# Evaluation of IMERG Precipitation Products in the Southeast Costal Urban Region of China
## 4 Results  
### Evaluation across Temporal Scales  
Figure 2 presents the comparison of the IMERG estimates with the gauge observations and the corresponding metrics at different temporal scales during the whole year. On an hourly basis, the performance differences between the three IMERG products are not significant. The overall \(Corr\) values range from 0.37 to 0.39, the negative \(MBE\) values range from 0.004 to 0.011 mm, and the \(RMSE\) values range from 1.50 to 1.54 mm. It was apparent that the accuracy of the three IMERG products was not good for hourly estimation. Many point-pairs were dispersed, close to or even located on the boundary axes, suggesting a big deficiency of IMERG in estimating light or high-intensity precipitation over short periods. The detection capability of IMERG was also insufficient, with a low hit rate (\(POD\) ranged between 0.52 and 0.56) and instead, a higher misdetection rate (\(FAR\) ranged between 0.59 and 0.60). As a result, none of the three IMERG products achieved \(ETS\) scores above 0.25, indicating a weak detection ability.  
As shown by the daily metrics (Figure 2d-f), the three IMERG products performed significantly better on a daily rather than hourly basis, but there was still much room for improvement. Probably due to the correction by the GPCC measurements, IMERG FR outperformed IMERG ER and IMERG LR in terms of agreement with gauge data and overall error magnitude. This calibrated product also avoided some apparently overestimated records (e.g., some points in the upper left corner of Figure 2d,e). For the detection of precipitation occurrence, the IMERG products showed a high hit rate in capturing actual precipitation events. However, there were many cases where IMERG misidentified non-precipitation events as precipitation events, probably due to the high sensitivity of the sensors. Among the different runs of IMERG, the detection ability of IMERG FR outperformed IMERG LR and IMERG ER.  
Figure 2: Comparison of estimates from IMERG ER (on **(a)** hourly and **(d)** daily scales), IMERG LR (on (**b**) hourly and **(e)** daily scales), IMERG FR (on (c) hourly and (f) daily scales) with gauge measurements. The blue lines in the figures are 1:1 lines and the red lines are the fitted trend lines.  
The performance of the three IMERG products differed over different seasons on an hourly basis (Figure 3). The results on a daily timescale can be referred to in the Supplementary Materials (Figure S1). The best scores for all metrics were achieved generally in the summer, with the second and third best scores occurring in the spring and autumn, and the worst in the winter. Similar results were found in previous studies across the Northern Hemisphere, including mainland China [69], Europe [55] and the United States [53]. The main reason may be that satellites are better at detecting and estimating local convection with a certain intensity [53], and this type of precipitation occurs mostly during summer in the Southeast China. In addition, weak precipitation is more common in winter compared to other seasons, while detecting weak precipitation has been a major challenge for satellite precipitation estimation. This issue was also pointed out by a study in the United States [34] that missed precipitation is a major error source during the winter. The three IMERG products showed no distinct differences for spring, summer, and autumn. But in the winter, IMERG FR outperformed the other two near-real-time products, showing the relatively good stability of IMERG FR performance across seasons.  
Figure 3: Variation of (**a**) \(MBE\), (**b**) \(Corr\), (**c**) \(RMSE\), (**d**) \(POD\), (**e**) \(FAR\), and (**f**) \(ETS\) across different seasons on an hourly scale.