spectral radiometer observations were obtained from the ER-2 high-altitude research aircraft during the 1986 FIRE cirrus missions. The dual polarization lidar measurements were nadir directed with 7.5 m vertical and 40 m horizontal resolution, and clearly depicted structure at the top and within the cirrus. Simultaneous radiometric cloud top images were acquired with 5 mrd resolution at ten visible channels, three infrared window channels, and four near-infrared channels. The combined lidar and radiometer data were analyzed for the cirrus structure, radiative parameters, and inferred microphysical properties. On 28 October 1986 a cirrus formation crossed Wisconsin. The results indicate that for the eastern edge of the formation there was a cirrus layer at 9 to 11 km altitude, and a separate lower cloud at 7 to 8 km. The lidar depolarization indicated the upper layer was ice crystals, the lower layer was ice in some areas, and water or possibly mixed phase in others. Split window thermal brightness measurements indicated the upper layer was principally particles of an effective radius less than 25 ?m. To the west, the cirrus formation was a denser layer extending between 6 to 11 km altitude. An equivalent height for the thermal IR emission of cirrus was defined. The equivalent height was found to be as much as 4 km below the true cloud top height. The average vertical structure of radiation parameters was derived. For the upward infrared radiance the strongest contribution was from 7 to 8 km altitude but higher cirrus were significant. Cloud visible reflectance approached 0.6 and the 10.84 ?m emittance ranged to 0.9. Distinct local vacations in the relation between reflectance and emittance were found, while a significant dispersion of the emittance to reflectance relation for the entire dataset was present. The dispersion was principally due to variations in surface albedo. An overall parameterization for the average measured relation between emittance and visible albedo is given.