Aliasing of atmospheric cloud water in time-variable gravity models from GRACE/-FO and next generation gravity missions?

Temporal aliasing of high-frequency mass variations poses, along with instrument noise, the biggest obstacle to improving the accuracy and resolution of satellite gravimetry products from the GRACE/-FO mission and next-generation gravity missions (NGGM). The current GRACE/-FO data processing strategy includes the removal of tidal and sub-monthly non-tidal mass variations in ocean and atmosphere from the level-1 data using Atmospheric Ocean Dealiasing (AOD) data sets. However, the atmospheric part, based on ERA5 reanalysis and ECMWF operational forecast data, considers only dry air and water vapor, neglecting the mass contribution of liquid and solid cloud water. According to ERA5 data, the total global mass of cloud water is about 0.5% of the atmospheric water vapor mass, which may seem insignificant. However, we found that highly localized mass variations of cloud water can exceed 1Gt during extreme convective weather events, and thus may significantly affect laser ranging interferometer (LRI) measurements of GRACE-FO and NGGM.

In this study, we examine the overlooked cloud water content during hydrometeorological extremes in current AOD products over the entire GRACE/-FO observation time span from 2002 to 2023. By employing a 3D connected component algorithm to quantify the duration, intensity, and affected area of these events using ERA5 cloud water data, we identified over 1,000 events annually that we expect to impact GRACE-FO’s LRI measurements. We also observe that while the duration of these events has decreased over time, their intensity has increased. Most concerning, our findings show that the number of events has doubled over the observation period from 2002 to 2023, which is evident across all continents and the ocean, aligning with what we expect due to rising temperatures and increased atmospheric water-holding capacity. Notably, cloud modeling remains one of the most significant challenges in atmospheric and climate science, as it spans the complexities of microphysics to the dynamics of the global Earth system, and therefore may not meet geodetic accuracy requirements. However, with new and more precise NGGM on the horizon, our findings suggest a growing need to reevaluate AOD strategies that were developed prior to GRACE launch.

This study is part of the research unit New Refined Observations of Climate Change from Spaceborne Gravity Missions (NEROGRAV), funded by the German Research Foundation (DFG) with the objective of improving GRACE/-FO and NGGM data processing strategies.