The European Space Agency (ESA)’s Aeolus mission, launched in August 2018, provides the first global horizontal line-of-sigh (HLOS) wind profile measurements. Many Numerical Weather Prediction (NWP) centres, including ECMWF, DWD, Météo-France, Met-Office and ECCC, have shown that assimilating Aeolus winds improves overall forecast skill, especially in the tropics and data-sparse regions. To better characterize the locations and drivers of improved skill from Aeolus, we use a series of Observing System Experiments (OSEs) with the ECCC Global Deterministic Prediction System (GDPS) covering the period July to September 2019 and December 2019 to March 2020. Three experiments are used: CNTRL, CNTRL+Aeolus, and CNTRL-winds. All the observations assimilated in the GDPS are included in the CNTRL experiment. The Aeolus winds are added in the CNTRL+Aeolus experiment and the operational wind observations are withheld in the CNTRL-wind experiment. The impact of the operational winds and Aeolus are quantified by comparing the forecast error of the CNTRL-winds and CNTRL experiments with the CNTRL and CNTRL+Aeolus experiments.

The operational winds improve the tropospheric forecast over the tropics the most, with a normalized forecast error of 8% for the wind field. By adding the Aeolus winds, which account for less than 1% of the observations, the tropospheric forecast error further improves by 0.7-0.9% over the tropics and the Arctic and by 0.5-0.6% over the data-sparse Southern Hemisphere. Furthermore, we study the impact on forecasts as a function of length scale using spherical harmonic decomposition. The impact is measured as the difference in the power spectra of the error variance of the 250-hPa kinetic energy field. The impact of Aeolus is mostly seen in the intermediate to large wavenumber range with a peak around wavenumber 9 (scales about 4000 km). The improvement is greatest in mid-latitudes over the storm track regions. The difference is the largest on day 1, then it decreases as forecast range increases. Global scale and scales smaller than wavenumber 60 have little to no impact.