Beyond GRACE: Evaluating the Benefits of NGGM and MAGIC for Rainfall Estimation on a European scale

The Gravity Recovery and Climate Experiment (GRACE) mission and its Follow-On (GRACE-FO) mission provide  observations of terrestrial water storage (TWS) dynamics on regional to global scales. However, they lack high spatio-temporal resolution, making them challenging to interpret different gravity field products. 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 ultimate aim of this collaboration to improve the current models and launch new high resolution missions in order to improve capacity for monitoring extreme events such as natural hazards, droughts and floods. The ultimate aim of this collaboration to improve the current models and launch new high resolution missions in order to improve capacity for monitoring extreme events such as natural hazards, droughts and floods. The primary objective of this study to examine the impact of improving spatial-temporal resolution of NGGM and MAGIC on rainfall estimation by developing multiple synthetic experiments on a European scale. The study employed SM2RAIN by inverting the soil water balance equation to estimate the rainfall accumulated between two consecutive TWS measurements. Initially, the ERA5L based TWSA at daily time scale was incorporated into SM2RAIN to check reliability of the model against ERA5L precipitation with spatial resolution of 100 km over Europe with range of latitudes 30 to 60°N and longitudes 10°W to 50°E.. The results shows SM2RAIN exhibited satisfactory performance at a daily temporal resolution, with mean values of R, RMSE, BIAS (0.85, 13.76, -0.29) against ERA5L precipitation. Based on statistical analysis, SM2RAIN-simulated rainfall shows good agreement across the most of Europe except in some areas of the northern Italy, northeastern states (Estonia, Latvia) and costal regions of Norway . Subsequently, synthetic experiments were developed by aggregating the daily ERA5 based TWS data into 5-day intervals which led to a decline in model performance against SM2RAIN-simulated rainfall as evidenced by all statistical measures with mean values of (0.73, 18.41 and -0.43) for CC, RMSE and BIAS respectively. In another experiment where inclusion of a target error 4.2 mm into 5-day TWS further reduce the model ability to access rainfall patterns, resulting in lower CC values across Europe, with the majority of areas showing below 0.3. At a threshold error 42 mm, the model’s performance of model significantly deteriorated in order to capture meaningful rainfall patterns with mean values of CC = 0.04 and RMSE 26.30. The results shows that degrading temporal resolution and larger error make the model quite difficult to capture and represent meaningful rainfall patterns, as the error completely overshadows the underlying dynamics captured in the SM2RAIN-simulated rainfall. The results of the study clearly highlight the benefit of NGGM and MAGIC in improving our capability to estimate various hydrological components relying on satellite data as inputs.