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Use of Aeolus observations at ECMWF

Michael Rennie, Lars Isaksen

 

ECMWF started to assimilate wind observations from the European Space Agency’s (ESA) pioneering Doppler wind lidar mission Aeolus on 9 January 2020, 16 months after the first wind profiles became available. ECMWF was the first numerical weather prediction (NWP) centre to go operational with Aeolus. This was possible due to the strong collaboration between ESA and the Aeolus Data Innovation and Science Cluster (Aeolus DISC), which includes the German Aerospace Center (DLR), the software company DoRIT, ECMWF, ESA, the Royal Netherlands Meteorological Institute (KNMI), and Météo-France. ECMWF was able to benefit from the data quickly because of its strong involvement with the mission since its conception in the 1980s and its selection as an ESA Earth Explorer Mission in 1998.

Aeolus is carrying the world’s first functioning space-based Doppler wind lidar and Europe’s first space-based lidar. The aim of the mission is to demonstrate this new technology in space for the benefit of weather forecasting and to improve the understanding of atmospheric dynamics, especially in the tropics. The mission is designed to last for at least three years.

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Aeolus wind profiles. The plot shows the Aeolus Level-2B Rayleigh-clear and Mie-cloudy horizontal line-of-sight wind speed profiles on 14 February 2020 over the North Atlantic. The blue area highlights the location of a jet streak with wind speeds up to about 100 m/s. Around this period, some records were broken for the speed of transatlantic flights due to the strength of the jet stream.

The path to assimilation

Within a few weeks after the launch of Aeolus on 22 August 2018, we were able to demonstrate the Doppler wind lidar’s ability to measure vertical profiles of horizontal line-of-sight winds from space, as seen in the first figure. But at that point it was still unclear whether the observations were of sufficient quality to improve operational weather forecasts.

Experimentation at ECMWF and several other NWP centres has since demonstrated positive impact on global weather forecasts from the assimilation of Aeolus data. This is despite the noise levels being significantly larger than pre-launch expectations, due to a number of issues with the novel Doppler wind lidar technology. ECMWF has played a major role in the DISC team, who have improved the quality of the Aeolus winds since the launch and now have a good understanding of wind bias issues. A series of research experiments in which Aeolus wind data were assimilated has been performed at ECMWF over the last 12 months. After several refinements in how the data were assimilated, the results were so promising that Aeolus was assimilated operationally from 9 January 2020.

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Impact on forecasts. Example of the impact of Aeolus Rayleigh-clear and Mie-cloudy winds on 2-, 3- and 5-day forecasts for the period 2 August to 31 December 2019. The figure shows the relative change in root-mean-square (RMS) error of the vector wind forecast with the assimilation of Aeolus data compared to without it, verified against operational analyses (Aeolus was not assimilated operationally in this period). The hatching shows statistical significance at the 95% confidence level. Blue colours indicate an improvement due to Aeolus. The impact is strongest in the tropics and near the poles.

Impact

Observing system experiments with Aeolus show that it improves ECMWF’s global forecasts of wind, temperature and humidity, with the largest impacts in the tropics and in polar regions (see the second figure). The good impact is also seen in measures such as short-range forecast improvements relative to other observation types and in the FSOI (Forecast Sensitivity to Observation Impact) metric.

The operational assimilation currently relies on a bias correction that assumes the model is bias free. However, this assumption is expected to be significantly relaxed very soon when the next generation of ground processing software for the horizontal line-of-sight winds is implemented in ESA’s ground processing environment. ECMWF plays a key role in ground processing as it produces the near- real-time Level-2B wind products. Thanks to careful investigations, in which ECMWF model winds were used as a reference, the Aeolus wind biases were found to be strongly correlated with the instrument’s main telescope temperature, which varies slightly with the Earth’s top of atmosphere radiation. The telescope temperatures are fortunately available in real time. A bias correction scheme using these temperatures as predictors has been developed.

The impact of Aeolus on forecasts is very good for a single instrument on a single satellite – it compares well with some well-established passive sounding instruments in FSOI statistics. Most of the positive impact of Aeolus so far is due to the clear-air Rayleigh winds, primarily in the tropics. However, recent tests of winds from cloudy areas (Mie winds) with more appropriate observation error modelling (accounting for representativeness error) are providing a larger impact in the extratropics than before.

Outlook

It is unclear for how long Aeolus will provide sufficiently strong atmospheric signal levels to maintain the level of positive impact that was demonstrated in July–December 2019, but ESA and its partners are still learning about the instrument’s behaviour in space and are working hard to increase the data quality and its useful lifetime. Aeolus was designed in the 1990s, so there are many aspects of the technology that could be improved in a possible future operational mission.