Eclipse Anomalies in Atmospheric Motion Vectors Derived from Meteosat Imagery and Their Implications on Data Assimilation

Abstract

This study examines atmospheric motion vectors derived by tracking features in image sequences from Meteosat for anomalies during the second satellite eclipse period of 2001. During eclipse periods (March?April; September?October), data from geostationary sensors are prone to anomalies caused by solar stray light entering the radiometer around local midnight. As a result, imagery from geostationary satellites can exhibit local anomalies for certain time slots, and these anomalies can change considerably from one time slot to the next. The effect of these anomalies on atmospheric motion vectors is characterized in this study by investigating the temporal consistency of the data and by monitoring the winds against short-term forecasts from the European Centre for Medium-Range Weather Forecasts (ECMWF) global model. Atmospheric motion vectors from the Meteosat water vapor channel exhibit spuriously fast winds for certain time slots around local midnight for extended periods around the satellite's eclipse. The anomalies are caused primarily through tracking problems, but height assignment is also affected. Less severe anomalies are found for infrared winds, in agreement with less severe image anomalies for the infrared channel. The European Organization for the Exploitation of Meteorological Satellites (EUMETSAT) and ECMWF quality control are capable of reducing the problem, but anomalous characteristics can still be found in the quality-controlled subsamples. As a result of these findings, atmospheric motion vectors from Meteosat are now excluded from the operational assimilation at ECMWF for certain time slots around the eclipse periods.