Analysis of the Sea-Land Contrast Bias in Sounding Channels of MWTS-III Onboard Fengyun-3E

The third-generation microwave temperature sounder (MWTS-III) onboard Fengyun-3E (FY-3E) can obtain the atmospheric temperature profile in the early morning time, which is valuable for numerical weather prediction (NWP), climate analysis, and other environment studies. However, calibrated brightness temperature (BT) of the surface-insensitive sounding channels (i.e.,channels 7-17) have a sea-land contrast bias. It is 0.2-0.3 K for channels 9-17 and -1.33 and -2.86 K for channels 7 and 8, respectively. The bias characteristic of channels 9-17 is similar to that of the corresponding channels of FY-3C, which is caused by the interference from a window channel and can be corrected by an empirical method. Moreover, the spectral response functions of channels 7 and 8 were examined over a wider frequency range by measuring the instrument of similar design and construction to the on-orbit MWTS-III. The measured data show that channels 7 and 8 have out-of-band response located between 50 and 52 GHz, which is close to the response range of channel 4. A linear function developed in this letter is effective for the BT bias correction. The sea-land contrast bias of these two channels has been decreased to -0.35 and -0.16 K. Also, the daily mean bias of channels 7 and 8 is improved from -1.8 and -2.3 K to -0.5 and -0.4 K, respectively. This correction function has been integrated into the FY-3E MWTS-III operational calibration program in December 7, 2022. Meanwhile, the subsequent MWTS-III has been further improved to avoid the sea-land contrast bias.