# Comparison of SEVIRI-Derived Cloud Occurrence Frequency and Cloud-Top Height with A-Train Data
## 3 Results
### Cloud-Top Height  
By utilizing AVAC-S, it is possible to plot all of the CTHs within the matchup dataset in the same figure, as demonstrated in the cross-section plot in Figure 3, which shows a section of CPR overpass 37,288 on 1 May 2013. CPR reflectivity is represented in the figure by coloured contours. The CALIOP results are represented by CTH values in dark green triangles, and white and red crosses indicate the  
Figure 2: Cloud occurrence frequency maps for Cloud Profiling Radar (CPR), Cloud-Aerosol LiDAR with Orthogonal Polarization (CALIOP), and Spinning Enhanced Visible and Infrared Imager (SEVIRI). The upper panel (**a**) is from 11 to 31 July 2013, and the lower panel (**b**) is from 1 to 31 December 2013. CTHs from CPR and SEVIRI, respectively. CALIOP is highly sensitive to small ice cloud particles and optically thin clouds, and therefore detects higher CTHs. A broad distribution of thin cirrus clouds just below the tropopause is clearly shown in the CALIOP CTHs. The CPR CTHs are plotted along the top of the CPR reflectivity contour. SEVIRI retrieved similar but slightly lower CTHs than those from CPR for convective clouds, where the CPR reflectivity became strong. SEVIRI captured some portion of thin cirrus clouds detected by CALIOP. However, it also produced CTHs in the middle of the troposphere below very thin cirrus clouds because they did not sufficiently affect the SEVIRI radiances. The CTH comparison among SEVIRI, CPR, and CALIOP was conducted by using the same matchup dataset as that was used in the COF analysis. In total, 329 CPR overpasses were selected. The spatial distribution of the matchup data is shown in Figure 4a. The data points are well distributed over the Meteosat-10 observation disk. Figure 4b,c show comparisons of SEVIRI CTHs with CPR and CALIOP, where the values were filtered by the spatial uniformity test and quality control threshold test using the CTH variance from SEVIRI algorithm for more rigorous analysis of the SEVIRI CTH product. In the case of the comparison with CPR, the total number of matchup data for which both CPR and SEVIRI had valid CTH values was 346,287. Applying the spatial uniformity filter, 217,895 (63%) data points were selected. After the CTH variance test, 193,555 data points were finally selected. That is, approximately 44% of the collocated data were eliminated by this data filtering process for rigorous quality control. The use of these filters removes some cases with small clouds and/or multi-layer cloud scenes. However, if these filters were not used, then issues related to establishing meaningful collocations (i.e., only using data where both SEVIRI and A-Train sensors are directed to the same clouds) would mean that the comparison dataset was too noisy to be useable. On the scatter plot of CTH retrieved from the SEVIRI algorithm against CALIOP measurements (Figure 4c), a large scatter of data points is shown. SEVIRI produced lower CTHs around 1-2 km, although CALIOP retrieved them as mid- or high-level clouds. This occurred mainly because the data corresponding to thin cirrus clouds, as previously shown in Figure 3, can be regarded as different clouds effectively measured by SEVIRI and CALIOP in this case. The data count density showed larger scattering than with CPR, where the axis of major density departed from the complete one-to-one matching line towards the lower SEVIRI values. The distribution was slightly broader with a correlation coefficient of 0.78, a bias of \(-1.61\) km (i.e., the lower SEVIRI CTH), and a standard deviation of 3.24 km from 220,199 data points. The bias appeared to be larger for higher clouds, whereas better coherence was shown for low clouds between the CALIOP and SEVIRI CTHs. Moreover, the scatter plot was relatively distinct between the CPR and SEVIRI CTH products with a correlation coefficient of 0.92, a bias of \(-0.20\) km (SEVIRI low), and a standard deviation of 1.67 km. The frequency was less than for the CALIOP comparison, although low SEVIRI CTH outliers were still detected.