Improvement of cloud detection near sunrise and sunset by temporal-differencing and region-growing techniques with real-time SEVIRI

The European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT), through the Satellite Application Facility for Nowcasting (SAFNWC), provides a software package suitable to generate real-time products derived from the Spinning Enhanced Visible and Infrared Imager (SEVIRI) on board the Meteosat Second Generation (MSG) satellite. A cloud mask is one of its key derived products. An analysis of the behaviour of version v2008 of this software using animation loops indicates a discontinuity in the detection of low clouds near the day-night transition. Thus, its use may have a negative impact when the scene includes such conditions. Our study provides solutions for a smoother continuity. It shows that a temporal-differencing technique combined with a region-growing technique can improve low-level cloudiness detection in the day-night transition area at the expense of a small increase of false alarms. The method and the resulting improvement are described and illustrated. A comparison of statistics of the cloudiness over Europe reported in Surface Synoptic Observations (SYNOP) and retrieved from the SEVIRI with and without the enhancement estimates a decrease of 50% of the frequency of missed clouds while the false alarm ratio increases only marginally. A comparison with another EUMETSAT operational SEVIRI-based cloud mask confirms the usefulness of the proposed technique at oblique sun angles.