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

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
in situ
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
$12.07 \mu \text{m}$
. 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
$3.71 \mu \text{m}$
, while TB biases from 04:00 to 16:00 UTC are relatively large at
$12.07 \mu \text{m}$
. Moreover, after the operational calibration and update in June 2020, the long-term TB biases at channel 08 (
$3.71 \mu \text{m}$
) obviously decrease from about 5–2.5 K, and the TB biases at channel 14 (
$13.54 \mu \text{m}$
) increase from about 0.5–3 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
in situ
drifter data can well monitor the on-orbit calibration performance of GEO satellite imaging sensor’s IR channels such as the FY-4A/AGRI, allowing an objective assessment on the effect of calibration update events.