# Evaluation of IMERG Precipitation Products in the Southeast Costal Urban Region of China
## 4 Results
### Evaluation for Spatial Distribution  
Figure 8 shows the spatial distributions of evaluation metrics on an hourly scale. Results on a daily scale are similar and can be found in the Supplementary Materials (Figure S8). The _MBE_ values for all IMERG products were mainly negative in the north and south, especially in the southernmost coastal areas (Figure 6a-c). The main difference in this metric among the IMERG products was reflected in the central region, showing the concentrated wetter estimates for IMERG ER and IMERG LR, which was not observed for IMERG FR. _Corr_ of the three IMERG products showed a spatial trend of gradually increasing from the north-western hills to the south-eastern plains, with the highest values in the southern coast (Figure 6d-f). The largest _RMSE_ values were shown along the southernmost coastlines, with the moderate and lowest values in the center and north, respectively (Figure 6g-i).  
In terms of detection capability (Figure 6j-r), the highest values of \(POD\) and \(FAR\) for the three IMERG products were both found in the central area with a high urban density, resulting in a significantly lower \(ETS\) in this area. In contrast, coastal areas such as Jiangmen, Zhuhai and southern Huizhou, which have higher \(POD\) and lower \(FAR\) values, showed better \(ETS\) scores. Generally, the estimation accuracy of IMERG was better for the southeast coastal region, while the strongest and weakest detection capabilities were found for the south coast and the central high-density urban areas, respectively. This finding was consistent with the evaluation results for different land covers, as urban areas are mainly located in the south-central plains, cultivated lands are evenly distributed across the study area, and extensive forests are in the northern hills (Figure 1c).  
Using \(Corr\) and \(ETS\) as proxies for accuracy and detection metrics, respectively, the spatial patterns of IMERG performance over the seasons was further explored (Figure 9). The spatial distributions of summer \(Corr\) and \(ETS\) were essentially similar to the year-round results because the annual precipitation in this region is dominated by summer precipitation. By winter, \(Corr\) reached its maximum in the east. In comparison, the areas of high \(Corr\) values of IMERG FR were wider, covering the east, northeast, and west. For detection capability, \(ETS\) values were relatively larger in the north, while the low values all occurred in the south.  
Figure 9: Spatial distribution of \(Corr\) of IMERG ER (in (**a**) summer and (**d**) winter), IMERG LR (in (**b**) summer and (**e**) winter), and IMERG FR (in (**c**) summer and (**f**) winter), as well as \(ETS\) of IMERG ER (in (**g**) summer and (**j**) winter), IMERG LR (in (**h**) summer and (**k**) winter), and IMERG FR (in (**i**) summer and (**l**) winter) on an hourly scale.  
Due to the abundant sources of water vapor as well as frequent tropical weather systems and local convection, coastal areas have more precipitation with certain intensity in summer, while precipitation in the northern mountains is usually relatively weak (Figure S9a). SPPs are deficient in detecting weak precipitation and therefore have overall better performance in the southeast during summer. In winter, the coastal convective activity decreases, and regional precipitation is mainly controlled by frontal troughs. Hence, the precipitation intensity in the south is not as strong as that in the north (Figure S9b). This led to the better performance of IMERG in the northern mountains during winter. It is noteworthy that the evaluated SPPs showed a lack of detection capability for central high-density urban areas in both summer and winter, suggesting a year-round urban influence on precipitation occurrence.