Improved determination of cloud-free radiances for oceans

Graduation date: 2000Improvements have been made to the spatial coherence method for automatically\ud determining cloud-free ocean radiances in satellite imagery by incorporating the spectral\ud signatures of reflecting surfaces. The spatial coherence method relies on the fact that\ud small-scale cloud-free regions typically exhibit uniform emission and uniform reflection.\ud While small-scale overcast regions typically exhibit uniform emission, they often exhibit\ud considerable variability in reflectance. On rare occasions the requirements of spatial\ud uniformity are not met and errors are produced in estimated cloud-free radiances. The\ud frequency of errors in identification of cloud-free and overcast pixels was assessed using\ud two years of Advanced Very-High Resolution Radiometer (AVHRR) data from six regions\ud of the globe. Significant improvement in the identification of cloud-free radiances is\ud obtained by including a test of Q, the ratio of the AVHRR channel 2 (0.83-μm) reflectance\ud to channel 1 (0.63-μm) reflectance. Q varies depending on whether the reflecting surface\ud is cloud-free ocean, cloud-free land, or overcast by clouds. A study was conducted to\ud determine the dependence of Q for overcast pixels on changes in season and geography.\ud While some variation is evident due to satellite viewing angle and differences in\ud atmospheric water vapor content, these effects are sufficiently small that constant\ud thresholds may be used to help separate cloud-free and overcast pixels. The modified\ud spatial coherence method uses the threshold for Q and radiance uniformity thresholds at\ud 0.63-μm and 11-μm to identify cloud-free and overcast pixels. A sensitivity study was\ud performed to determine the dependence of cloud-free ocean radiance estimates on the\ud values of the uniformity thresholds. The results of the study indicate that using thresholds\ud of 0.5% for the 0.63-μm reflectivity and 0.5 mWm⁻²sr⁻¹cm for the 11-μm radiance,\ud produces cloud-free radiances that are rarely biased by more than 0.4% for reflectances at\ud 0.63 μm and 0.4 K for the 11-μm brightness temperature. The uniformity and Q\ud thresholds may be used for a large variety of scenes from different seasons and geographic\ud areas