Description: The main objective of the Gravity Recovery and Climate Experiment (GRACE) Atmospheric and Oceanic De-Aliasing Level-1B product (AOD1B) is the removal of high-frequency non-tidal mass variations due to sub-monthly mass transport in the atmosphere and oceans. Application of AOD1B shall avoid aliasing of these high-frequency signals into monthly gravity models derived from modern gravity missions and shall help to derive consistent orbit solutions for altimetry and Satellite Laser Ranging missions. The AOD1B 6-h series of spherical harmonic coefficients up to degree and order 100 are routinely generated at the German Research Centre for Geoscience and distributed to the GRACE Science Data System and the user community. Inputs for this product are acquired from numerical weather prediction models which are regularly revised and consequently not stable in time. The latest AOD1B release 5 (RL05) is based, as all other releases, on input from ECMWF and does not resolve this problem of discontinuities present in the surface pressure and surface geopotential input data. This might contaminate the gravity field variations derived from atmospheric mass variations. In this paper we present a method to overcome this problem during future AOD1B product generation, as well as two new Level-2 products (GAE and GAF) that, over land, fix a posteriori the two jumps present in the already distributed Level-2 RL05 monthly gravity models which were based on AOD1B RL05. The impact of the proposed correction on the variations and long-term trend of the total mass of the atmosphere and on the ice mass balance over Antarctica and over Greenland is also illustrated. We found that the GAE/GAF-corrected trend of the global atmospheric mass over the GRACE mission lifetime significantly decreased from –0.05 to –0.02 mm yr –1 in terms of geoid height. A considerable effect (33 per cent) was also found in the quadratic term of ice mass loss over Antarctica which results in an acceleration of 3.2 Gt yr –1  yr –1 smaller than without applying this correction.

Description: We have extended backwards from 2001 to 1979 the current release 05 (RL05) of the Gravity Recovery and Climate Experiment (GRACE) Atmospheric and Oceanic De-aliasing Level-1B (AOD1B) product and studied the impact of this and a previous release 04 (RL04) of the AOD1B product on precise orbits of five altimetry satellites (ERS-1, ERS-2, TOPEX/Poseidon, Envisat and Jason-1) for the time span 1991–2012, as compared to the case when no AOD1B product is used. We have found that using AOD1B RL05 product reduces root mean square (RMS) fits of satellite laser ranging (SLR) observations by about 1.0–6.4 per cent, 2-d arc overlaps in radial, cross-track and along-track directions by about 1.3–12.0, 0.3–10.0 and 2.0–10.0 per cent, respectively, for various satellites tested, as compared to the case without AOD1B product. Using AOD1B RL05 product instead of RL04 one reduces SLR RMS fits by 0.1–0.7 per cent, 2-d arc overlaps in radial, cross-track and along-track directions by 0.1–0.6, 0.1–1.3 and 0.2–1.2 per cent, respectively, for the satellite orbits tested. The multi-mission crossover analysis shows that the application of an AOD1B product reduces the scatter of radial errors by 0.4–2.8 per cent for the satellite missions studied. At the regions with the most pronounced changes the use of the AOD1B products improves the consistency between the sea level as measured by the TOPEX and ERS-2 missions and by the Jason-1 and Envisat missions by 5 to 10 per cent (globally by about 2 per cent). The results of our study show that extended AOD1B RL05 product performs better than the AOD1B RL04 and improves orbits of altimetry satellites and consistency of sea level products.

Description: This study examines present-day changes of the Antarctic ice sheet (AIS) by means of different data sets. We make use of monthly gravity field solutions acquired by the Gravity Recovery and Climate Experiment (GRACE) to study mass changes of the AIS for a 10-year period. In addition to "standard" solutions of release 05, solutions based on radial base functions were used. Both solutions reveal an increased mass loss in recent years. For a 6-year period surface-height changes were inferred from laser altimetry data provided by the Ice, Cloud, and land Elevation Satellite (ICESat). The basin-scale volume trends were converted into mass changes and were compared with the GRACE estimates for the same period. Focussing on the Thwaites Glacier, Landsat optical imagery was utilised to determine ice-flow velocities for a period of more than two decades. This data set was extended by means of high-resolution synthetic aperture radar (SAR) data from the TerraSAR-X mission, revealing an accelerated ice flow of all parts of the glacier. ICESat data over the Thwaites Glacier were complemented by digital elevation models inferred from TanDEM-X data. This extended data set exhibits an increased surface lowering in recent times. Passive microwave remote sensing data prove the long-term stability of the accumulation rates in a low accumulation zone in East Antarctica over several decades. Finally, we discuss the main error sources of present-day mass-balance estimates: the glacial isostatic adjustment effect for GRACE as well as the biases between laser operational periods and the volume-mass conversion for ICESat.

Description: Short term mass variations cannot be measured adequately by GRACE due to undersampling. Therefore, they are removed from the measurements beforehand using geophysical models (de-aliasing). As GRACE data analysis has not yet reached ultimate accuracy, one presumes that inadequate de-aliasing and geophysical model uncertainties are one possible reason for this. The standard GRACE de-aliasing procedure disregards geophysical model (atmospheric and oceanic) errors, therefore a procedure has been developed to take atmospheric and oceanic uncertainties into account. Thereby, we expect to improve the de-aliasing product and the resulting GRACE gravity field models. As the GRACE results are used for geophysical interpretations, any increase in accuracy will lead to a better understanding of geophysical processes. The paper summarizes the results of this work. First, the standard de-aliasing process as well as the new procedure, which is able to take geophysical model uncertainties into account, is summarized. After the definition of the performed error scenarios, an overview and discussion of the applied atmospheric and oceanic error maps is given. Finally, the impact of the previously defined error scenarios on the de-aliasing coefficients as well as on a GRACE gravity field solution is investigated. K-band range-rate satellite-to-satellite tracking (KBRR) residuals, as an intermediate gravity field result and gravity field solutions based on the applied error scenarios are analyzed. From the results obtained so far it can be concluded that GRACE results or rather KBRR residuals are sensitive to atmospheric and oceanic model errors. Depending on the error structure of the introduced model uncertainties, KBRR residuals could have been reduced. However, it has to be stated that with respect to the current GRACE error budget, atmospheric and oceanic model uncertainties seem not to play a prominent role in the accuracy of current GRACE gravity field solutions. Nevertheless, further and deeper analysis of the KBRR residuals is needed, as (positive) effects of model uncertainties are visible here.

Description: The GFZ German Research Centre for Geosciences as part of the GRACE Science Data System (SDS) is currently reprocessing the complete GRACE mission data. This new Level-2 data release (RL06 in the SDS nomenclature) will be based on reprocessed Level-1B instrument data (RL03), updated processing standards and background models and will take care of limitations known from previous RL05. Examples are the application of the latest RL06 Atmosphere and Ocean Dealiasing Model, update of the ocean tide model, implementation of the most recent IERS conventions or improvements in GFZ´s GPS data processing. This 15+ year time series of monthly Level-2 spherical harmonics and underlying processing standards will then serve for the continuation with GRACE-FO (Follow-on) data expected for early 2018. In parallel a team of GFZ, the Alfred-Wegener-Institute Bremerhaven and TU Dresden has developed and implemented a portal at GFZ where users can download dedicated Level-3 products for hydrological, oceanic and polar research activities. This portal is expected to be made public by the end of 2017. The presentation will show the status and examples of these new RL06 Level-2 products and prototype Level-3 products based on GFZ’s RL05a Level-2 monthly solutions.