# Comparison of SEVIRI-Derived Cloud Occurrence Frequency and Cloud-Top Height with A-Train Data
## 3 Results
### Cloud Occurrence Frequency

Table 2: Summary of cloud occurrence frequencies over all matchup cases per month among Cloud Profiling Radar (CPR), Cloud-Aerosol LiDAR with Orthogonal Polarization (CALIOP), and Spinning Enhanced Visible and Infrared Imager (SEVIRI). All results are given in cloud occurrence frequency units (%). December and April (not shown here). This means that the SEVIRI algorithm detected clouds very rigorously over the ocean surface. Table 4 provides POD, FAR, PC, and KSS scores derived from the matchup dataset, for the different surface types. In comparison between SEVIRI and CPR, POD\({}_{\text{cloudy}}\) was high with values close to 0.90 and large FAR\({}_{\text{cloudy}}\) values around 0.32. In addition, both POD\({}_{\text{clear}}\) and FAR\({}_{\text{clear}}\) were low, with values of 0.67 and less than 0.1, respectively. Poor POD\({}_{\text{clear}}\) and FAR\({}_{\text{cloudy}}\) values were expected because SEVIRI detected more clouds than CPR with a COF bias of 14.35%. Over land, POD\({}_{\text{cloudy}}\) decreased, and both POD\({}_{\text{clear}}\) and FAR\({}_{\text{clear}}\) increased, which can be attributed to the relatively weak cloud detectability of SEVIRI over land. Moreover, for the comparison with CALIOP, POD\({}_{\text{cloudy}}\) and FAR\({}_{\text{clear}}\) showed less accurate results of 0.82 and 0.25, respectively, than those of the CPR case. However, POD\({}_{\text{clear}}\) and FAR\({}_{\text{cloudy}}\) improved to 0.82 and 0.12, respectively. Again, these results are attributed to the high sensitivity of CALIOP to clouds. According to PC, KSS, and the scores previously described, the SEVIRI COF showed closer agreement with the CALIOP COF than the CPR COF. However, the SEVIRI COF appears to have a relative weakness in detecting clouds over land in general. In order to infer the spatial distribution of the mean COFs, the matchup data were grouped into latitude-longitude grid boxes of \(2^{\circ}\times 2^{\circ}\). The estimated mean COF distributions and the differences are presented in Figure 1. The grey-scale brightness in the upper panel of the Figure 1 presents the mean COF maps over the grid box for CPR, CALIOP, and SEVIRI. The values range from 0 (dark) to 1 (white); the boxes with white \"x\" symbols with a black background in the figure show that no data were collected in those areas. The coloured maps in the lower panel show the differences in the mean COF among SEVIRI, CPR, and CALIOP. The red (blue) colour indicates that the SEVIRI mean COF is larger (smaller). A high frequency of clouds occurred most often in the inter-tropical convergence zone (ITCZ) and mid-latitude storm belts along with lower concentrations occurred in subtropical deserts and oceanic subtropical highs. The COF distribution patterns for the CPR, CALIOP, and SEVIRI products showed generally good agreement. However, CPR (CALIOP) had the lowest (highest) frequencies over most areas. These trends were more distinct over the broad region near the ITCZ. Again, the SEVIRI COFs were generally between those of CPR and CALIOP. It is clear \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Reference Data** & **Surface Type** & \(\text{POD}_{\text{cloudy}}\) & \(\text{POD}_{\text{clear}}\) & FAR\({}_{\text{cloudy}}\) & FAR\({}_{\text{clear}}\) & **PC** & **KSS** & **Number of Data** \\ \hline \multirow{3}{*}{CPR} & All & 0.9019 & 0.6743 & 0.3239 & 0.0988 & 0.7722 & 0.5763 & 815961 \\  & Land & 0.8340 & 0.7669 & 0.3351 & 0.1072 & 0.7909 & 0.6009 & 291108 \\  & Ocean & 0.9305 & 0.6120 & 0.3196 & 0.0916 & 0.7618 & 0.5425 & 524853 \\ \hline \multirow{3}{*}{CALIOP} & All & 0.8231 & 0.8213 & 0.1209 & 0.2538 & 0.8224 & 0.6444 & 815961 \\  & Land & 0.7147 & 0.9051 & 0.0914 & 0.2936 & 0.7968 & 0.6199 & 291108 \\ \cline{1-1}  & Ocean & 0.8769 & 0.7661 & 0.1322 & 0.2196 & 0.8366 & 0.6430 & 524853 \\ \hline \hline \end{tabular}
\end{table}