(1) Temporal Variation

Table 7 compares the mean values of the three categorical metrics (POD, FAR, and CSI) over the seven months from April to October and seasonally (spring, summer, fall) among the SPPs at the hourly scale. Similar to the daily scale, three categorical metrics are used to assess the hourly precipitation detection capabilities of the SPPs. An hourly rainfall threshold of 0.1 mm/d is used in calculating the metrics. As seen from the table, with lower PODs and CSIs, as well as higher FARs, all five SPPs are poorer at detecting hourly rainfall than daily rainfall.


**Table 7.** Mean categorical evaluation metrics of the SPPs at hourly scale.

<sup>a</sup> Spring extends from April to May; Summer extends from June to August; Fall extends from September to October.

Seasonally, the five SPPs have exhibited somewhat similar patterns of change in hourly rainfall detection performance. In summer, they all have the highest correct rainfall detection rates (PODs), but also the highest false alarm rates (FARs). Between the rest two seasons, all five SPPs have higher PODs as well as lower FARs, therefore better rainfall detection performance, in spring. In fact, the seasonal CSIs indicate that the overall rainfall detection performance of the SPPs all tops in spring, followed by summer, and then winter.

Figure 10 further examines the changes in mean categorical metrics over a diurnal cycle in three seasons. Among the three categorical metrics, PODs, especially those of IMERG\_L and IMERG\_F, exhibit relatively less hourly variations through the diurnal cycle. The only discernible pattern in the metric is that the IMERG products tend to have the highest correct rainfall detection rates (> 0.7) in early evening, while the TMPA products have the lowest at midnight in spring. Unlike POD, FAR exhibits more diurnal variations. In spring, FARs of all five SPPs tend to peak around noon. Whereas, in summer and fall, they all tend to bottom in the morning and climb to the peak at around midnight. Similar to FAR, CSI exhibits distinct diurnal variations. In spring, all SPPs have the lowest CSI, i.e., the poorest hourly rainfall detection performance at noon and midnight. In summer and fall, the performance of the SPPs tends to peak in the morning, and bottom out around midnight.

**Figure 10.** Changes in mean categorical metrics over a diurnal cycle in three seasons: (**a**) *POD* in spring; (**b**) *POD* in summer; (**c**) *POD* in fall; (**d**) *FAR* in spring; (**e**) *FAR* in summer; (**f**) *FAR* in fall; (**g**) *CSI* in spring; (**h**) *CSI* in summer; and (**i**) *CSI* in fall.

Similar to the results of the continuous metrics at the hourly scale, the values of all three categorical metrics are not much different among the SPPs, indicating the IMERG products are no longer superior to the TMPA products in hourly rainfall detection.

The values of the categorical metrics obtained in this study lie at the medium level among those obtained in previous limited studies. For example, Omranian and Sharif [56] evaluated the Early, Late, and Final runs of IMERG products for May 2015 in the Lower Colorado River Basin of Texas in U.S. At the hourly scale, they obtained a POD value of 0.74, a FAR value of 0.45 and a CSI value of 0.46. However, in their evaluation of the 3-hour daytime satellite rainfall estimates in the Chindwin

River basin, Myanmar, Yuan et al. [55] obtained a POD value of <0.3 for all three IMERG and two TMPA products.
