First PWV Retrieval Using MERSI‐LL Onboard FY‐3E and Cross Validation With Co‐Platform Occultation and Ground GNSS.

FY‐3E plays a vital role in the meteorological global earth observing system. Precipitable water vapor (PWV) is an essential parameter for the water cycle and global climate change. Here, we carry out the PWV retrieval using the MERSI‐LL sensor onboard the FY‐3E satellite for the first time. The retrieval accuracy under different cloudage conditions is validated by the extra PWV from ground‐based GNSS and spaceborne occultation. For the results against ground‐based GNSS, the total accuracy shows an RMSE of 2.69–3.36 mm as the clouds increase, and correlation coefficients higher than 0.95. The spatial accuracy distribution indicates that inland stations have higher accuracy than the coast and island stations. As for the results against spaceborne occultation, the verification accuracy varies with the spatial pairing distance, showing poor accuracy in the low latitude area. This study can provide an essential reference for the community to understand the current water vapor inversion performance of MERSI‐LL. Plain Language Summary: Fengyun 3E (FY‐3E), as the world's first civilian early morning (EM) orbit meteorological remote sensing satellite, plays an important role in filling the EM‐orbit gap in the global earth observing system. Precipitable water vapor (PWV) is crucial for understanding the process of water cycle, global climate change, and numerical weather prediction. This study exhibits the first attempt to retrieve PWV using the MERSI‐LL sensor onboard the FY‐3E satellite. The accuracy of the PWV retrieval under different cloud conditions is validated by ground‐based GNSS and spaceborne occultation data. Results from the comparisons with ground‐based GNSS data reveal that RMSE ranges from 2.69 to 3.36 mm as cloud increases, with correlation coefficients higher than 0.95. Furthermore, the spatial distribution of accuracy indicates that inland stations exhibited higher precision than coastal and island stations. The verification accuracy against spaceborne occultation decreases as the spatial pairing distance increases, and the results reveal that poor accuracy occurs in the low latitude area. This study provides crucial insights into the water vapor retrieval of MERSI‐LL, which will be valuable for the scientific community in understanding the current capabilities of this sensor. Key Points: First time to retrieve Precipitable water vapor (PWV) with FY‐3E MERSI‐LL data under all weather conditionsRetrieval accuracy was cross‐validated both with co‐platform occultation and ground GNSSRandom Forest (RF) method with the advantage of high accuracy and stability was applied to retrieve PWV [ABSTRACT FROM AUTHOR]