## 3 Results and Discussion
### Spatial Distributions of Continuous Error Metrics
Additionally, IMERG-E measures 33% higher extreme precipitation rates than IMERG-F. Figure 3 presents the bias in the IMERG estimates as compared to the IMD rain-gauge-based dataset. Near-real-time IMERG products (e.g., IMERG-E and IMERG-L) show a larger magnitude of bias as compared to IMERG-F estimates. IMERG-E and IMERG-L show similar bias features such as underestimation of the monsoon rainfall over the west coast, northeast, and northern India, and overestimation over the remaining regions. However, the magnitude of the bias is about 0.5-1 mm day\({}^{-1}\) less in IMERG-L than IMERG-E across the country. The bias features are similar but with lower magnitude in IMERG-F estimates as compared to IMERG-E and IMERG-L over most parts of the country except along the west coast. The magnitude of bias in IMERG-F V6 is about 4 mm day\({}^{-1}\) less than IMERG-E and IMERG-L estimates. However, IMERG-F V6 does not show any notable improvement in bias from IMERG-F V5. It can also be seen that magnitude of the bias is higher by about 4 mm day\({}^{-1}\) in IMERG-F V6 over the west coast and southernmost parts of the country during the pre-GPM period (e.g., 2010-2013) than the GPM period (2014-2017). Unlike IMERG-E and IMERG-L estimates, the IMERG-F product shows an underestimation of monsoon rainfall along the west coast (location shown in Figure 1a) followed by an overestimation. The topography, mean monsoon rainfall from the rain-gauge-based dataset, and bias in IMERG-F V6 product over the west coast for 2014-2017 are shown in Figure 4. The coastal region has very low elevation, and elevation increases drastically eastward, known as the Western Ghats mountain range. The rain-gauge-based dataset shows that higher rainfall occurs over the coastal belts (upto about 100 km to the coast) and becomes very less or negligible eastward. The windward side of the Western Ghats receives higher monsoon rainfall associated with low-level monsoon jet and orography,  
Figure 2: Spatial distributions of 4-year mean monsoon rainfall (mm day\({}^{-1}\)) across India from the IMD rain-gauge-based data and IMERG estimates for the GPM (2014–2017) and pre-GPM (2010-2013) periods. whereas the leeward side receives very low rainfall [49]. Heavy rainfall activities along these coastal areas during the southwest monsoon season are also associated with offshore troughs/vortices and offshore convective systems linked to atmospheric conditions over the equatorial Indian Ocean [51; 52]. However, warm rain events dominate over the Western Ghats due to rather shallower clouds [53], and IMERG products have limited accuracy in warm precipitation estimation than ice-initiated precipitation [54]. As IMERG-F utilizes monthly rain gauge observations for bias correction, the magnitude of the bias is reduced as compared to near-real-time products. However, highly varied topography and rather coarser resolution of rain-gauge-based input dataset (e.g., GPCC) to the IMERG-F introduce contrasting bias features in the IMERG-F product along the west coast. Additionally, it was shown that rain gauge correction has no impact on the accuracy of IMERG V6 products over the global complex terrain areas [34]. The distinct diurnal cycle of precipitation over the coastal and complex topographic regions associated with local-scale effects is not adequately captured by the satellite-derived precipitation products, and this error propagates in the daily precipitation estimates [55; 56]. The combined use of satellites, rain gauges, and weather radars would be beneficial for better precipitation estimation over such regions [57; 58; 59]. The spatial distributions of CV from the rain-gauge-based dataset and IMERG products across India for the southwest monsoon season are illustrated in Figure 5. As expected, a larger magnitude of CV over the lower mean monsoon rainfall regions and smaller CV over the higher mean rainfall regions are depicted by both rain-gauge-based and multi-satellite products qualitatively.