Abstract. Atmospheric water vapor plays a key role in Earth's radiation
balance and hydrological cycle, and the precipitable-water-vapor (PWV)
product under clear-sky conditions has been routinely provided by the
advanced Medium Resolution Spectral Imager (MERSI-II) on board Fengyun-3D
since 2018. The global evaluation of the PWV product derived from MERSI-II is
performed herein by comparing it with PWV from the Integrated Global
Radiosonde Archive (IGRA) based on a total of 462 sites (57 219 matchups)
during 2018–2021. The monthly averaged PWV from MERSI-II presents a
decreasing distribution of PWV from the tropics to the polar regions. In
general, 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 is
concentrated around zero and mostly located within the range from −1.00 cm
to 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 is
mostly distributed in the area surrounding the Black Sea and the middle
of South America. There is a slight underestimation of MERSI-II PWV for all
seasons with the MB value below −0.18 cm, with the bias being the largest
magnitude in summer. This is probably due to the presence of thin clouds,
which weaken the radiation signal observed by the satellite. We also find
that there is a larger bias in the Southern Hemisphere, with a large value
and significant variation in PWV. The binned error analysis revealed that
the MB and RMSE increased with the increasing value of PWV, but there is an
overestimation for PWV smaller than 1.0 cm. In addition, there is a higher
MB and RMSE with a larger spatial distance between the footprint of the
satellite 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 deviations
between MERSI-II and IGRA PWV products.