Abstract
Abstract. Winds measured by lidar from the Aeolus satellite are compared with winds measured by two ground-based radars – MARA in Antarctica (70.77∘ S, 11.73∘ E) and ESRAD (67.88∘ N, 21.10∘ E) in Arctic Sweden – for the period 1 July–31 December 2019. Aeolus is a demonstrator mission to test whether winds measured by Doppler lidar from space can have sufficient accuracy to contribute to improved weather forecasting. A comprehensive programme of calibration and validation has been undertaken following the satellite launch in 2018, but, so far, direct comparison with independent measurements from the Arctic or Antarctic regions have not been made. The comparison covers heights from the low troposphere to just above the tropopause. Results for each radar site are presented separately for Rayleigh (clear) winds, Mie (cloudy) winds, sunlit (“summer”) and non-sunlit (“winter”) seasons, and ascending and descending satellite tracks. Horizontally projected line-of-sight (HLOS) winds from Aeolus, reprocessed using baseline 2B10, for passes within 100 km of the radar sites, are compared with HLOS winds calculated from 1 h averaged radar horizontal wind components. The agreement in most data subsets is very good, with no evidence of significant biases (<1 m s−1). Possible biases are identified for two subsets (about −2 m s−1 for the Rayleigh winds for the descending passes at MARA and about 2 m s−1 for the Mie winds for the ascending passes at ESRAD, both in winter), but these are only marginally significant. A robust significant bias of about 7 m s−1 is found for the Mie winds for the ascending tracks at MARA in summer. There is also some evidence for increased random error (by about 1 m s−1) for the Aeolus Mie winds at MARA in summer compared to winter. This might be related to the presence of sunlight scatter over the whole of Antarctica as Aeolus transits across it during summer.
Highlights
The Aeolus satellite is a European Space Agency (ESA) mission which aims to demonstrate the possibility of providing global wind measurements throughout the troposphere and lower stratosphere using Doppler wind lidar, with good enough accuracy for use in assimilations for numerical weather prediction
Validation of the Aeolus winds against direct independent wind measurements at polar latitudes offers a possibility to begin to see to what extent polar conditions might affect measurements, whether scattered sunlight effects are as theoretically predicted, since the long summer days result in Atmospheric Laser Doppler Instrument (ALADIN) measurements being made in full sunlight in those regions even with a dawn–dusk orbit
To compare radar and Aeolus winds, we first calculate what the Horizontally projected line-of-sight (HLOS) wind should be according to the zonal (U ) and meridional (V ) wind components measured by the radar. (We could in principle include the vertical wind component measured by the radar, but this was found to be negligible in the 1 h averages.)
Summary
The Aeolus satellite is a European Space Agency (ESA) mission which aims to demonstrate the possibility of providing global wind measurements throughout the troposphere and lower stratosphere using Doppler wind lidar, with good enough accuracy for use in assimilations for numerical weather prediction. Corrections have had to be made for “hot pixels”, which are increased dark current rates for specific ALADIN ACCD (accumulation charge coupled device) detector pixels (Weiler et al, 2020) and for biases in line-of-sight winds of up to 5 m s−1, which were found to differ between ascending and descending nodes and between different geographic regions (Martin et al, 2021; Rennie and Isaksen, 2020). Validation of the Aeolus winds against direct independent wind measurements at polar latitudes offers a possibility to begin to see to what extent polar conditions might affect measurements, whether scattered sunlight effects are as theoretically predicted, since the long summer days result in ALADIN measurements being made in full sunlight in those regions even with a dawn–dusk orbit
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