# Performance Evaluation of IMERG GPM Products during Tropical Storm Imelda
## 3 Results and Discussion
### Probabilistic Statistical Indices  
In the POD analysis (Figure 10), Final-run was found to have the best results, with the highest POD values out of the three GPM products. The spatial distribution of the POD in all three products reveals that, as expected, the POD was found to be higher in counties that registered higher precipitation. The average POD for all three products was around 0.90, with a higher median of 0.92. For the Final-run product, the POD was observed to be greater than 0.80 in about 95% of the study area. However, the short study period and small study area (8 days with 192 observations) could have skewed the POD values. Moreover, the timeframe included in the study covers an extreme event that can be easily detected by both radar and satellite products. Hence, to normalize the presence of the extreme event, three days before landfall and after dissipation were included in the analysis. In Figure 11, the false alarm ratio (FAR) followed similar patterns to those of the POD. Lower FAR values were registered in the areas heavily affected by the storm, and higher FAR values were seen in the periphery of the study area. All three IMERG products showed a similar spatial pattern. The Early- and Late-run products showed a relatively better FAR, with an average value of 0.40, however, the Final-run product registered a FAR value of 0.44. The Early product showed its advantage in FAR by registering a lower median, of about 0.35, compared to 0.38 and 0.42 in the Late- and Final-run products, respectively. Nearly two-thirds (67%) of the study area for the Early-run product showed a FAR of less than 0.4, whereas, only 40% of the areas in the Final-run showed a FAR of less than 0.4. Generally, the FAR by all the IMERG products was found to be very high, especially when considering the analysis was performed for a short period (8 days) and in an extreme event condition. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Study Period** & **Product** & **CC** & **RMSE** & **RBIAS (\%)** & **KGE** \\ \hline
14–21 & Early & 0.574 & 3.74 & \(-\)4.0 & 0.36 \\ September & Late & 0.600 & 3.66 & \(-\)11.00 & 0.39 \\
2019 & Final & 0.595 & 3.88 & 41.00 & 0.16 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The areal average of the basic statistical indices of the satellite products. Figure 9: Spatial distribution of KGE for the IMERG (**A**) Early-, (**B**) Late-, and (**C**) Final-run products during Tropical Storm Imelda. The spatial distribution of the critical success index (CSI) again supported that the near-real-time IMERG products (Early and Late) outperformed the Final-run, especially in the areas where the storm was most severe (Figure 12). The Early-run product showed the best performance in terms of CSI, with an average CSI of 0.57 and a median of 0.60. The Final-run product was found to have the lowest CSI, having an average value of 0.53 and a median of 0.55. To put this into perspective, in the Early-run product, about 20% of the study area showed a CSI of more than 0.7, but only 5% of the area recorded a similar CSI in the Final-run product. Figure 11: Spatial distribution of FAR for the IMERG (**A**) Early-, (**B**) Late-, and (**C**) Final-run products during Tropical Storm Imelda. Figure 10: Spatial distribution of POD for the IMERG (**A**) Early-, (**B**) Late-, and (**C**) Final-run products during Tropical Storm Imelda. The Peirce skill score (PSS) helps by providing insights into the ability of satellite products to separate the actual occurrences of precipitation from the no-occurrence instances. PSS shows the overall robustness of the system to accurately predict the tropical storm event. The Early- and Late- products again outperformed the Final-run in the PSS analysis (Figure 13), in a similar way as for FAR and CSI. The spatial distribution of PSS from the three products showed a PSS of more than 0.70 in case of the Early and Late products, however, only 36% of the area was found to have a PSS higher than 0.70 in the Final-run product.