Intercalibrating MetOP/AVHRR and AQUA/MODIS with improved SNO accuracy

Although the Simultaneous Nadir Overpass (SNO) method has been used in several previous studies, the uncertainty of this method has not been well quantified for the solar reflective bands. In this study, the accuracy of the SNO method is evaluated first based on a four year time series of SNO data between AVHRR/NOAA16 and MODIS/AQUA. It was found that the nominal accuracy for the 0.63 mum band is better than 0.9%, which can unambiguously identify the large offset (~9%) between AVHRR/NOAA16 and MODIS/AQUA. This discrepancy is thought to be caused by the calibration traceability and standard used for AVHRR. For the 0.84 mum band, the uncertainty varies with a variety of factors, including atmosphere, water vapor, and landcover types. Therefore, although a 10-20% difference is found between AVHRR/NOAA16 and MODIS/AQUA, the uncertainty in this analysis is estimated to be greater than +1-5%. Based on these baseline uncertainty assessments, a preliminary analysis of the calibration differences between AVHRR/MetOP and MODIS/AQUA is also performed, and small biases are found when the AVHRR prelaunch calibration coefficients are used. The use of global full resolution area coverage (FRAC) AVHRR data reduced the uncertainty in the analysis but a longer time series is needed in order to evaluate the ability of the SNO method for quantifying the relative degradation rate, since this rate appears to be relatively small for the 0.63 mum channel for newer instruments than that the previous AVHRR series. It is expected that the accuracy of the SNO method can be further improved with the FRAC data which can contribute to the generation of fundamental climate data records.