We examined the sensitivity of GRACE Follow‐On (GRACE‐FO) laser ranging interferometer (LRI) measurements to sub‐monthly time‐variable gravity (TVG) signals caused by transient, high‐frequency mass changes in the Earth system. GRACE‐FO LRI provides complementary inter‐satellite ranging measurements with higher precision over a wider range of frequencies than the baseline K‐band microwave ranging system. The common approach for studying mass variation relies on the inverted TVG or mascon solutions over a period of, for example, one month or 10 days which are adversely affected by temporal aliasing and/or smoothing. In this article, we present the alternative along‐orbit analysis methodology in terms of line‐of‐sight gravity difference (LGD) to fully exploit the higher precision LRI measurements for examination of sub‐monthly mass changes. The discrepancy between “instantaneous” LGD LRI observations and monthly‐mean LGD (from Level‐2 data) at satellite altitude indicates the sub‐monthly gravitational variability not captured by monthly‐mean solutions. In conjunction with the satellite ocean altimetry observations, high‐frequency non‐tidal atmosphere and ocean models, and hydrology models, we show that the LGD LRI observations detect the high‐frequency oceanic mass variability in the Argentine Basin and the Gulf of Carpentaria, and sub‐monthly variations in surface (river) water in the Amazon Basin. We demonstrate the benefits gained from repeat ground track analysis of GRACE‐FO LRI data in the case of the Amazon surface water flow. The along‐orbit analysis methodology based on LGD LRI time series presented here is especially suitable for quantifying temporal and spatial evolution of extreme, rapidly changing mass variations.
For nearly 20 years, the Gravity Recovery and Climate Experiment (GRACE;Tapley et al., 2004) and its successor mission GRACE Follow-On (GRACE-FO;Landerer et al., 2020) have provided global gravity fields monitoring large-scale mass re-distributions. These measurements have found a wide array of applications such as in monitoring terrestrial water storage and droughts (Boergens et al., 2020) or estimating the mass loss of the Greenland ice sheet (Sasgen et al., 2020). Bottom pressure anomalies estimated from GRACE data have also been used to infer changes in the North Atlantic Overturning Circulation (Landerer et al., 2015). As such, GRACE and GRACE-FO have made vital contributions to our understanding of environmental changes on our planet (Rodell et al., 2018;Tapley et al., 2019).
<p>The Atmosphere and Ocean non-tidal De-aliasing Level-1B (AOD1B) product is widely used in satellite gravimetry to correct for transient effects of atmosphere-ocean mass variability that would otherwise alias into monthly-mean global gravity fields. The most recent release is based on the global ERA5 reanalysis and ECMWF operational data together with simulations from the general ocean circulation model MPIOM consistently forced with fields of the same atmospheric data-set.</p>
<p>The talk focusses on the current status of the new release RL07. We present the changes made for RL07 with respect to the previous release RL06, the current status of the processing as well as the ongoing work in terms of consistency and uncertainty assessment.</p>
<p>Surface mass anomalies on Earth modify the external gravity field via both Newtonian attraction and elastic deformation of the underlying crust. Time-variable mass transport divergence leading to quickly changing surface mass distributions induces additional horizontal pressure gradients that feed back into the dynamics of the transport process. In view of the present-day accuracy of geodetic observations, this feedback is well known to be important for global ocean tide modelling (Ray, 1998). The same feedback, however, is also affecting the barotropic response of the global oceans to surface wind stress and atmospheric pressure loading. It is typically termed as "Self Attraction and Loading" and can be seen as one contribution to sea-level variability induced by "Gravity, Rotation, and Deformation (GRD)" as defined by Gregory et al. (2019).</p><p>In this presentation, we will specifically discuss the contribution to sea-level variability induced by surface pressure variations over the continents, which are by now often ignored in numerical ocean modelling. Induced ocean bottom pressure signals are specifically prominent at the shortest periods between hours and days, and frequently exceed 1 hPa in coastal regions. The signals are found to be relevant for the satellite gravimetry missions GRACE and GRACE-FO, and the process will be therefore included in the next release of the AOD1B non-tidal de-aliasing product. &#160;</p>
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