# A Random Forest-Based Precipitation Detection Algorithm for FY-3C/3D MWTS2 over Oceanic Regions
## 4 Results  
### A Single-Case Analysis  
#### 4.1.1 Matching \(RF\_SI\) to AMSU-A Pixels  
Figure 9a shows the \(RF\_SI\) results produced for the FY-2F pixels obtained via multispectral precipitation detection. Figure 9b shows the FY-2F VISSR pixel-by-pixel CLC product on 6 June 2017, at 21:00 UTC. The red box corresponds to the scanning region of NOAA-19 AMSU-A during transit, with a 7 min observation time difference relative to that of the FY-2F satellite. Figure 9a contains regions with \(RF\_SI\) values exceeding 45 K along the equator between 120\({}^{\circ}\)E and 160\({}^{\circ}\)E and in the region between 80\({}^{\circ}\)E and 90\({}^{\circ}\)E and 5\({}^{\circ}\)N and 15\({}^{\circ}\)N, which usually corresponds to the cumulonimbus clouds shown in Figure 9b. Near the region at approximately 80\({}^{\circ}\)E, 10\({}^{\circ}\)S, the \(RF\_SI\) values, which range between 30 K and 40 K, also correspond to cumulonimbus clouds. However, a large area of cirrus clouds is contained in the Sea of Japan region near the edge of the image, where the average \(RF\_SI\) is 35 K, which is significantly different from the assumption of convective scattering in the \(SI\) description. According to the results of the sensitivity analysis (Figure 8), cirrus information can affect the \(RF\_SI\) values, and the large number of cirrus clouds contained in the Sea of Japan region with many ice crystal particles may have a complicating effect on microwave scattering, which leads to the calculation of higher \(RF\_SI\) values. The \(RF\_SI\) values obtained at approximately 30 K and 35 K in the red box correspond to mixed clouds (equatorial region), but in the region of 30\({}^{\circ}\)S, a large area of mixed clouds is present (Figure 9b), corresponding to an average \(RF\_SI\) of 15 K in Figure 9a, with some differences between the two. Overall, in the matched spatial and temporal regions, \(RF\_SI\) has a good correlation with VISSR convective systems, such as cumulonimbus clouds. However, complex microwave  
Figure 9: Comparison between FY-2F-retrieved cloud parameters and those of \(RF\_SI\) at 21:00 UTC on 6 June 2017. (**a**) \(RF\_SI\) values derived from random forest model simulations (the red box corresponds to the scanning region of NOAA-19 AMSU-A during transit) and (**b**) FY-2F VISSR cloud classification products. scattering caused by large cirrus clouds may bias the _RF_SI_ values. Additionally, may be misclassifications may be observed over the edge areas of VISSR observations. Figure 10a-c show the pixel analysis results obtained within the scanning region of AMSU-A (corresponding to the red box in Figure 9). The original resolution of the _RF_SI_ products can reach 5 km, whereas the traditional _SI_ and _CLWP_ products have resolutions of only 45 km. To compare _RF_SI_ with the _SI_ at the same resolution, it is necessary to map the high-resolution _RF_SI_ data onto the pixels of AMSU-A, as shown in Figure 10a. Figure 10b displays the AMSU-A _SI_ data, and Figure 10c shows the AMSU-A _CLWP_ data. In the microwave observation images, high _SI_ values (Figure 10b) are present in the regions of the Bay of Bengal (17 degN, 92 degE and 10 degN, 90 degE) and the Indian Ocean (10 degS, 90 degE), with a maximum _SI_ value of 50 K. The corresponding _CLWP_ in Figure 10c also exhibits good correspondence, with a maximum _CLWP_ of 0.9 g/kg, which aligns with the location of the highest _SI_ value. The _RF_SI_ values in the three strong scattering areas also correspond well with the SIs. The _RF_SI_ values produced over the maximum _SI_ value regions are 50 K, demonstrating a largely good degree of location overlap. Additionally, for the weak scattering conditions near 8 degS, 82 degE, with _SI_ values falling between 30 K and 35 K, the _SI_ and _CLWP_ \(>\) 0.5 g/kg correspond well in terms of location. The _RF_SI_ values over these regions are approximately 30 K, so they are largely within the range of the scattering threshold.