Global and regional data assimilation

Monthly total water storage anomalies (mm) from the Global Land Water Storage data set produced by assimilation (left) or GRACE/-FO observations (right). The TWSA from assimilation is derived by assimilating 4° GRACE/-FO TWSA into the global hydrological model WaterGAP. © Helena Gerdener

The satellite missions GRACE and GRACE-FO provide the opportunity to measure total water storage anomalies (TWSA) globally from space, which includes surface and subsurface water. However, the spatial resolution of 300 km is too coarse for many applications and there is a temporal gap of about one year between the two missions. To overcome these issues, GRACE/-FO data have been assimilated into global hydrological models to spatially downscale the TWSA to the model’s resolution and vertically disaggregate the TWSA into water storages such as, for example, groundwater. At the same time, assimilation improves the model’s realism, which is inevitably imperfect due to simplifications and biased forcing data. 

In the SING project, we aim to assess the impact of an improved spatial resolution of the NGGM and MAGIC missions on two existing assimilation systems and on potential new level-4 data products. The first one globally assimilates observations into the WaterGAP model (WaterGAP-DA, Gerdener et al. 2023) at a spatial resolution of approximately 50 km. In the second framework observations are assimilated into the Community Land Model (CLM-DA, Springer et al. 2019) over Europe, which simulated processes at the land surface at 12 km grid scale. Our simulations will quantify how the new missions enable more realistic water storages via assimilation approaches, in terms of improved spatial resolution as compared to GRACE, while also offering temporally consistent fluxes and water storages.

Reference: Gerdener, H., Kusche, J., Schulze, K. et al. The global land water storage data set release 2 (GLWS2.0) derived via assimilating GRACE and GRACE-FO data into a global hydrological model. J Geod 97, 73 (2023). https://doi.org/10.1007/s00190-023-01763-9