(3) Spatial Variation

Figure 5 compares the spatial distribution of the four annual continuous evaluation metrics at the daily scale among the SPPs. The spatial distribution of the CCs varies considerably among the IMERG products, while staying similar between the TMPA products. Topography does not seem to be a significant influencing factor of the CCs, although all five SPPs have lower CCs at some stations of higher altitude (station 12 and 13). In terms of RMSEs and MADs, however, topography plays a more eminent role. For all five SPPs, both metrics tend to get larger at higher altitude. In particular, RMSEs and MADs of three stations at high altitude (stations 10, 12, and 13) consistently surpass those of the rest stations. In addition, topography seems to affect the RBs of the IMERG products considerably, which tend to underestimate daily rainfall more seriously at high altitude. The impact of topography on the RBs of the TMPA products, however, is rather mixed. The absolute RB of the 3B42RT product is actually smaller at higher altitude. Similar to our study, Wang et al. (2019) [47] also observes more serious underestimation of daily rainfall by the IMERG products at high altitude in the Hexi region deep in the hinterland of the Eurasian continent. The underestimation of rainfall at high altitude by the SPPs could owe to local precipitation augmentation induced by topographical lift.

**Figure 5.** Spatial distribution of the annual continuous evaluation metrics at daily scale: (**a**) *CC*; (**b**) *RMSE*; (**c**) *RB*; and (**d**) *MAD*.
