Evaluation and Correction of the FY-3D MWHS-2 On-Orbit Calibration Biases Against S-NPP ATMS

This paper presents the evaluation and correction of the on-orbit calibration biases of the Microwave Humidity Sounder 2 (MWHS-2) aboard Fengyun 3D (FY-3D) against the Advanced Technology Microwave Sounder (ATMS) on Suomi National Polar-orbiting Partnership (S-NPP) satellite using the double difference method. First, a microwave radiative transfer model (RTM) and a calibration bias correction method are developed. Then, the matching observations in the year of 2022 between the MWHS-2 and the ATMS channels are collected over both sea surfaces and land surfaces. Next, the brightness temperatures at top of atmosphere in the MWHS-2 and ATMS channels are simulated using the RTM constructed in this work. After that, the double differences and the theoretical observations are computed. Finally, the calibration biases in the MWHS-2 channels are corrected. The results show that the RTM in this work is valid and accurate. The simulations of the RTM in this work basically agree with that obtained by the latest version of Radiative Transfer for TOVS (RTTOV). The impact of the simulation differences between the two RTMs and the combined influence of the simulation errors and spectral response differences on the final intercalibration results are quantitatively analyzed. In June of 2022, the calibration biases in FY-3D MWHS-2 channels 1, 11∼15 are -1.89±5.13 K, -2.78±1.02 K, -1.21±1.05 K, -0.54±1.14 K, 1.35±0.88 K, and -1.19±1.47 K, respectively. In the whole year of 2022, the calibration biases in the MWHS-2 channels are generally stable: in the MWHS-2 channels 1, 11∼15, the maximum variations in two consecutive months are 1.42 K, 0.31 K, 0.36 K, 0.43 K, 0.37 K, and 0.39 K, respectively, while the maximum variations in 2022 are 2.03 K, 0.37 K, 0.38 K, 0.53 K, 0.60 K, and 0.63 K, respectively. The on-orbit calibration biases in the MWHS-2 channels are corrected against the ATMS channels.