# Comparison of SEVIRI-Derived Cloud Occurrence Frequency and Cloud-Top Height with A-Train Data
## 2 Materials and Methods  
### Datasets  
The UK Met Office has utilised Meteosat data in real time to retrieve products for various applications. Meteosat-10 data have been operational since 21 January 2013 after its launch on 5 July 2012. The SEVIRI instrument on the MSG satellites has a set of channels designed to provide accurate cloud-top properties [9].  
The first step in deriving SEVIRI cloud parameters is to produce a cloud mask that identifies clear, partly cloudy and fully cloudy fields of view using the techniques previously developed for AVHRR [10]. However, an important development for processing MSG radiances is the inclusion of clear sky simulated radiances calculated using the Unified Model (UM) profiles input into the Radiative Transfer for Television Infrared Observation Satellite Operational Vertical Sounder (RTTOV),which is a fast radiative transfer model [11]. This allows for an excellent opportunity for comparing the differences between observed and simulated radiances, which should be small for clear sky scenes. The cloud detection tests encompass spectral differencing by using various channel combinations and comparing the measured and simulated differences. Spatial coherence and visible reflectance information is also used. Additionally, for low cloud detection in twilight conditions, the temporal differencing technique is applied in accordance with the frequent observation merit of geostationary satellites [12].  
The UK Met Office SEVIRI-derived cloud-top products contain information about the cloud-top pressure (CTP), temperature (CTT), and height (CTH) relative to the local topography for pixels identified as cloudy in the satellite scene. These cloud-top products were derived by using three independent algorithms in the following order: Minimum Residual (MR), Stable Layers (SL), and Profile Matching (PM) methods. First, the identified cloudy pixels are assigned to a cloud-top pressure and ECA by using an MR technique [13] based on an NWP forecast profile and SEVIRI radiances. Currently, three SEVIRI channels, including two infrared windows at 10.8 and 12.0 \(\upmu\)m, and a CO\({}_{2}\) absorption channel at 13.4 \(\upmu\)m, are used in this technique. The atmospheric stability from the background NWP model profile is also considered to ensure that the cloud is not placed at the bottom of an unstable layer. The CTP from the MR method is used mainly for the assignment of high clouds, of which CTP is less than 480 hPa. An SL method is employed if a suitable solution is not found by the MR method, that is, if the uncertainty in the MR solution is greater than a prescribed threshold, which is normally the case for low cloud. The SL method uses the 10.8 \(\upmu\)m window channel radiance and attempts to find the best level for the cloud-top to obtain an agreement between observed and simulated radiances. The concept of this method is based on the knowledge that clouds are normally topped by a stable layer that prevents the air parcels from rising further [14; 15]. In practice, this means that the upper-level clouds tend to use the solution of the MR method, and the lower-level clouds apply the SL method with a priori constraints. If no proper CTP solution is obtained from these two methods, the PM method, which has no a priori constraint, is used as a fallback solution; however, such cases are rare. This approach begins at the bottom of the profile and moves the cloud layer upward until agreement with the observed radiance at 10.8 \(\upmu\)m is achieved. After the ECA and CTP are determined, CTT and CTH are obtained through the conversion of the UM background vertical profile.