# Comparison of SEVIRI-Derived Cloud Occurrence Frequency and Cloud-Top Height with A-Train Data
## 4 Discussion  
### Cloud Occurrence Frequency  
The COF distributions analysed here show consistency with previous studies. For example, [PERSON] et al. [25] used one year of International Satellite Cloud Climatology Project (ISCCP) data from March 1985 to February 1986 and analysed the cloud types categorised by cloud-top pressures into high, middle, and low clouds using intermediate boundaries of 440 hPa and 680 hPa, and by cloud optical thickness into optically thin and thick clouds using 9.38 as a reference. They reported that high-cloud types occurred preferentially where convection occurred in the tropics and in mid-latitude storm tracks. In addition, they found that low clouds were predominant over ocean and were most abundant in the subtropical eastern oceans, where the sea surface temperature is comparatively low and the mean air motion is downward. Total cloud cover was the greatest over high-latitude oceans, where stratus regimes are dominant, and in regions of intense tropical convection. They also showed that thin, high-level cloud frequency features decreased in summer relative to winter over the Atlantic Ocean, Africa, and Europe for the Meteosat-10 observation area [25] (Figures 2 and 3). [PERSON] et al. [26] investigated the spatial and temporal variability of sub-visual clouds (SVC) and opaque clouds by using Stratospheric Aerosol and Gas Experiment (SAGE) II data from 1985-1990. They found that the latitudinal movements of the tropical maxima are in concert with the seasonal shift of the ITCZ. Cloud occurrence in the subtropical regions has a distinct seasonal variation, with maximum frequency in local summer and minimum during local winter. However, for the Meteosat-10 observation area, a slight decrease of SVC occurrence was found [26] (Plate 2). Similar cloud spatial variability was reported in sequential papers of [PERSON] [27; 28; 29]. Three COF cloud products investigated in this study showed similar spatial distribution and variability to the previous studies, although some systematic differences were noted in COF values. The difference shown in the statistical scores can be attributed to the different characteristics and sensitivity of the sensors. CALIOP is more sensitive to the small ice particles in clouds because it uses beams at a shorter wavelength of 532 nm. Thus, it senses thin, high-level cloud much better than the other sensors. On the contrary, CPR, which uses a microwave wavelength of 3.2 mm (94 GHz), is effective in detecting rain droplet-size cloud water particles. It is essentially insensitive to very thin cirrus clouds, which have small sizes and low concentrations of  
Figure 7: Spinning Enhanced Visible and Infrared Imager (SEVIRI) cloud-top height (CTH) scatter plots compared with Cloud Profiling Radar (CPR) CTH for each period used a different CO\({}_{2}\) bias correction increment. The periods and CO\({}_{2}\) increment values are (**a**) 1 May–26 June 2013, CO\({}_{2}\) BC: \(-\)1.15 K; (**b**) 17 July–18 September 2013, CO\({}_{2}\) BC: 0.85 K; (**c**) 25 September–13 November 2013, CO\({}_{2}\) BC: \(-\)0.15 K; and (**d**) 20 November 2013–30 April 2014, CO\({}_{2}\) BC: \(-\)0.65 K.  
ice crystals, because the radar backscatter in the cirrus volume is insufficient for generating a return signal that rises above the CPR detection threshold. CPR is also relatively ineffective for detecting clouds in the lowest kilometre of the atmosphere owing to radar contamination from ground clutter. As indicated by the different characteristics of various sensors, SEVIRI detects clouds over a wider area than CPR, and it shows lower COFs than CALIOP near the ITCZ and mid-latitudes, where thin, high clouds containing small ice particles are dominant. The COF difference between the SEVIRI and the CALIOP products decreases in summer, particularly for thin, high-level clouds of the Meteosat-10 area, as shown in previous studies.