## 3 Results
### Validation of IMERG Data Quality Based on Interannual Variability  
Figure 7 shows the comparisons of the interannual variability of annual precipitation total, frequency, and intensity between IMERG data and gauge observations for the four regions examined. IMERG overestimated total precipitation in individual years, but the data were consistent with gauge observations for all the four regions. This may be due to the fact that the IMERG data had been corrected based on monthly ground precipitation observations. The discrepancy was slightly larger in Europe, possibly because of the sparse grid network used in this study leading to larger errors in IMERG data. The average annual precipitation total in North America, Europe, and North Asia varied between 500 mm/y and 1000 mm/y. A significant upward trend was detected in this parameter in North America, which increased from over 500 mm/y in 2001 to around 1000 mm/y in 2020, possibly due to climate change. Indeed, the water cycle or specific regional topography and circulation patterns in this region would be strengthened under global warming. Conversely, the annual precipitation total in North Asia decreased from 1000 mm/y in 2001 to 750 mm/y in 2020. This could be attributed to widespread human activities associated with land use and land cover changes. Due to its geographical location and monsoon system, South Asia exhibited a considerably higher annual precipitation total compared with the other three regions, with values fluctuating around 1200 mm/y and significant interannual variability. Figure 7: Interannual variations in regional annual precipitation total (**first row**), precipitation frequency (**second row**), and hourly precipitation intensity (**third row**). In these plots, the red and blue lines represent gauge observations and IMERG data, respectively. Panels (**a–d**) correspond to North America, Europe, North Asia, and South Asia, respectively. IMERG overestimated the annual precipitation frequency in North America and North Asia by 2.5% and 2%, respectively, and the two regions showed an increasing and decreasing trend in this parameter, also respectively. The IMERG data obtained for Europe in terms of interannual precipitation variability matched well to the gauge observations, although these were slightly higher than the IMERG estimates. In terms of the hourly precipitation intensity, IMERG data performs best in South Asia, fluctuating around 1.7 mm/h. However, in North Asia and North America, IMERG generally underestimated precipitation intensity, while it overestimated precipitation frequency. In North Asia, the difference was minimal (1.25 mm/h), while in North America, it was pronounced, exceeding 1 mm/h. In Europe, as observed for total precipitation, IMERG overestimated precipitation intensity by 0.25 mm/h compared to gauge observations. Figure 8 shows the interannual variations in precipitation in the four regions examined as measured by the three above-mentioned qualitative indicators, i.e., POD, FAR, and CSI. POD exhibited an upward trend in each region, with the values in North America, South Asia, and North Asia all exceeding 50% in the later years of the study period, while a lower POD of around 40% was observed in Europe. FAR showed a decreasing trend in North America and Europe, and this may attribute to the small number of meteorological stations in the early stage of the study period. Conversely, a clear increasing trend in this parameter was observed in North Asia and South Asia, possibly due to the increased difficulty in accurately measuring precipitation amid the rise of extreme precipitation events associated with global warming. CSI, which reflects the trends in both POD and FAR, showed an increasing trend in North America and Europe. In the Asian regions, this index exhibited a downward trend, particularly in North Asia, where it decreased from around 30% in 2001 to approximately 26% in 2020.