*3.1. Mean Status*

Figure 1a shows the spatial distribution of precipitation over Taiwan, averaged from summers of 2014–2017. From the CWB data, it is noted that larger precipitation is observed in southwest Taiwan than in the other sub-regions. This is due to the interaction between the prevailing southwesterly summer monsoonal flow (not shown) and the local topography (Figure 1b), which can lead to more precipitation occurring on the windward side of the mountains (i.e., southwest Taiwan) [30]. Visually, all four of the SPPs can qualitatively depict the feature with more significant precipitation over southwest Taiwan. However, GSMap7 seems better than the others in illustrating the location of maximum precipitation. This might due to the inclusion of an orographic effect for additional upward motion and moisture flux convergence in the GSMaP algorithm [29,42].

Also worth noting in Figure 1a, all four of the SPPs tend to underestimate the amount of summer mean precipitation over most areas of Taiwan. This might be because that satellite methods underestimate heavy precipitation associated with shallow orographic precipitation systems [42]. Despite the weakness in validating the magnitude of precipitation, a further comparison between the values of precipitation area-averaged over Taiwan indicates that IMERG6 (~8.9 mm·d<sup>−</sup>1), relative to the other SPPs (TRMM7~8.0, IMERG5~8.5, and GSMaP7~8.6 mm·d−1), is closer to the CWB data (~11.0 mm·d<sup>−</sup>1).

Based on Figure 1a, the Scorr and the RMSE are then calculated for the comparison between the SPPs and the CWB data. As seen in Figure 1c, TRMM7 has the largest RMSE and the smallest Scorr. Compared to the performance of TRMM7 (i.e., Scorr <sup>=</sup> 0.53, RMSE <sup>=</sup> 15.5 mm·d<sup>−</sup>1), the Scorr of IMERG6 (~0.73) is increased by approximately 37.7% [= (0.73 – 0.53)/0.53 × 100%] and the RMSE of IMERG6 (~9.4 mm·d<sup>−</sup>1) is reduced by approximately 39.4% [<sup>=</sup> (15.5 – 9.4)/15.5 <sup>×</sup> 100%]. This suggests that using IMERG6 to replace TRMM7 can increase the performance by approximately 35%~40% in depicting the spatial distribution of summer mean precipitation over Taiwan.

In addition to IMERG6, IMERG5 (i.e., Scorr = 0.72, RMSE = 10.9 mm·d−1) and GSMaP7 (i.e., Scorr <sup>=</sup> 0.82, RMSE = 10.2 mm·d−1) also outperform TRMM7 in illustrating the spatial distribution of summer mean precipitation over Taiwan (Figure 1c). Part of the reason for this might be that the original spatial resolution of IMERG5, IMERG6, and GSMaP7 (0.1◦ × 0.1◦) is higher than that of TRMM7 (0.25◦ × 0.25◦). Therefore, although TRMM7 has been re-gridded into the same spatial resolution (0.1◦ × 0.1◦) for the comparison (see Section 2), the performance of TRMM7 is still worse than the other higher resolution SPPs for depicting the precipitation that occurs over complex terrain.

To reveal whether the above suggestion is dependent on the altitude, we further compared the performance of SPPs at different altitudes. Worth noting in Figure 1d, there is an obvious increase in the difference between the SPPs and the CWB data, as the altitude increases. Among the four SPPs, the performance of IMERG6 (GSMaP7) is closer to the CWB data at most altitudes below 1000 m (higher than 1500 m), while TRMM7 has the largest bias over most altitudes. These features indicate again that the new GPM SPPs (including IMERG5, IMERG6, and GSMaP7) outperform TRMM7 in depicting the distribution of summer mean precipitation over Taiwan; this finding is not dependent on the altitude.
