# Performance Evaluation of IMERG GPM Products during Tropical Storm Imelda
## 3 Results and Discussion
### Temporal Evolution of the Tropical Storm  
Figure 3 illustrates the temporal evolution (17-20 September 2019) of the storm after its landfall and the spatial patterns captured by IMERG Final-run product (Figure 3A) and Stage-IV (Figure 3B). On the day of the landfall (17 September), the Final-run and Stage-IV showed a similar pattern. On the next day (18 September), the two products reported different spatial distributions of the storm. The satellite product missed the hotspot in the southern part of the region, as shown in Figure 3. On the day of the heavy precipitation event (19 September), the satellite-based product failed to capture the intensity of the tropical storm, with a relatively smaller area (4200 km\({}^{2}\)) getting more than 250 mm. However, the radar product estimated almost twice (7700 km\({}^{2}\)) the area reported by IMERG to have received precipitation of more than 250 mm on that day. Moreover, the maximum daily rainfall observed on that day by radar was 2.3 times the maximum daily rainfall observed by IMERG, which was 380 mm. Lastly, the dissipation of the tropical storm was observed on 20 September from both products with quite different spatial patterns.  
Figure 2: The spatial distribution of the cumulative precipitation estimation (\(\geq\)10 mm) captured by IMERG (**A**) Early-, (**B**) Late-, (**C**) Final-run products, and (**D**) Stage-IV products during Tropical Storm Imelda from 14 to 21 September 2019.  
A time series plot of an area average of the tropical storm shows the temporal evolution at a higher temporal resolution as shown in Figure 4. The time series of the event showed that the precipitation data during Tropical Strom Imelda had a bimodal distribution. The first peak was reached at about 11:00 AM on 18 September UTC time zone. The second and largest peak came more than 24 h later at 2:00 PM on 19 September UTC time zone (Figure 4). The second peak was captured by all the products with reasonable accuracy in its rise and fall. However, the first peak was overestimated significantly by the IMERG Final-run product, with an average of 1.5 mmhr\({}^{-1}\) over the study area, which included an area of around 41,100 km\({}^{2}\). This was approximately 62 million cubic meters of water in an hour spread across the entire study area. The Early-run product showed the best fit among the other IMERG products matching both of the peaks of the radar product. The Late-run also showed a similar underestimation in both of the peaks. However, the highest correlation coefficient of the time series was shown between the radar and the Late-run, with a value of 0.98. Moreover, the other two products also showed high correlation coefficients of 0.97 and 0.96 for the Early- and Final-run, respectively. A recent study conducted over the Sichuan Basin of China revealed that the precipitation deviation of the Final-run products was mainly observed during moderate precipitation events (1-10 mmhr\({}^{-1}\)) [67]. Final-run products showed better detection over Early- and Late-run products for light precipitation (<1 mmhr\({}^{-1}\)) events [67]. This might explain the slight underestimations on 17 September and 18 September when precipitation was lower than 1 mmhr\({}^{-1}\), and the overestimations on the following days when precipitation ranged between 1.25-10 mmhr\({}^{-1}\).  
Figure 3: Evolution of the Tropical Storm Imelda after its landfall on 17 September 2019 (daily rainfall \(\geq\)30 mm are shown) and spatial pattern captured by (**A**) IMERG Final-run, (**B**) Stage-IV products.