An improved cirrus detection algorithm MeCiDA2 for SEVIRI and its evaluation with MODIS

摘要

In this study, a substantially improved version of the Meteosat cirrusdetection algorithm (MeCiDA2) will be presented, which now allows applicationto the full earth disc visible by the Meteosat satellite. As cirrus cloudshave an influence on the radiation budget of the earth, their opticalproperties and their global coverage has to be monitored at the global scaleusing instruments aboard geostationary satellites. Since MeCiDA was optimisedfor the area of Europe only, various changes were necessary to handle thevariable conditions found over the full Meteosat disc. Required changesinclude the consideration of the viewing angle dependency and of thesensitivity of the 9.7 μm channel to the ozone column. To thisend, a correction is implemented that minimises the influence of thevariability of the stratospheric ozone. The evaluation of the proposedimprovements is carried out by using MeCiDA applied to MODIS (moderate resolution imaging spectrometer) data to addressviewing angle-dependent cirrus detection, and by additionally comparing it tothe cloud optical properties MOD06 cirrus product. The new MeCiDA versiondetects less cirrus than the original one for latitudes larger than40°, but almost the same amount elsewhere. MeCiDA's version for MODISis more sensitive than that for SEVIRI (spinning enhanced visible and infrared imager) with cirrus occurrences higher by10%, and the new MeCiDA provides almost the same cirrus coverage(±0.1) as given by the cloud phase optical properties from MODIS forlatitudes smaller than 50°. Finally, the influence of sub-pixelclouds on the SEVIRI cirrus detection has been examined: more than 60%of the undetected SEVIRI cirrus pixels have a cirrus coverage smaller than0.5.