Synoptic meteorological signal in daily and five-day GRACE/GRACE-FO solutions

The sub-monthly gravity solutions derived from the Gravity Recovery and Climate Experiment (GRACE), and its Follow-On (FO) missions at daily and five-day interval represent a new opportunity to investigate rapid sub-monthly changes in terrestrial hydrology and help to map and understand the progression of hydrological extremes. In this research, we report on a comparison of the five-day mascon solution developed by the Center for Space Research (CSRm-5d), University of Texas at Austin, and the daily spherical harmonics solution from the Institute of Geodesy, Graz University of Technology (ITSG-2018). We compare the two GRACE/GRACE-FO products with atmospheric reanalysis data for the net meteorological changes in the water balance over grid and hydrological basin scales using an independent NASA-Modern Era Retrospective-analysis for Research and Applications (MERRA) and fifth generation ECMWF atmospheric reanalysis of the global climate  (ERA -5) data covering the period 2004-2010. Water fluxes and net changes in water balance at a 5-day rate, and 1-degree grid were derived, and the signal at sub-monthly timescale was isolated using one-dimensional inverse wavelet transform. Statistical matrices were used to evaluate the phase, amplitudes, and variability of the rapid signal. Results show that both GRACE/GRACE-FO data uncover valuable information of net meteorological changes for time scale as short as 11-day. Higher variability and stronger amplitude of the signal are predominantly in areas where the change in the meteorological fluxes is high (e.g., North India, South Africa, and Eastern U.S). While the ITSG-2018 solutions are statistically and process model-based, the CSRm-5d solutions are purely based on gravity fields. Both datasets show consistent agreement relative to the MERRA-2 over the globe. Rapid sampling of GRACE/GRACE FO gravity fields provides a new avenue to infer and understand rapid global and local geophysical processes.