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. Atmospheric water vapor plays a key role in Earth's radiationbalance and hydrological cycle, and the precipitable-water-vapor (PWV)product under clear-sky conditions has been routinely provided by theadvanced Medium Resolution Spectral Imager (MERSI-II) on board Fengyun-3Dsince 2018. The global evaluation of the PWV product derived from MERSI-II isperformed herein by comparing it with PWV from the Integrated GlobalRadiosonde Archive (IGRA) based on a total of 462 sites (57 219 matchups)during 2018–2021. The monthly averaged PWV from MERSI-II presents adecreasing distribution of PWV from the tropics to the polar regions. Ingeneral, a sound consistency exists between PWV values of MERSI-II and IGRA; their correlation coefficient is 0.951, and their root mean squared error(RMSE) is 0.36 cm. The histogram of mean bias (MB) shows that the MB isconcentrated around zero and mostly located within the range from −1.00 cmto 0.50 cm. For most sites, PWV is underestimated with the MB between−0.41 and 0.05 cm. However, there is also an overestimated PWV, which ismostly distributed in the area surrounding the Black Sea and the middleof South America. There is a slight underestimation of MERSI-II PWV for allseasons with the MB value below −0.18 cm, with the bias being the largestmagnitude in summer. This is probably due to the presence of thin clouds,which weaken the radiation signal observed by the satellite. We also findthat there is a larger bias in the Southern Hemisphere, with a large valueand significant variation in PWV. The binned error analysis revealed thatthe MB and RMSE increased with the increasing value of PWV, but there is anoverestimation for PWV smaller than 1.0 cm. In addition, there is a higherMB and RMSE with a larger spatial distance between the footprint of thesatellite and the IGRA station, and the RMSE ranged from 0.33 to 0.47 cm.There is a notable dependency on solar zenith angle of the deviationsbetween MERSI-II and IGRA PWV products.

Global evaluation of the precipitable-water-vapor product from MERSI-II (Medium Resolution Spectral Imager) on board the Fengyun-3D satellite