## 3 Results
### Validating the Quality of IMERG Data Based on Diurnal Variations  
Figure 5 illustrates the diurnal variation of the average annual cycle of hourly precipitation amount, frequency, and intensity for the four regions examined. In terms of total precipitation (top row in Figure 5), both gauge observations and IMERG data showed distinct diurnal patterns across the four regions. In North America, IMERG data were quite consistent with gauge observations, with precipitation peaking at 23:00 UTC and reaching minimal values around 15:00-16:00 UTC. However, IMERG tended to overestimate the peak-hour precipitation (41 mm/h for gauge observations and 44 mm/h for IMERG data) and underestimate the trough-hour precipitation (30 mm/h for gauge observations and 28 mm/h for IMERG data). IMERG could adequately reflect the diurnal cycle of hourly precipitation in spite of the above-mentioned overestimation and underestimation, which may be related to local effects associated with the terrain. In Europe, IMERG generally overestimated the diurnal cycle of hourly precipitation, especially at peak hours (35 mm/h for gauge observations and 45 mm/h for IMERG data), with mismatched phases between these two datasets. Distinct bimodal patterns of precipi  
Figure 5: Diurnal variation of average annual cycle of hourly precipitation amount (**top row**), precipitation frequency (**middle row**), and precipitation intensity (**bottom row**) for different regions; the red and blue lines represent gauge observations and IMERG data, respectively; (**a**) North America, (**b**) Europe, (**c**) North Asia, and (**d**) South Asia. The shadow represents the nighttime (20:00–08:00) for the local time. tation intensity were detected in South Asia, but IMERG overestimates a small variation in North Asia. The peak structures observed in IMERG was close to observation in South Asia but showed a large disparity at around 09:00 in North Asia. This disparity may be related to the systematic problem with satellite inversion in terms of precipitation frequency and intensity. Many drizzles can evaporate into the air before the small raindrops arriving at the ground, thus not being captured by the rain gauge, especially at daytime. However, the satellite estimates precipitation from the scattering of ice crystals in the clouds, resulting much larger frequency but smaller intensity for IMERG data than gauge observation. The ability of the IMERG product to detect precipitation at different hours for the four regions examined is shown in Figure 6. The regional mean values of POD, FAR and CSI are listed in Table 2. POD values fluctuating around approximately 50% throughout the day were detected in each region. Specifically, in North America, the peak daily precipitation occurred at 16:00 local time, with POD reaching about 55%. High precipitation rates were maintained between 16:00 and 23:00 local time, declining thereafter until reaching a minimum around 10:00 the next day (POD of approximately 45%). In contrast, in Europe, the lowest POD for hourly precipitation (around 35%) was detected around 11:00 local time and then increased to a maximum of 45% at 11:00 local time. Then, the POD values remained near 45% until 06:00 the next day. In North Asia, three peak POD values were detected at 19:00, 23:00, and 04:00 local time. The difference in POD between daytime and nighttime, which ranged from 50% to 55%, was not significant, possibly due to the high nighttime precipitation in the Sichuan Basin boosting the POD values at this time. In contrast, a distinct diurnal variation was detected in North Asia, with a peak POD value of approximately 57% at around 16:00 local time and dropping to a minimum of 45% at around 02:00 the next day.