Intercalibration of FY-4A AGRI Thermal Infrared Channels Against AHI Channels Using the Double Difference Method

Abstract

This letter addresses the intercalibration of the thermal infrared (TIR) channels 11 (8.0–9.0 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> ), 12 (10.3–11.3 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> ), 13 (11.5– <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$12.5~\mu \text{m}$ </tex-math></inline-formula> ), and 14 (13.2–13.8 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> ) of the Advanced Geostationary Radiation Imager (AGRI) on Chinese Fengyun 4A (FY-4A) satellite against the Advanced Himawari Imager (AHI) on the Himawari 8 using the double difference (DD) method with the data in January and July of 2020. To transfer the radiometric calibration from AHI to FY-4A AGRI, an accurate TIR radiative transfer model and intercalibration equations are constructed. The results show that AGRI channel 12 is well calibrated and keeps stable in the two months. However, in other AGRI TIR channels, radiometric calibration biases are obviously observed, especially AGRI channel 11, in which the calibration biases vary by hemisphere and month. The observations in AGRI channels 13 and 14 are about 0.83 and 0.50 K underestimated, respectively. In AGRI channel 11, the observations are averagely 2.36 and 4.08 K overestimated in Northern Hemisphere and Southern Hemisphere, respectively. Moreover, in Southern Hemisphere, the observations in AGRI channel 11 in July are averagely 0.55 K warmer than those in January. The intercalibration coefficients were obtained by linear regression, and finally, the radiometric calibration biases in the AGRI TIR channels were successfully removed.