# Evaluation of IMERG Precipitation Products in the Southeast Costal Urban Region of China
## 5 Discussion
### Performance of IMERG  
The evaluation shows that IMERG cannot accurately represent hourly and daily precipitation in the southeast coastal urban areas of China. As presented in Figure 2, the IMERG products have poor estimation accuracy and weak detection ability (low hit rate and high misdetection rate), which is consistent with some previous works conducted in Guangdong province [37; 71; 85; 86]. Poor performance of IMERG is also found in coastal regions such as Brazil [49], India [87], Mediterranean islands [50] and eastern China [88]. This may be attributed to the complex radiation conditions in coastal regions [52]. The ocean surface is radiometrically cold and homogeneous, while the land surface is warm and heterogeneous [21]. As the transitional zones, coastal regions include radiative contributions from both land and ocean surfaces, which poses a great challenge for coastal precipitation retrieval.  
For different precipitation intensities, IMERG significantly overestimates weak precipitation and underestimates high-intensity precipitation, which has been previously reported in regions such as the Guangdong Province [37; 71; 86], eastern China [70; 72], North America [43], and Central Asia [73]. The overestimation of weak precipitation is probably due to the misdetection of dynamic land surface signals by PMW [51]; while the underestimation of high-intensity precipitation comes from the insignificant association of some heavy precipitation types with the atmospheric signatures relied upon by PWM and IR retrieval [86]. Besides, the Bayesian averaging of the GPROF retrieval algorithm and the morphing process of the IMERG algorithm are also causes of wetter weak precipitation and drier heavy precipitation, respectively [51; 78].  
Analysis for urban and non-urban areas demonstrates a year-round negative impact of the urban areas on IMERG detection ability. Within the urban areas, IMERG performs better in areas with lower urbanization levels. Similar results were found in another study for Southeast China, which showed a larger uncertainty in IMERG estimates for urban precipitation [88]. This results from the greater uncertainty in urban precipitation occurrence caused by aerosol emissions, artificial surface roughness, urban thermal effects, and water vapor changes [80; 81; 82].  
In addition, the gauge-corrected IMERG FR does not show a significant improvement over IMERG ER and IMERG LR, which is likely related to the limited temporal resolution and number of GPCC observations in the study area. This finding is also noted in the studies of [PERSON] et al. [37] and [PERSON] et al. [86] in similar regions.  
Notably, limited by the accessibility of gauge data, we only performed the validation for the year of 2018. However, this one year of hourly data does not affect our evaluation results. First, we focused mainly on the IMERG performance in describing precipitation occurrence and amount on an hourly basis. Second, each precipitation intensity level had a sufficient number of rainfall events which could be used for evaluation. Third, the study area also experienced tropical storms during the year of 2018, such as typhoon Ewiniar in June and typhoon Mangkhut in September, which provided an opportunity to evaluate large high-intensity precipitation. Therefore, we consider our results to be robust and also reliable for other years.