# A Process-Oriented Method for Tracking Rainstorms with a Time-Series of Raster Datasets
## 4 Results and Discussion
### Comparisons of PoAIR and TITAN  
The results of the PoAIR and TITAN algorithms are shown in Figure 9. PoAIR combines spatial similarity and thematic evolution to track rainstorm objects at successive time snapshots, which resolves the problem of excessively fast rainstorm movement. Additionally, PoAIR uses the evolution sequence, i.e., all the objects from their generation to T, to extrapolate the spatial location of the object at the time snapshot T+1, instead of only at snapshot T, which is used by TITAN. Thus, PoAIR tracked all the simulated rainstorm processes except process 1, process 6, and process 9 (Figure 9a). As it uses the overlapped-area threshold to track an object at successive time snapshots, TITAN has great difficulty tracking rainstorm processes, and identifies each of the simulated rainstorm processes as two or more independent processes (Figure 9b). The main reason for this is that each simulated rainstorm process has at least one object that moves too fast. Compared with TITAN, PoAIR has an improved capability to track fast-moving rainstorms. However, when a rainstorm reaches its maximum threshold of movement speed, i.e., 58 km/h in this \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Name** & **Start –End Time** & **Number of Objects** & **Detailed Information** \\ \hline Rainstorm 1 & T2–T10 & 9 & A basic process has no merging, splitting, or merging-splitting objects, and turns dramatically at T4. \\ \hline Rainstorm 2 & T1–T13 & 22 & A complicated process has three splitting objects and one merging object. \\ \hline Rainstorm 3 & T2–T14 & 19 & A complicated process has two splitting objects, one merging object, and one merging-splitting object. \\ \hline Rainstorm 4 & T3–T15 & 17 & A complicated process has one splitting object, two merging objects, and one merging-splitting object. \\ \hline Rainstorm 5 & T4–T14 & 11 & A basic process has no merging, splitting, or merging-splitting objects. \\ \hline Rainstorm 6 & T4–T14 & 22 & A complicated process has one splitting object, one merging object, and one merging-splitting object. \\ \hline Rainstorm 7 & T3–T14 & 19 & A complicated process has one splitting object, one merging object, and one merging-splitting object. \\ \hline Rainstorm 8 & T1–T14 & 25 & A complicated process has two splitting objects, two merging objects, and one merging-splitting object. \\ \hline Rainstorm 9 & T5–T12 & 8 & A basic process has no merging, splitting, or merging-splitting objects, and moves with a speed of 65 km/h at T9 (exceeding the speed threshold). \\ \hline Rainstorm 10 & T4–T14 & 18 & A complicated process has one splitting object and two merging-splitting objects. \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Detailed information of the simulated rainstorm dataset shown in Figure 8.  
Figure 9: Results of the PoAIR and TITAN algorithms. (**a**) Results of the PoAIR algorithm using simulated data. (**b**) Results of the TITAN algorithm using simulated data. paper, PoAIR has no ability to track it. For example, PoAIR identified Rainstorm 9 as two independent processes since at T9 the rainstorm moved with a speed of 65 km/h. Additionally, PoAIR also has difficulty tracking rainstorms which dramatically change their movement direction. The reason for this is that the spatial location of the rainstorm at time T+1 is extrapolated from its generation to time T, and when it changes direction, the distance between the rainstorm's actual location and the extrapolated one exceeds the maximum search radius of PoAIR. For example, Rainstorm 1 turns dramatically to the right at T4, which leads to the position of the rainstorm at T5 exceeding the maximum search radius of PoAIR, and thus Rainstorm 1 was identified as two independent processes. As PoAIR uses a process-sequence-state semantic to reconstruct rainstorm processes, it is more insensitive to noise than the TITAN algorithm (Figure 9b). In this paper, noise is defined as an independent object, which is neither connected with other objects in space nor evolves in successive time snapshots.