Would the 2021 Western Europe Flood Event Be Visible in Satellite Gravimetry?

Abstract The primary objective of the GRACE Follow‐On satellite mission is to measure temporal changes in the Earth's gravitational field. Distance variations between the two GRACE‐FO satellites, recorded by a K‐Band Ranging system and a new Laser Ranging Interferometer (LRI), are significantly influenced by atmospheric mass redistribution. We investigate whether the sub‐monthly variations in atmospheric water mass, precipitation, and changes in total water storage during the extreme flood event in western Europe in 2021 were sufficiently large to influence the satellite gravity field measurements, if the GRACE‐FO satellites would have passed directly over the region. We use several data sets such as weather forecasts (ICON‐D2 model), hydrological simulations (ParFlow/CLM), observations as well as reanalyses, showing the high uncertainty between different estimations of the considered variables: total precipitable water, total precipitation, and total water storage. Our estimates suggest a potentially noticeable impact of the 2021 flood event on the GRACE‐FO satellites. Although it was globally seen a rather small event, even the atmospheric water mass beyond water vapor, which is not considered within the de‐aliasing process, is close to the LRI detection accuracy. This is particularly relevant for future gravity missions, which will use the LRI with potentially higher sensitivity as their main instrument. Sub‐monthly variations in the total atmospheric water mass, that is, beyond water vapor of huge extreme precipitation events should be investigated further to reduce potential future aliasing errors.