The precise monitoring of the Earth's water cycle and the mass balance of glaciers and ice caps has become a cornerstone of contemporary geodesy, with satellite gravimetry playing a pivotal role in advancing our understanding of these processes. Before the GRACE era, the determination of monthly gravity field models relied on alternative methods, such as Satellite Laser Ranging (SLR) to passive spherical satellites. While early applications of SLR data were limited to assessing the Earth's oblateness, advancements in satellite constellations, observational techniques, orbit, and background models eventually enabled the derivation of gravity field models to a degree and order 10 of spherical harmonics based on SLR. However, these models still do not match the spatial resolution achieved with GRACE data. Nevertheless, the long-term gravity field models derived from SLR data are a fundamental information source of large-scale global changes in ice mass, ocean and land hydrology, especially for periods predating 2002.

This study addresses the challenge of limited high-resolution satellite gravimetry data available before the GRACE era by leveraging SLR data for large-scale changes and fitting GRACE data using an empirical function with stochastic parameters to enhance spatial resolution. By analyzing these combined datasets, we delimit global areas affected by significant accelerations in water storage and identify the dates for the maxima and minima of the function for the period from 1995 to 2024. We found that in the Svalbard region, ice mass accumulation reached its maximum in the middle of the first decade of the 21st century, followed by a significant acceleration of ice mass loss due to climate warming, which continues to the present day. Such a change in trend cannot be identified using solely GRACE data, therefore, the SLR+GRACE combinations are indispensable. A similar trend is observed in the Gulf of Alaska Glaciers, where ice mass loss has accelerated substantially since the beginning of the observations, particularly intensifying after 2012. In contrast, the Antarctic Peninsula saw a complete deceleration in ice mass loss, with the trend reversing around 2021. The results of our study show strong agreement with external validation datasets, including satellite altimetry and climate parameters such as Sea Surface Temperature Anomalies.

How to cite: Gałdyn, F., Sośnica, K., Zajdel, R., Meyer, U., and Jäggi, A.: How well can we derive the time-variable gravity field before the GRACE mission based on SLR data?, GRACE/GRACE-FO Science Team Meeting, Potsdam, Germany, 8–10 Oct 2024, GSTM2024-39, https://doi.org/10.5194/gstm2024-39, 2024.

Global mean sea level (GMSL) exhibits an average secular increasing trend of 3.1 +/- 0.3 mm yr-1 over the last 30 years [NOAA, 2023]. However, more than 43% of the variability in the de-trended global satellite sea level record occurs on 2-3 year time scales in rapid global water cycle ‘events’, during which GMSL can rise or fall at more than twice the ambient secular rate, causing changes as large as 12 mm over a relatively short period. This means that rapid changes in the natural movement of water from the oceans to the land, which appear on the continents as strong mass-change hydrologic signals, can be of sufficient amplitude to offset (i.e. equal and opposite) the mass additions to the oceans from the ice sheets over 2-3 year periods, or to offset those contributions by as much as 50% over a decade. In this talk, we examine the long GRACE/GFO record of global mass budget closure to better characterize the history of these global water cycle events, describe their apparent nature, and quantify their future potential impact on apparent GMSL.

How to cite: Reager, J. T.: Global water cycle ‘events’ and global mass redistribution at interannual to decadal time-scales, GRACE/GRACE-FO Science Team Meeting, Potsdam, Germany, 8–10 Oct 2024, GSTM2024-25, https://doi.org/10.5194/gstm2024-25, 2024.

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.

How to cite: Mielke, C., Springer, A., and Kusche, J.: Aliasing of atmospheric cloud water in time-variable gravity models from GRACE/-FO and next generation gravity missions?, GRACE/GRACE-FO Science Team Meeting, Potsdam, Germany, 8–10 Oct 2024, GSTM2024-51, https://doi.org/10.5194/gstm2024-51, 2024.