Post calibration of channels 1 and 2 of long-term AVHRR data record based on SeaWiFS data and pseudo-invariant targets

Abstract

Advanced very high resolution radiometer (AVHRR) data provide a global, long-term, consistent time series with favorable spectral and spatial resolution. Such measurements are necessary for climate change studies. However, significant problems with the AVHRR calibration have been seen in two separate sources of early AVHRR data — lack onboard calibration and observed trends and biases when the AVHRR radiances are compared with other accurate top-of-atmosphere (TOA) radiances sources. In this paper, new calibration coefficients for channels 1 and 2 of the AVHRR sensors onboard 16 NOAA satellites from TIROS-N, NOAA-6 to NOAA-19, and Metop-A, B are reported. The calibration slopes are derived based on several pseudo-invariant sites endorsed by the Committee on Earth Observation Satellites (CEOS). Sea-Viewing Wide Field-of-View Sensor (SeaWiFS) channels 6 and 8 radiance data are applied in characterizing the bidirectional reflectance distribution functions (BRDF) of the TOA reflectance over the employed targets, and MODTRAN simulations are carried out to parameterize the spectral channel adjustment factors (SBAFs) between SeaWiFS and each AVHRR sensor to eliminate the spectral response differences over different targets considering the impacts of observing geometries and gaseous absorption. The BRDFs and SBAFs are then applied to filtered AVHRR time-series to generate time series of calibration slopes presented in the form of quadratic polynomials as a function of time and applied in the recalibration of AVHRR data records. Comparisons with calibration coefficients generated from other products exhibit preferable validity and consistency, and notable incoherence and trend in the TOA reflectance time series of the L1B AVHRR data over each site from all the launched NOAA series polar orbiting satellites are largely eliminated after the recalibration. The long-term and inter-sensor consistent AVHRR data records after this post calibration can provide valuable insight into global climate changes since 1978.