## 5 Results and Analysis of Experiment
### Rainfall Detection Results
In Figure 13a, the yellow dotted line denotes that the detection threshold is set as 30%. Among the 360 radar images, 180 rain-contaminated radar images and 160 rain-free radar images are detected by the ZPP method, and the total accuracy is 94.4%. Therefore, the detection performance of the ZPP method with redefined threshold is higher than that of the traditional ZPP method with threshold set at 50%. Figure 13c shows the rainfall detection results of the RZE method when the rainfall detection area is defined in the wave area. The blue solid line denotes the curve trend of the calculated RZE, and the selected area with green solid line denotes the radar image in the presence of rainfall. The red dotted line denotes the detection threshold K determined in Section 4.1, and the yellow dotted line denotes the calculated threshold Z when the threshold of the ZPP is set as 30%. Among the 360 radar images, 200 rain-contaminated radar images and 108 rain-free radar images are detected under the detection threshold K, and 182 rain-contaminated radar images and 159 rain-free radar images are detected under the detection threshold Z. The total detection accuracies of the two thresholds are 85.6% and 94.7%, respectively, which is similar to the detection accuracy of the ZPP method, but the detection speed of the RZE method is faster. The reason is that the RZE parameter is \(1/\mu\) times of the ZPP parameter when the detection area of the two parameters is defined in the same wave area. Therefore, the RZE method can also complete the task of rainfall detection when the detection area is defined in the wave area, but the detection performance of the RZE method in the wave area is far less than that in the occlusion area. To better reflect the advantages of the RZE method for rainfall detection in the occlusion area, the performance of the RZE method and ZPP method are compared in Figure 14. The RZE method for rainfall detection in the wave area is not compared in Figure 14, because its detection performance is similar to the performance of the ZPP method. The horizontal coordinate denotes the radar image sequence, the left vertical coordinate denotes the rainfall intensity, and the right vertical coordinate denotes the rainfall detection results of three methods. The value of 1 denotes the radar image in the presence of rainfall, and the value of 0 denotes the radar image in the absence of rainfall. The blue solid line denotes the rainfall intensity, and the yellow, green and red solid lines denote the rainfall detection results of the ZPP method (50% threshold), the ZPP method (30% threshold) and the RZE method. It can be seen that rain-free images are detected as rain-contaminated images by the ZPP method (50% threshold) in radar image sequence of 69 to 120, and rain-contaminated images are detected as rain-free images by the ZPP method (30% threshold) in radar image sequence of 295 to 320. The RZE method has the highest accuracy of rainfall  
Figure 13: The rainfall detection results of the X-band marine radar images. **(a)** The ZPP method; **(b)** The RZE method for rainfall detection in the occlusion area; **(c)** The RZE method for rainfall detection in the wave area. detection, so the proposed method has better rainfall detection performance than that of the ZPP method. The detection results of the two methods are statistically analyzed, as shown in Table 3. The results show that the accuracy of the RZE method (occlusion area) is 11.7% higher than that of the traditional ZPP method (50% threshold) and 2.3% higher than that of the ZPP method (30% threshold). The accuracy of the RZE method (wave area and threshold K) is approximately equal to that of traditional ZPP method (50% threshold), and the accuracy of the RZE method (wave area and threshold Z) is approximately equal to that of ZPP method (30% threshold).