# Comparison of SEVIRI-Derived Cloud Occurrence Frequency and Cloud-Top Height with A-Train Data
## 3 Results
### Cloud-Top Height
The discontinuity of the SEVIRI CTH distribution occurred near 5 km in height, which resulted from the SEVIRI CTH  
Figure 3: Colour-coded Cloud Profiling Radar (CPR)-derived radar reflectivity and cloud-top heights (CTHs) along CPR overpass 37,288 on 1 May 2013. CTHs from Cloud-Aerosol LiDAR with Orthogonal Polarization (CALIOP), CPR, and Spinning Enhanced Visible and Infrared Imager (SEVIRI) are plotted as dark green triangles and white and red crosses, respectively. algorithm that selectively used the MR or SL methods near 480 hPa. As shown in Figure 4, average CTHs derived from SEVIRI are closer to CPR measurements than to CALIOP measurements. Thus, further comparisons with CPR products were conducted. Observed and simulated clear sky radiances differ because of the cumulative effects of instrument noise, spectral response function errors, inadequate knowledge of the atmospheric and surface state, and radiative model approximations. Thus bias corrections are critical for the proper use of infrared channels for cloud height estimation [23]. [PERSON] and [PERSON] [24] reported ice contamination of the SEVIRI sensor, which causes a change in the spectral response as well as the sensitivity of the detector. They found that the CO\({}_{2}\) channel had a significant sensitivity to ice contamination and showed larger bias. The UK Met Office has monitored the satellite observed radiances and adapted the monitoring results periodically for operational product retrievals and NWP applications. During the period analysed in this study, the SEVIRI CO\({}_{2}\) channel bias correction increment (BC) was updated three times for SEVIRI CTH processing: on 10 July, 24 September, and 15 November 2013. Figure 5 shows the time series of SEVIRI CTH validation scores compared with CPR CTHs. The uppermost dotted line represents the standard deviation; the solid line in the middle represents the correlation coefficient; and the dashed line at the bottom shows the bias. The number of data used for each validation is shown as a bar graph. Constant and good correlation scores of 0.9 were noted during the validation period. However, the variation of bias score was outstanding along with the changes in CO\({}_{2}\) BC value. Prior to the update in July, when the \(-1.15\) K CO\({}_{2}\) BC increment was used, SEVIRI CTHs  
Figure 4: (**a**) Geographical distribution of collocation dataset used in this study and scatter plots for Spinning Enhanced Visible and Infrared Imager (SEVIRI) cloud-top height (CTH) with comparison to (**b**) Cloud Profiling Radar (CPR) CTH and (**c**) Cloud-Aerosol LiDAR with Orthogonal Polarization (CALIOP) CTH. had larger negative biases of approximately \(-0.5\) km. The biases increased to positive values after the large BC change to 0.85 K in July. Note that the data on 10 July are the results from Meteosat-9 instead of Meteosat-10 because of its decontamination process. The CO\({}_{2}\) BC change in September from 0.85 K to \(-0.15\) K led to a decrease in the bias values down to about \(-0.25\) km. Moreover, another change to \(-0.65\) K in November enhanced the negative biases slightly larger, but to make correlation coefficient and standard deviation slightly better as 0.93 and 1.61, respectively. Without the large variability of biases and standard deviations shown in January and February, when small numbers of data were collected, SEVIRI produced CTHs with more stable bias compared to CPR CTHs. The variation of the standard deviation score is not apparent relative to the CO\({}_{2}\) BC changes, although it fluctuated slightly more between September and November. To investigate the effect of CO\({}_{2}\) BC updates on CTH retrieval, the CTH results derived from SEVIRI applying the previous and the updated BC values are shown in Figure 6. Figure 6 shows a section of CPR overpass 39,413, when CO\({}_{2}\) BC increment was updated from 0.85 K to \(-0.15\) K on 24 September 2013.