(1) Temporal Variation

Table 3 compares the mean values of the continuous evaluation metrics among the SPPs at the daily scale. Annually, all four continuous evaluation metrics except RB have indicated a relatively better performance by the IMERG products in estimating daily rainfall. For example, the annual RMSEs of the IMERG products range from 9.66 mm/d (IMERG\_F) to 11.30 mm/d (IMERG\_E), while those of the TMPA products both exceed 11.50 mm/d. Seasonally, however, while IMERG\_F generally remains the best product for estimating daily rainfall, the other two near-real-time IMERG products tend to perform better than the TMPA products in spring and summer, but often worse in fall and winter (except for RB).


**Table 3.** Mean continuous evaluation metrics of the SPPs at daily scale.

<sup>a</sup> Spring extends from March to May; Summer extends from June to August; Fall extends from September to November; Winter extends from December to the following February.

Within each SPP family, the order of the daily rainfall estimation accuracy is largely consistent, i.e., IMERG\_F > IMERG\_L > IMERG\_E (except for RB) and 3B42 > 3B42RT. Moreover, except for RB, the five SPPs have exhibited rather similar seasonal patterns of change in their daily metrics. For example, the CCs of the five SPPs all peak in summer and fall to the bottom in fall. The RMSEs and MADs of the SPPs all tend to peak in summer, decline in spring, and down to the lowest in fall and winter. Throughout the year, the Shuaishui River Basin is affected by different climatic systems. Mainly under the influence of the high-altitude trough, precipitation in fall and winter is mostly brought by stratiform clouds, which tends to be stable and therefore easier to measure. In spring and summer, however, the convective component in the precipitation system increases due to the Meiyu front and shear line system. Both thermal convection precipitation under the control of the Western Pacific Subtropical High (WPSH) and rainstorms caused by the typhoon system increase the difficulty of obtaining accurate measurement of rainfall because of their characteristics of short duration and high spatial heterogeneity. The differences in climatic systems have led to different seasonal rainfall characteristics, with more rainy days and higher rainfall intensity in spring and summer than in fall and winter. Between 2009 and 2017, there have been 529 and 589 rainy days in spring and summer,

compared to 425 and 452 days in fall and winter. Days with precipitation < 1 mm account for 44.2% and 41.8% in fall and winter, compared to 30.6% and 26.7% in spring and summer. Meanwhile, days with precipitation > 50 mm account for 4.9% and 8.3% in spring and summer, compared to around 1.5% in fall and winter. The CCs' peaking in summer could be attributed to the season's large variability in daily rainfall, whose general pattern of change is relatively well captured by the SPPs. However, because there are more days with heavy precipitation in summer, the absolute errors of SPP estimates remain the largest in the season. Similarly, the lowest RMSEs and MADs in fall and winter are probably owing to their dominance of days with lower precipitation.

Similar to our findings, Su et al. [46] concluded that the post-time IMERG-F product, with a CC of 0.79, RMSE of 6.31 mm/d, and RB of 9.04%, was the best IMERG product for estimating daily rainfall in the Upper Huai River Basin of China. Meanwhile, Anjum et al. [43] found the post-time 3B42 V7 product, with a CC of 0.70 and RB of 14.77%, performed better than the real time 3B42RT product for estimating daily rainfall in Pakistan.

Compared to those at the monthly scale, the CCs of all five SPPs have decreased considerably at the daily scale. For instance, the annual CC of IMERG\_F drops from 0.97 at the monthly scale to 0.81, while the annual CCs of the other SPPs all drop further to around 0.75. In contrast to the CCs, the RBs of the SPPs at the daily scale are more similar to those at the monthly scale in terms of both their signs and magnitude. Annually, all three IMERG products tend to underestimate daily precipitation with the lowest RB of −9.99%, while both TMPA products tend to overestimate with the lowest RB of 1.21%. Seasonally, the IMERG family products tend to underestimate daily rainfall in all four seasons except the two near-real-time products in winter. In contrast, 3B42RT exhibits a strong tendency of overestimation in summer and winter, while 3B42 only in winter.
