New wind observations in polar regions can be derived by tracking clouds in consecutive 1 1/2-hourly swaths taken from the polar-orbiting MODIS instrument. These satellite-derived wind vectors provide unprecedented temporal and spatial observational coverage in an area otherwise very poorly sampled by the conventional observation network. They therefore provide an important new dataset to analyse polar wind fields in global data assimilation systems.

In this study, an experimental MODIS winds dataset has been assimilated within ECMWF's variational data assimilation system over a trial period of 30 days. The impact of the new wind information on the analyses and forecasts is investigated in detail, including the impact on the mean wind analysis and on the resulting fit of the model data against other observations.

The new MODIS winds have a significant positive forecast impact over the Northern Hemisphere, particularly in the polar region. The assimilation considerably modifies the mean polar wind analyses, with a weakening of the Arctic vortex in the model fields. Case studies are discussed to further highlight the impact of the new observations on the forecast performance.