Water Vapor Retrievals from Near-infrared Channels ofthe Advanced Medium Resolution Spectral ImagerInstrument onboard the Fengyun-3D Satellite

Water vapor plays a key role in weather, climate and environmental research on local and global scales. Knowledge about atmospheric water vapor and its spatiotemporal variability is essential for climate and weather research. Because of the advantage of a unique temporal and spatial resolution, satellite observations provide global or regional water vapor distributions. The advanced Medium Resolution Spectral Imager (MERSI) instrument—that is, MERSI-II—onboard the Fengyun-3D (FY-3D) meteorological satellite, has been one of the major satellite sensors routinely providing precipitable water vapor (PWV) products to the community using near-infrared (NIR) measurements since June 2018. In this paper, the major updates related to the production of the NIR PWV products of MERSI-Ⅱ are discussed for the first time. In addition, the water vapor retrieval algorithm based on the MERSI-Ⅱ NIR channels is introduced and derivations are made over clear land areas, clouds, and sun-glint areas over the ocean. Finally, the status and samples of the MERSI-Ⅱ PWV products are presented. The accuracy of MERSI-Ⅱ PWV products is validated using ground-based GPS measurements. The results show that the accuracies of the water vapor products based on the updated MERSI-Ⅱ instrument are significantly improved compared with those of MERSI, because MERSI-Ⅱ provides a better channel setting and new calibration method. The root-mean-square error and relative bias of MERSI-Ⅱ PWV products are typically 1.8-5.5 mm and -3.0％ to -14.3％, respectively, and thus comparable with those of other global remote sensing products of the same type.