## 4 Discussion
### Discrepancies in Precipitation Frequency and Intensity  
The analysis in this study revealed that IMERG data and gauge observations exhibited similar spatial distribution characteristics, particularly in Asia and the United States. This was mainly due to the improved inversion ability of the new-generation GPM product, which can detect light rain and solid precipitation [34; 35; 42]. However, IMERG slightly overestimated annual precipitation amounts (especially in regions experiencing lower annual precipitation), with data exhibiting relatively larger deviations compared to ground station observations. Similar overestimations of IMERG data in precipitation have also been reported [34]. Significant deviations in precipitation frequency were observed in northern China and western United States, while IMERG performed well in estimating this parameter in the European region. This may be related to the fact that, due to oceanic climate condition, the seasonal distribution of precipitation frequency is relatively even [73; 74]. In terms of precipitation intensity, IMERG data showed noticeable deviations from gauge observations across different regions, underestimating high precipitation intensity in China and the United States but overestimating it in Europe. This disparity may be related to the evaporation of small raindrops before arriving at the ground, and therefore many drizzles cannot be captured by the rain gauge, which can be identified by the remote sensors from the scattering of ice crystals in the clouds [12]. Past studies in a predominantly tropical region also attributed the low performance of IMERG precipitation intensity to the difficulty in estimating precipitation from warm clouds [75]. The results are in line with previousstudies where precipitation frequency was also overestimated while the intensity was underestimated, leading to annual precipitation amounts that were comparable with those derived from gauge observations [21; 37]. The diurnal cycle of precipitation is an important feature in hydrometeorological studies and also is a key aspect in the evaluation of satellite precipitation products with a high spatial-temporal resolution [21; 72]. This study also evaluated the detectability of GPM IMERG products in reproducing the diurnal cycle of precipitation using several quantitative and qualitative statics. It was found that IMERG data and gauge observations described very similar diurnal precipitation cycles in South Asia. However, the former overestimated precipitation at peak hours but underestimated it at trough hours in North America, and generally overestimated the diurnal variations (with a large degree of overestimation at peak hours) in Europe and North Asia. This may be related to the influence of climatic factors and the fact that these areas are prone to experience heavy rainfall, especially in the summer [25; 72]. The two datasets showed similar patterns of fluctuation of the hourly precipitation frequency in the four regions examined, although the daily fluctuation for this parameter in IMERG data was large, while gauge observations fluctuated more stably. The POD of hourly precipitation in each region generally fluctuated around 50%. Gauge observations have been demonstrated to reflect rather stable precipitation frequencies and intensities during the diurnal cycle, while IMERG data have shown strong fluctuations mainly because of inaccurate estimates from the cross-track microwave sensors at individual hours [21]. In terms of the interannual variation in annual precipitation total, IMERG data and gauge observations showed similar trends due to the correction of the former dataset using monthly ground observations [7; 33; 55]. Both datasets detected a clear increasing trend in annual precipitation total in North America and a downward trend in North Asia. This is basically consistent with the variation in the annual precipitation frequency in these areas.