REMOVAL OF THE SOLAR COMPONENT IN AVHRR 3.7-MU-M RADIANCES FOR THE RETRIEVAL OF CIRRUS CLOUD PARAMETERS

A numerical scheme has been developed to remove the solar component in the Advanced Very High Resolution Radiometer (AVHRR) 3.7-mu m channel for the retrieval of cirrus parameters during daytime. This method us es a number of prescribed threshold values for AVHRR channels 1 (0.63 mu m), 2 (0.8 mu m), 3 (3.7 mu m), 4 (10.9 mu m), and 5 (12 mu m) to s eparate clear and cloudy pixels. A look-up table relating channels 1 a nd 3 solar reflectances is subsequently constructed based on the presc ribed mean effective ice crystal sizes and satellite geometric paramet ers. An adding-doubling radiative transfer program has been used to ge nerate numerical values in the construction of the look-up table. Remo val of the channel 3 solar component is accomplished by using the look -up table and the measured channel I reflectance. The cloud retrieval scheme described in Ou et al. has been modified in connection with the removal program. The authors have applied the removal-retrieval schem e to the AVHRR global area coverage daytime data, collected during the First ISCCP (International Satellite Cloud Climatology Project) Regio nal Experiment cirrus intensive held observation (FIRE IFO) at 2100 UT C 28 October 1986 over the Wisconsin area. Distributions of the retrie ved cloud heights and optical depths are comparable to those determine d from Geostationary Operational Environmental Satellite visible and I R channels data reported by Minnis et al. Moreover, verifications of t he retrieved cirrus temperature and height against lidar data have bee n carried out using results reported from three FIRE IFO stations. The retrieved cloud heights are within 0.5 km of the measured lidar value s.