The impact of geostationary clear sky water vapour radiances (CSRs) of the Spinning Enhanced Visible Infra-Red Instrument (SEVIRI) on board Meteosat 8 and 10 on specific humidity, temperature and wind analyses and forecasts of the operational global ensemble variational assimilation system of the German Weather Service (DWD) has been investigated. The study uses a set of observing system experiments performed under idealised conditions with synthetic observations. The verification scores show that the assimilation of CSRs from the two water vapour (WV) channels at 6.25 and 7.35 μm improve the forecast skills not only for humidity but also for temperature and wind, especially in the high and middle troposphere and in the region covered by the MSG satellites. The different results obtained from the idealised experiments show that the improvement, especially for temperature and wind fields, is due to the interaction between specific humidity, temperature and wind through the model dynamics and physics during the forecast step as well as through the improved spatial correlations and cross-correlations between variables of the updated background error covariance matrix derived from the ensemble.

Keywords --- 3-D ensemble variational assimilation, geostationary clear sky radiances,  water vapour, wind