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 uses 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 separate clear and cloudy pixels. A look-up table relating channels 1 and 3 solar reflectances is subsequently constructed based on the prescribed mean effective ice crystal sizes and satellite geometric parameters. An adding-doubling radiative transfer program has been used to generate numerical values in the construction of the look-up table. Removal 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 scheme to the AVHRR global area coverage daytime data, collected during the First ISCCP (International Satellite Cloud Climatology Project) Regional Experiment cirrus intensive held observation (FIRE IFO) at 2100 UTC 28 October 1986 over the Wisconsin area. Distributions of the retrieved cloud heights and optical depths are comparable to those determined from Geostationary Operational Environmental Satellite visible and IR channels data reported by Minnis et al. Moreover, verifications of the retrieved cirrus temperature and height against lidar data have been carried out using results reported from three FIRE IFO stations. The retrieved cloud heights are within 0.5 km of the measured lidar values.