Abstract
Abstract. Time variable gravity fields, reflecting variations of mass distribution in the system Earth is one of the key parameters to understand the changing Earth. Mass variations are caused either by redistribution of mass in, on or above the Earth's surface or by geophysical processes in the Earth's interior. The first set of observations of monthly variations of the Earth gravity field was provided by the US/German GRACE satellite mission beginning in 2002. This mission is still providing valuable information to the science community. However, as GRACE has outlived its expected lifetime, the geoscience community is currently seeking successor missions in order to maintain the long time series of climate change that was begun by GRACE. Several studies on science requirements and technical feasibility have been conducted in the recent years. These studies required a realistic model of the time variable gravity field in order to perform simulation studies on sensitivity of satellites and their instrumentation. This was the primary reason for the European Space Agency (ESA) to initiate a study on ''Monitoring and Modelling individual Sources of Mass Distribution and Transport in the Earth System by Means of Satellites''. The goal of this interdisciplinary study was to create as realistic as possible simulated time variable gravity fields based on coupled geophysical models, which could be used in the simulation processes in a controlled environment. For this purpose global atmosphere, ocean, continental hydrology and ice models were used. The coupling was performed by using consistent forcing throughout the models and by including water flow between the different domains of the Earth system. In addition gravity field changes due to solid Earth processes like continuous glacial isostatic adjustment (GIA) and a sudden earthquake with co-seismic and post-seismic signals were modelled. All individual model results were combined and converted to gravity field spherical harmonic series, which is the quantity commonly used to describe the Earth's global gravity field. The result of this study is a twelve-year time-series of 6-hourly time variable gravity field spherical harmonics up to degree and order 180 corresponding to a global spatial resolution of 1 degree in latitude and longitude. In this paper, we outline the input data sets and the process of combining these data sets into a coherent model of temporal gravity field changes. The resulting time series was used in some follow-on studies and is available to anybody interested.
Highlights
The primary goal of the recently completed European Space Agency (ESA) study entitled “Monitoring and Modelling in-dividual Sources of Mass Distribution and Transport in the Earth System by Means of Satellites” was to find the most advantageous approach for using satellites to track the individual components of mass redistribution in the Earth System
The twin-satellite GRACE mission has shown that time variable gravity field is observable from space on a monthly base with spatial resolutions of a few hundred kilometres (Tapley et al, 2004)
In the following sub-chapters we provide a detailed description for the pre-processing of the geophysical mass fields and the conversion to spherical harmonics
Summary
The primary goal of the recently completed European Space Agency (ESA) study entitled “Monitoring and Modelling in-dividual Sources of Mass Distribution and Transport in the Earth System by Means of Satellites” (see Acknowledgements) was to find the most advantageous approach for using satellites to track the individual components of mass redistribution in the Earth System. The second step was to use the realistic Earth mass model to develop satellite orbits and configurations to monitor the changes such that we are able to separate the different components of the mass variability. The European Alps, Himalaya, Karakoram and Gulf of Alaska (Fig. 4), we prescribed plausible secular mass loss trends that covered a range of signal magnitude from ∼15 to 75 Gt yr−1 These fluxes were subsequently incorporated into the global hydrology model Two processes that have different spatial and temporal behaviours control the ice sheet fluxes: surface mass balance (SMB) and ice dynamics (ID). The former is determined by the balance between solid precipitation and runoff and responds almost instantaneously to changes in external forcing. Inter-annual variability in SMB is large compared to, for example, secular trends in mass loss and it is important, that this is adequately simulated in addition to any underlying trends
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