Impact of MODIS Polar Winds in ECMWF's 4DVAR Data Assimilation System

Abstract

This study investigates the impact of a new satellite-derived wind product over the polar regions on ECMWF's global four-dimensional variational assimilation system. The winds are derived at the University of Wisconsin? Madison by tracking structures in successive swaths from the polar-orbiting Moderate Resolution Imaging Spectroradiometer (MODIS) on the Terra satellite. The new wind product provides unprecedented coverage of the polar wind field. The data are assimilated subject to cautious quality control at lower levels and over land. The MODIS winds have a positive impact on medium-range global weather forecasts, particularly over the polar regions, but also elsewhere over the Northern Hemisphere. The mean polar wind analysis can be considerably altered as a result of the MODIS winds assimilation. Sensitivity calculations highlight the importance of polar regions for forecasts in the midlatitudes and indicate how MODIS winds can reduce key analysis errors in this region. The MODIS winds are now assimilated operationally at ECMWF.