Aeolus + Processes, Technical Note 5

We analyzed the scale-dependent effects of Aeolus winds in the ECMWF system in<br> relation to flow properties by evaluating analysis increments, defined as the differences<br> between Aeolus analyses and associated short-range forecasts, in observing system experi-<br> ments (OSEs) used in the project. Additionally, we assessed the impact of Aeolus data on<br> predictability improvements in comparison with COSMIC2 data.<br> Our results show that data assimilation leads to similar systematic increments in the<br> CTRL, Aeolus, and COSMIC2 experiments. Near the equator, tropospheric biases are<br> largely associated with balanced zonal flow, whereas in the outer tropical latitudes they<br> are primarily related to unbalanced meridional flow. The assimilation consistently enhances<br> the vertical shear of the zonal wind in CTRL, an effect further amplified by Aeolus assim-<br> ilation, particularly during the QBO disruption of 2019/2020. Based on these findings, we<br> recommend that Aeolus2 maximizes the vertical resolution of its measurements.<br> On average, the assimilation of Aeolus winds produces the largest analysis increments<br> in planetary-scale zonal winds, especially in Kelvin waves and the n = 1 Rossby waves.<br> The 4D-Var system generates comparable effects on meridional and zonal winds from both<br> Aeolus and COSMIC2 data. However, the increments exhibit different balance properties;<br> Aeolus assimilation decreases unbalanced meridional wind in the stratosphere, whereas<br> COSMIC2 assimilation increases it. Forecast errors, defined as the differences between OSE forecasts and analyses, exhibit<br> distinct growth patterns in the zonal and meridional winds across different layers. In the<br> early stages, error growth is primarily influenced by model error. After approximately three<br> days, subsynoptic-scale errors become saturated, and the growth is increasingly dominated<br> by planetary-scale errors in the zonal winds, particularly in the stratosphere. Predictability<br> gains from the assimilation of Aeolus winds were estimated at approximately 2.7 hours in<br> the tropopause layer and 5 hours in the lower stratosphere for 3-day forecasts (Fig. 1). For<br> comparison, assimilation of COSMIC2 data in the same layers yielded predictability gains<br> of 7 and 13 hours, respectively, during the studied WP4 period (1–14 December 2022).