## 3 Results and Discussion
### Error Metrics for Different Rainfall Intensity Intervals  
In this section, domain-mean daily monsoon rainfall from the IMERG products has been evaluated against the rain-gauge-based dataset for different rain intensity intervals. IMD categorizes daily monsoon rainfall intensity into seven intervals. In this study, very light rainfall (or traces) of less than 2.5 mm day\({}^{-1}\) is not considered due to the sensitivity of precipitation-related satellite sensors. Additionally, two different groups, namely, very heavy and extremely heavy rainfall intervals, have been grouped as one category for this study due to rather less number of such events. Therefore, five rainfall intervals defined in Table 4 have been used in this study. The evaluation is carried out for domain-mean rainfall for all-India, central India (20\({}^{\circ}\)-25\({}^{\circ}\) N and 75\({}^{\circ}\)-85\({}^{\circ}\) E) and west coast (10\({}^{\circ}\)-20\({}^{\circ}\) N and 73\({}^{\circ}\)-78\({}^{\circ}\) E). Two domains, namely, central India and the west coast, are shown in Figure 1a. Central India and the west coast receive a fairly good amount of southwest monsoon rainfall, and rain gauge density over these sub-regions is good enough (e.g., Figure 1b). Central India is a plain region, whereas the west coast is an orographic region. Central India receives substantial monsoon rainfall associated with large-scale features such as monsoon trough, lows or/and depressions, whereas the west coast monsoon precipitation process is a combination of both large-scale and local-scale features. As poor rain gauge density leads to larger uncertainty in the error characteristics [(63)], northeast India is not considered. Only three IMERG estimates (e.g., V6 of IMERG-E, IMERG-L, and IMERG-F) for 2014-2017 have been considered for this analysis. \begin{table}
\begin{tabular}{c c} \hline \hline
**Rain Category** & **Daily Rainfall Range (mm)** \\ \hline Light & 2.5–7.5 mm \\ Moderate (Mod) & 7.5–35.5 mm \\ Rather heavy (RHvy) & 35.5–64.5 mm \\ Heavy (Hvy) & 64.5–124.5 mm \\ Very heavy (VHvy) & \(\geq\)124.5 mm \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Five rainfall categories based on daily rainfall amounts have been used in this study. The daily rainfall ranges for these five rain categories are adopted from the IMD. Daily rainfall less than 2.5 mm is not considered, and two rain categories—very heavy and extremely heavy—are grouped into very heavy rain category in this study. Figure 10: Spatial distributions of frequency bias index and Peirce skill score in the detection of rainy days from the IMERG estimates as compared to the rain-gauge-based data over India for the GPM (2014–2017) and pre-GPM (2010–2013) periods. Figure 11 presents the frequency of daily monsoon rainfall of distinct intensity intervals and their contributions to the total monsoon rainfall for all-India and two sub-regions. The frequency of light and moderate rain is altogether about 55-60% based on the rain-gauge-based data, which are 40-42% from the IMERG estimates. Both near-real-time and research products of IMERG V6 underestimate the frequency of light to very heavy rainfall over all-India and two sub-regions. This is due to considerable overestimation of frequency of very light rainfall of less than 2.5 mm day\({}^{-1}\) by the IMERG estimates as compared to the rain-gauge-based dataset. Although the frequency of heavy to very heavy rainfall is 2-3%, it contributes 20-23% of the total monsoon rainfall as inferred from the rain-gauge-based dataset. Moderate rainfall contributes 40-50% of total monsoon rainfall. The contribution of different rainfall intensity intervals to the total monsoon rainfall is captured well by the IMERG estimates. However, near-real-time IMERG products overestimate the contribution of moderate rainfall to the total monsoon rainfall over the west coast of India.