# Preliminary Validation of Surface Reflections from Fengyun-3C Radio Occultation Data
## 3 Methods and Results  
### Comparison of FY-3C Refractivity Profiles with COSMIC  
The GNSS RO reflections frequently happen on smooth land, ocean and ice surfaces. Globally, the reflected occultations present seasonal and latitudinal patterns [11]. For example, the percentage of reflections from middle latitude to polar regions is higher than that from the low latitude regions. In the winters of the northern and southern hemispheres, the percentage of reflections is also higher due to the snow and ice cover on the land's surface [11]. Figure 1 shows the global distribution of FY-3C- and COSMIC-matched event groups from January to February (top) and August to September (bottom) during 2015. As previously reported, the reflected signal tend to occur in colder and drier conditions, and are strongly related with the total column water [12]. In our study, the distribution of matched events between FY-3C and COSMIC is identical with a pattern that shows that the reflections prefer to occur in colder and drier conditions. As is shown in Figure 1, the matched events between FY-3C and COSMIC are mainly gathered at both hemispherical latitudes between 40-50\({}^{\circ}\), while the matched events at the tropics and polar regions are sparsely scattered. At the tropical oceans, the number of matched events falls as the water vapor above the ocean surface increases. Over the larger ocean area in the southern hemisphere, where it is more suitable for reflections to occur, there are more matched events in the southern hemisphere than in the northern hemisphere. Besides, the numberof matched events over land in January and February (top) is larger than in August and September (bottom) at a latitude of >60\({}^{\circ}\). Such distribution patterns may be caused by the cover of snow and ice over land in winter; thus, the reflections occur. As shown in Figure 2, the number of COSMIC RO reflection events in each month is around 7000 to 19,000, while the number of FY-3C RO events is around 6000 to 14,000 in each month in 2015. No data are available for FY-3C from June to July 2015. Based on the criteria which we mentioned in Section 2.3, we obtained 2372 matched events between FY-3C and COSMIC with reflections, and 2709 matched events between FY-3C and COSMIC without reflections (no-reflection). All these matched events will be used for subsequent comparisons. Figure 1: Global distribution of the matched RO events between FY-3C and COSMIC. The red marks represent COSMIC events and the blue marks represent FY-3C events. The top panel shows the distribution in January and February 2015, the bottom panel shows the distribution in August and September 2015. After obtaining the collocated refractivity profiles and data quality control, we used mean relative bias and relative standard deviation to evaluate the accuracy index between COSMIC and FY-3C events. The comparison indexes are defined as follows [36]:  
\[\mu_{\text{k}}=\frac{1}{N_{k}}\sum_{i=1}^{N_{k}}\frac{R_{i}^{\text{F}}(k)-R_{i}^ {\text{C}}(k)}{R_{i}^{\text{C}}(k)} \tag{1}\]  
\[\sigma_{\text{k}}=\sqrt{\frac{1}{N_{k}}\sum_{i=1}^{N_{k}}}(\frac{R_{i}^{F}(k)-R _{i}^{\text{C}}(k)}{R_{i}^{\text{C}}(k)}-\mu_{\text{k}})^{2} \tag{2}\]  
where \(k\) is the individual altitude layer, \(N_{k}\) is the total number of profiles in each layer, \(R\) is the refractivity, \(F\) and \(C\) represent FY-3C and COSMIC, respectively, \(\mu\) and \(\sigma\) represent mean relative bias and relative standard deviation, respectively. As the seasonal patterns of COSMIC RO reflection events were verified significantly [11], we selected four representative months with more reflection events for the subsequent comparison (Figure 3). The statistical differences of mean bias and standard deviation bias are substantially consistent. For the comparisons between FY-3C and COSMIC with reflected signals across the four representative months (Figure 3a), from 5 km to 25 km, the mean relative biases are less than 1%, and the corresponding relative standard deviations are less than 2%.