## 5 Results and Analysis of Experiment
### Rainfall Intensity Level Retrieval Results  
The radar image detected as rainfall in Section 5.1 is used to determine the rainfall intensity level through the fitting function of the rainfall intensity obtained in Section 4.2. The RZE parameter calculated by the Equation (5) is substituted into the fitting function, and the theoretical value of rainfall intensity is 0.4411 mm/10 min. According to Table 2, the rainfall intensity level is moderate rain, which is consistent with the actual rainfall intensity measured by the rain gauge. Therefore, the result of the rainfall intensity level retrieval by the fitting function is correct. In order to further verify the performance of the rainfall intensity level retrieval, a total of 600 rain-contaminated radar images are selected for experiments. Among them, due to the lack of data of micro rain and heavy rain, 100 radar images are selected by the two classification standards, respectively. The data of light rain and moderate rain are relatively large, and 200 radar images are selected by the two classification standards, respectively. The actual rainfall intensity of 600 rain-contaminated radar images is shown in Figure 15a. As the measurement accuracy of the rain gauge is 0.1 mm, the actual rainfall intensity measured by the rain gauge can produce a certain range of error. According to Table 2, the  
Figure 14: The radar image sequence comparison among the RZE method, the ZPP method (30% threshold), the ZPP method (50% threshold) and the rainfall intensity. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & \multicolumn{2}{c}{**Rain-Free**} & \multicolumn{2}{c}{**Rain-Contaminated**} & \multirow{2}{*}{**Total Images (360)**} \\  & **Images (160)** & & & **Images (200)** & \\ \cline{2-5}  & **Correct** & & & **Correct** & \\  & **Number** & **Accuracy** & **Number** & **Accuracy** & **Total Accuracy** \\ \hline ZPP (50\% threshold) & 106 & 66.3\% & 200 & 100\% & 85\% \\ ZPP (30\% threshold) & 160 & 100\% & 180 & 90\% & 94.4\% \\ RZE (occlusion area) & 160 & 100\% & 188 & 94\% & 96.7\% \\ RZE (wave area and threshold K) & 108 & 67.5\% & 200 & 100\% & 85.6\% \\ RZE (wave area and threshold Z) & 159 & 99.4\% & 182 & 91\% & 94.7\% \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The results of rainfall detection by the ZPP and RZE method. actual rainfall intensity is divided into four levels: micro rain, light rain, moderate rain and heavy rain, which can better verify the accuracy of the rainfall intensity calculated by the fitting function, as shown in Figure 15b. Figure 16a shows the RZE value of 600 selected rain-contaminated radar images calculated by the Equation (5), and the corresponding value of the rainfall intensity calculated by the fitting function is shown in Figure 16b. It can be seen that the retrieval data of the rainfall intensity level in the color classification area is correct, and the retrieval data outside the color classification area is incorrect. The statistical results of the rainfall intensity level are shown in Table 4. When the rainfall intensity level is light rain, the rainfall in the occlusion area of the radar image is unstable and the rainfall distribution is uneven, which makes the accuracy of rainfall intensity retrieval low. When the rainfall intensity level is moderate rain or heavy rain, the occlusion area of the radar image is seriously disturbed by rainfall and the brightness changes greatly, which makes the accuracy of rainfall intensity retrieval higher. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Actual Rainfall Intensity Level** & **Micro Rain** & **Light Rain** & **Moderate Rain** & **Heavy Rain** \\ \hline standard/mm/10 min & 0–0.1 & 0.1–0.25 & 0.25–0.7 & 0.7–1.5 \\ \hline Total data & 100 & 200 & 200 & 100 \\ Retrieved as micro rain & 88 & 19 & & \\ Retrieved as light rain & 12 & 156 & 16 & \\ Retrieved as moderate rain & 25 & 170 & 10 \\ Retrieved as heavy rain & & & 14 & 90 \\ \hline Accuracy & 88\% & 78\% & 85\% & 90\% \\ Total accuracy & & & 84\% & \\ \hline \hline \end{tabular}
\end{table}