Beyond GRACE: Evaluating the benefits of NGGM and MAGIC for precipitation estimation over Europe

Abstract. The Gravity Recovery and Climate Experiment (GRACE) mission and its Follow-On (GRACE-FO) mission provide observations of terrestrial water storage (TWS) dynamics from regional to global scales. However, they lack high spatio-temporal resolution, which is essential for hydrological applications. A join collaboration between the National Aeronautics and Space Administration (NASA) and the European Space Agency (ESA), initiated a decade ago, is known as the Mass change And Geosciences International Constellation (MAGIC). The aim of this collaboration is to launch a new paired mission, i.e., GRACE-C and NGGM (Next Generation Gravity Mission), to improve the monitoring of extreme events such as floods and droughts.

The primary objective of this study is to examine the impact of the expected improvement in the spatial-temporal resolution and accuracy of NGGM and MAGIC on precipitation estimation by developing multiple synthetic experiments on a European scale. The study employed the well-known SM2RAIN algorithm to estimate the precipitation accumulated between two consecutive TWS measurements. The total amount of water in the soil from the fifth generation ECMWF reanalysis for the land (ERA5L) is used as a proxy of TWS for the period of 2003–2012. Firstly, the reliability of SM2RAIN to obtain precipitation from TWS measurements is tested by using ERA5L precipitation as reference. The results showed that SM2RAIN exhibited satisfactory performance at a daily temporal resolution, with mean values of the correlation coefficient, R, equal to 0.86. Good agreement was obtained across most of Europe except in some areas of the northern Italy, northeastern states (Estonia, Latvia) and coastal regions. Secondly, synthetic experiments `were developed by degrading the temporal resolution of TWS data and by introducing error ranging from 1 to 40 mm. The results showed that while the SM2RAIN algorithm maintains robust performance under moderate temporal degradation (5–10 days) and low measurement errors (<5 mm), with correlations remaining above 0.75, performance deteriorates significantly when errors exceed 10–20 mm, with correlations dropping below 0.4 at 42 mm error levels. These findings identify critical design thresholds for NGGM/MAGIC missions, demonstrating that achieving a measurement accuracy better than 5–10 mm is crucial for reliable precipitation estimation across diverse European hydroclimatic conditions.