Nonnegligible Diurnal and Long-Term Variation Characteristics of the Calibration Biases in Fengyun-4A/AGRI Infrared Channels Based on the Oceanic Drifter Data

Abstract

This study mainly focuses on the diurnal and long-term variation characteristics of the calibration performance of the geostationary (GEO) meteorological satellite Fengyun-4A/ Advanced Geostationary Radiation Imager (FY-4A/AGRI) infrared (IR) channels from June 1, 2017 to December 31, 2020. An improved algorithm is developed in this research based on the <i>in situ</i> observations from ocean drifters, which can capture the diurnal variations of the calibration performance of FY-4A/AGRI IR channels. The results suggest that there are significant and nonnegligible diurnal variations of the uncertainties and biases for the brightness temperature (TB) observed by the FY-4A/AGRI, especially the IR channels at 3.71, 8.61, 10.83, and <inline-formula> <tex-math notation="LaTeX">$12.07 \mu \text{m}$ </tex-math></inline-formula>. Among them, the calibration performance from 16:00 to 18:00 UTC (around the time of local midnight at the subsatellite point of FY-4A) is worse at <inline-formula> <tex-math notation="LaTeX">$3.71 \mu \text{m}$ </tex-math></inline-formula>, while TB biases from 04:00 to 16:00 UTC are relatively large at <inline-formula> <tex-math notation="LaTeX">$12.07 \mu \text{m}$ </tex-math></inline-formula>. Moreover, after the operational calibration and update in June 2020, the long-term TB biases at channel 08 (<inline-formula> <tex-math notation="LaTeX">$3.71 \mu \text{m}$ </tex-math></inline-formula>) obviously decrease from about 5&#x2013;2.5 K, and the TB biases at channel 14 (<inline-formula> <tex-math notation="LaTeX">$13.54 \mu \text{m}$ </tex-math></inline-formula>) increase from about 0.5&#x2013;3 K, implying a possible positive or negative impact of the calibration update on the calibration performance of IR channels. Overall, this method based on the <i>in situ</i> drifter data can well monitor the on-orbit calibration performance of GEO satellite imaging sensor&#x2019;s IR channels such as the FY-4A/AGRI, allowing an objective assessment on the effect of calibration update events.