ALBERT

Description: GRACE/GRACE-FO Level-3 product based on GFZ RL06 Level-2B products (Dahle & Murböck, 2019) representing Ocean Bottom Pressure (OBP) variations provided at 1° latitude-longitude grids as defined over ocean areas. The OBP grids are provided in NetCDF format divided into yearly batches. The files each contain seven different variables: 1) 'barslv': gravity-based barystatic sea-level pressure 2) 'std_barslv': gravity-based barystatic sea-level pressure uncertainties 3) 'resobp': gravity-based residual ocean circulation pressure resobp 4) 'std_resobp': gravity-based residual ocean circulation pressure uncertainties 5) 'leakage': apparent gravity-based bottom pressure due to continental leakage 6) 'model_ocean': background-model ocean circulation pressure 7) 'model_atmosphere': background-model atmospheric surface pressure These Level-3 products are visualized at GFZ's web portal GravIS (http://gravis.gfz-potsdam.de). Link to data products: ftp://isdcftp.gfz-potsdam.de/grace/GravIS/GFZ/Level-3/OBP

Description: Based on the latest GFZ release 06 of monthly gravity fields from GRACE satellite mission, area-averaged barystatic sea-level is found to rise by 2.02 mm/a during the period April 2002 until August 2016 in the open ocean with a 1000 km coastal buffer zone when low degree coefficients are properly augmented with information from satellite laser ranging. Alternative spherical harmonics solutions from CSR, JPL and TU Graz reveal rates between 1.94 and 2.08 mm/a, thereby demonstrating that systematic differences among the centers are much reduced in the latest release. The results from the direct integration in the open ocean can be aligned to associated solutions of the sea-level equation when fractional leakage derived from two differently filtered global gravity fields is explicitly considered, leading to a global mean sea-level rise of 1.72 mm/a. This result implies that estimates obtained from a 1000 km coastal buffer zone are biased 0.3 mm/a high due the systematic omission of regions with below-average barystatic sea-level rise in regions close to substantial coastal mass losses induced by the reduced gravitational attraction of the remaining continental ice and water masses.

Description: This study has been run in the context of the European Union research project G3P (Global Gravitybased Groundwater Product) on developing Groundwater storage (GW) as a new product for the EU Copernicus Services. GW variations can be derived on a global scale by subtracting from total water storage (TWS) variations based on the GRACE/GRACE-FO satellite missions variations in other water storage compartments such as soil moisture, snow, surface water bodies, and glaciers. Due to the nature of data acquisition by GRACE and GRACE-FO, the data need filtering in order to reduce North-South-oriented striping errors. However, this also leads to a spatially smoothed TWS signal. For a consistent subtraction of all individual storage compartments from GRACE-based TWS, the individual data sets for all other hydrological compartments need to be filtered in a similar way as GRACE-based TWS. In order to test different filter methods, we used compartmental water storage data of the global hydrological model WGHM. The decorrelation filter known as DDK filter that is routinely used for GRACE and GRACE-FO data introduced striping artifacts in the smoothed model data. Thus, we can conclude that the DDK filter is not suitable for filtering water storage data sets that do not exhibit GRACE-like correlated error patterns. Alternatively, an isotropic Gaussian filter might be used. The best filter width of the Gaussian filter is determined by minimizing the differences between the empirical spatial correlation functions of each water storage and the spatial correlation function of GRACE-based TWS. We also analyzed time variations of correlation lengths such as seasonal effects. Finally, the selected filter widths are applied to each compartmental storage data set to remove them from TWS and to obtain the GW variations.

Description: Groundwater is one of the most important freshwater resources for mankind and for ecosystems. Assessing groundwater resources and developing sustainable water management plans based on this resource is a major field of activity for science, water authorities and consultancies worldwide. Due to its fundamental role in the Earth’s water and energy cycles, groundwater has been declared as an Essential Climate Variable (ECV) by GCOS, the Global Climate Observing System. However, within Copernicus - the European Earth Observation Programme – there is no service available yet to deliver data on this fundamental resource, nor is there any other data source worldwide that operationally provides information on changing groundwater resources in a consistent way, observation-based, and with global coverage. Filling this gap is the goal of the G3P (Global Gravity-based Groundwater Product) project, funded since the beginning of 2020 by the European Union. G3P aims at (1) capitalizing from the unique capability of GRACE and GRACE-FO satellite gravimetry as the only remote sensing technology to monitor subsurface mass variations and thus groundwater storage change for large areas with global coverage, and (2) incorporating and advancing a wealth of products on storage compartments of the water cycle that are already part of the Copernicus portfolio and will be used for separating out the groundwater storage variations from the gravity-based total terrestrial water storage, to finally (3) developing an operational global groundwater service prototype as a cross-cutting extension of the existing Copernicus portfolio. In this contribution, we present the concept of G3P and first results.

Description: This work presents a new extension to B-Splines that enables them to model functions on directed tree graphs such as non-braided river networks. The main challenge of the application of B-splines to graphs is their definition in the neighbourhood of nodes with more than two incident edges. Achieving that the B-splines are continuous at these points is non-trivial. For both, simplification reasons and in view of our application, we limit the graphs to directed tree graphs. To fulfil the requirement of continuity, the knots defining the B-Splines need to be located symmetrically along the edges with the same direction. With such defined B-Splines, we approximate the topography of the Mekong River system from scattered height data along the river. To this end, we first test and validate successfully the method with synthetic water level data, with and without added annual signal. The quality of the resulting heights is assessed besides others by means of root mean square errors (RMSE) and mean absolute differences (MAD). The RMSE values are 0.26 m and 1.05 m without and with added annual variation respectively and the MAD values are even lower with 0.11 m and 0.60 m. For the second test, we use real water level observations measured by satellite altimetry. Again, we successfully estimate the river topography, but also discuss the short comings and problems with unevenly distributed data. The unevenly distributed data leads to some very large outliers close to the upstream ends of the rivers tributaries and in regions with rapidly changing topography such as the Mekong Falls. Without the outlier removal the standard deviation of the resulting heights can be as large as 50 m with a mean value of 15.73 m. After the outlier removal the mean standard deviation drops to 8.34 m.