Calibration and Validation of the 36-year NOAA/AVHRR Imager Visible Channel Data record in support of the NOAA Climate Data Records program.

Abstract:

The NOAA/AVHRR (Advanced Very High Resolution Radiometer) series of polar-orbiting earth-imagers have been flying since 1978 to the present and provide an opportunity to derive a long-term consistent set of well calibrated visible channel radiances for cloud, aerosol, and land use retrievals. This will allow climate modelers to investigate climate natural variability, intra-seasonal oscillations such as the ENSO, and feedback mechanisms over a 36-year record. Large climate perturbations, such as the 1982 and 1998 El Ninos as well as the 1982 El Chichon and 1992 Mt Pinatubo volcanic eruptions, have not been observed since 2000. The vicarious calibration method relies on temporally well characterized multiple pseudo-invariant calibration sites (PICS) referenced to the Aqua-MODIS calibration. The PICS are characterized by NOAA-16 TOA reflectances, over the full range of observed solar zenith angles of a NOAA degrading orbit culminating in a terminator orbit. The NOAA-16 reflectances are first calibrated against Aqua-MODIS using the simultaneous nadir overpass (SNO) method. Site characterization with NOAA-16 has the advantage of reducing the uncertainties associated with spectral band adjustments, since the AVHRR sensor spectral responses are similar. Consistent calibration between the individual desert, polar ice and deep convective cloud PICS approaches validates the methodology. The individual calibration gains are combined to provide the final merged calibration by weighting them by the inverse of their temporal variance. By combining by site stability ensures that site anomalous reflectance drifts do not adversely impact the calibration. Also the merged gain has a lower temporal variability than any individual PICS. In this study we describe the methodology used to derive a new set of calibration coefficients for Channel-1 0.65 (um) and Channel-2 (0.86 um) of the NOAA/AVHRR series of Polar-Orbiting imagers beginning in 1978. We will demonstrate the consistency of the individual PICS calibrations referenced to the merged gain. During the MODIS time frame, we will validate the contemporary AVHRR sensor calibration coefficients against Aqua-MODIS radiances using the simultaneous nadir overpass (SNO) method. We also discuss some of the key AVHRR calibration challenges and how they were overcome.