A Uniform Calibration Approach Using Multiple Invariant Earth Targets for the 35-year AVHRR Visible Record

• Main Authors: Doelling, D., Bhatt, R., Scarino, B., Gopalan, A., Haney, C.
• Format: Text
• Language: unknown
• Published: DigitalCommons@USU 2015
• Subjects: North Pole
• Online Access: https://digitalcommons.usu.edu/calcon/CALCON2015/All2015Content/7; https://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=1207&context=calcon

The 35-year Advanced Very High Resolution Radiometer (AVHRR) satellite record provides the longest time series of consistent imager data suitable for decadal climate studies. Critical to summing the observations from multiple AVHRR sensors is their consistent radiometric calibration. Due to lack of on-board calibration system, the calibration of the complete AVHRR record is possible only through vicarious approaches using invariant Earth targets. However, owing to the degrading orbits of the NOAA satellites housing the AVHRR instruments, the calibration becomes more challenging and requires the invariant targets to be characterized with a full range of solar zenith angles, which is not possible with MODIS and VIIRS observations as their satellite orbits are well-maintained. We present NOAA-16 AVHRR-based calibration models for multiple invariant Earth targets, including four desert sites, two polar regions, and tropical deep convective clouds, that account for the degrading satellite orbits and uniformly calibrate the AVHRR visible and near-infrared record. All the invariant targets are referenced to the Aqua-MODIS Collection-6 calibration by transferring the Aqua-MODIS calibration to NOAA-16 AVHRR using simultaneous nadir overpass (SNO) radiance pairs over the North Pole. Spectral band adjustment factors (SBAF) based on SCanning Imaging Absorption SpectroMeter for Atmospheric CartograpHY (SCIAMACHY) spectral radiances over the invariant targets are computed to account for the spectral band differences between AVHRR and MODIS sensors. In order to derive optimal AVHRR gains, the results derived from individual targets are combined using an inverse-variance-weighted regression approach. The consistency of the AVHRR calibration is validated by comparing with the Aqua-MODIS-to AVHRR SNO calibration during the MODIS timeframe, as well as by analyzing the temporal variance of the 35-year AVHRR reflectance time series over the invariant Earth targets for the entire record.