# Performance Evaluation of IMERG GPM Products during Tropical Storm Imelda
## 3 Results and Discussion
### Basic Statistical Indices  
Figure 5 shows the CC spatial patterns between Stage-IV radar data and each of the three IMERG-GPM satellite products during Tropical Storm Imelda. The CC spatial pattern shows the consistency of the IMERG-GPM products with more detail. As there is high spatial variability during extreme events, the CC pattern helps to evaluate the performance of the satellite products and illustrate their characteristics. The spatial patterns of CC show a higher consistency in the coastal areas and in places hit hardest by the storm (Figure 5). The average CC was almost the same for the three IMERG products, at 0.574, 0.600, and 0.595 for the Early-, Late-, and Final-run products, respectively. The spatial distribution also shows similar behaviors for all of the IMERG products. Higher CC values were observed for all comparisons over the counties of Jefferson, Liberty, and Montgomery in the northeastern part of the study area (Figure 5). Generally, coastal areas with a high amount of precipitation had relatively higher correlation coefficients (CC) than the areas further inland. Two-thirds of the study area (above 67%) showed a CC higher than 0.5 for all IMERG products. Though the Late-run product showed the highest consistency of CC throughout the study area, the Final-run product showed a better consistency in the regions near the center of the storm. An earlier study [53] on the same region during hurricane Harvey found that the complex  
Figure 4: Time series plot of areal average precipitation estimates captured by the IMERG and Stage-IV products during Tropical Storm Imelda (17–21 September 2019). Figure 5: Spatial distributions of the correlation coefficient (CC) between Stage-IV radar data and (**A**) IMERG Early-, (**B**) Late-, and (**C**) Final-run products over 17 Counties in Texas (\(n=192\); hourly from 14–21 September 2019). structure of hurricane Harvey might have significantly affected the performance of the satellite products near the center of the hurricane. Figure 6 shows scatterplots of IMERG Early-, Late- and, Final-run satellite products with Stage-IV radar product during Tropical Storm Imelda. The scatterplot helps to visually inspect the alignment of the individual pixels of the two products during the event). Similar to the CC values, all products followed similar patterns, with almost the same linear model fit (R\({}^{2}\)) with minor variations in the slopes (0.46, 0.43, and 0.44 for Early-, Late-, and Final-run, respectively). All the slopes were found to be significantly different than zero at a 0.01 significance level. There were significant differences between the radar estimates and those of the satellite at a pixel scale. Moreover, overestimation of light precipitation and underestimation of heavy precipitation is evident from the scatterplots. However, there was a clear improvement in the quality of the data from the Early-run to the Final-run. In Figure 6, the systematic error is seen as an apparent point cloud, appearing as a ceiling in the top part (Figure 6A). That ceiling of the point cloud is much lower in the Late-run and disappears in the Final-run. The coefficient of determination (R\({}^{2}\)) with a value around 0.4 shows a poor fit for the IMERG products. The data visualization and analysis in Figure 6 shows that the IMERG algorithm needs further improvements to minimize systematic errors for the Early- and Late-run products. The spatial distribution of the RMSE (Figure 7) shows an almost similar pattern in all the IMERG products. It is clearly seen that areas highly affected by Tropical Strom Imelda (southeast coastal regions stretching up to the border with Louisiana) show a higher RMSE, with two hot spots in all three products. The RMSE of the products suggests that the products were improved, in minimizing the average error compared to a similar study conducted in the same area earlier [53]. The mean and median RMSE was 3.74 mm and 3.67 mm, 3.88 mm and 3.27 mm, and 3.15 mm and 3.42 mm for the Early-, Late-, and Final-run, respectively.