Solar Stray Light Effects in Meteosat Radiances Observed and Quantified Using Operational Data Monitoring at ECMWF

Abstract

Radiances in the water vapor (WV) and infrared channels of the Meteorological Satellite (Meteosat)-5 and -7 are being continuously monitored in the form of clear-sky radiances using the operational European Centre for Medium-Range Weather Forecasts (ECMWF) data assimilation system. The clear-sky radiances are provided by the European Organization for the Exploitation of Meteorological Satellites (EUMETSAT), using their scenes analysis to eliminate pixels affected by clouds. The monitoring time series at hourly resolution show that anomalies are present in the clear-sky radiances derived from some images close to satellite local midnight. Particularly strong effects are observed in the WV channel during the eclipse seasons but also for several weeks before and afterward. The effects are caused by solar stray light entering the radiometer and creating different anomaly patterns such as warm bows; small, very intense warm spots; warm and cold horizontal stripes; and calibration anomalies, resulting in too-low radiances in large parts of certain images. Such effects of stray light have been known since the early years of the Meteosat program, and efforts have always been made to diminish the impact by not producing products from such Meteosat image data. However, this study, which analyzes data from the 2-yr period from April 2000 to May 2002 in detail, shows that the effect is discernable in radiance products for periods much longer than those considered in the operational system. Far from the eclipse season, warm bows are present in the WV radiances at 0100 UTC that have an amplitude of 2?4 K. In extreme cases, however, closer to eclipse season, local anomalies may exceed 10?25 K. The WV data at 2300, 0000, 0100, 0200, and 0300 UTC are affected to a different extent and at different times. Some of the time slots are shown to be contaminated for extended periods before and after the spring and autumn eclipses.