Relative merits of the 1.6 and 3.75 μm channels of the AVHRR/3 for cloud detection

A study was performed to investigate the potential impacts of the cloud-masking capability of the advanced very high resolution radiometer (AVHRR) on board the National Oceanic and Atmospheric Administration (NOAA) polar orbiting satellites because of the addition of the 1.6 mum channel (ch3a) and the removal of the 3.75 mum channel (ch3b) during daylight operation. Both channels are measured by the AVHRR, but only one is available in the data stream. Because the AVHRR presents the longest time series of global imager data, this change could impact a critical source of climate data. Specifically, changes in its cloud-masking capability may introduce a discontinuity in its derived clear-sky data records. To study the relative cloud-detection capabilities of the AVHRR with ch3a or ch3b, data from the moderate resolution imaging spectroradiometer (MODIS) on the National Aeronautics and Space Administration (NASA) TERRA mission was used because it offers ch3a, ch3b, and the other AVHRR channels simultaneously. The MODIS data analysis indicated that ch3b offered more capability in separating cloud from snow. Cloud-masking results with ch3a and ch3b were comparable with respect to the separation of cloud from aerosol and the detection of cloud over a desert scene. Although these results are preliminary and based on a limited analysis, they do indicate that the switch from ch3b to ch3a may have significant impacts on the cloud-detection capability of the AVHRR.