*4.2. Improvement of the OIMERG Precipitation Data Achieved via OI*

Figure 5 shows comparisons of the CC, MAE, and RMSE values calculated before and after the OI-based merging of the OIMERG data and the observation data. After OI, the CC values increased in almost all months; this effect was the most obvious for September 2017, with an increase of 0.27. The CC was slightly worse after OI for only two months: by 0.01 for May 2014 and by 0.02 for August 2014. In addition, the RMSE generally decreased by 0.37–5.16 mm, with slight increases observed for only two months (by 0.06 mm for September 2016 and by 0.23 mm for August 2014). The MAE decreased in all months except August 2014, for which it increased by 0.17 mm. Overall, compared with the OIMERG precipitation data, the CIMERG data obtained after OI showed improvements in the mean CC, RMSE, and MAE values from 0.516, 28.83 mm, and 19.73 mm to 0.616, 26.56 mm, and 17.59 mm, respectively. Therefore, the OI processing of the OIMERG and observed precipitation data greatly improved the precision of the original satellite precipitation product.

**Figure 5.** Comparisons of the (**a**) CC, (**b**) mean absolute error (MAE) and (**c**) root-mean-square error (RMSE) before and after OI processing for the warm seasons of 2014–2018.
